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A
MANUAL OF MAGNETISM,
INCLUDING
GALVANISM, MAGNETISM, ELECTRO-MAGNETISM,
ELECTRO-DYNAMICS, NMAGNETO-ELECTRICITY,
AND TIIERilO-ELECTRICITY.
WITH 180 ORIGINAL ILLUSTRATIONS.
ff ourtib abftfon.
B O S T ON:
PUBLISHED BY DANIEL DAVIS, JR.,
MAGNETICAL INSTRUMENT MAKER,
428 WASINGTON STREET.
1852.




The instruments described in the following treatise are manufactured by Daniel Davis, Jr., and sold by him, No. 428 Washington
Street, and also by Joseph M' ilgntman, No. 33 Cornhill, Boston.
Entered according to Act of C"ongress in the year 1847, by
1)ANIEL  DAVIS$  JR.,
fn the Clerk's Office of the District Court of the District of Massachusetts








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PRE FAEA.
THi present work is principally occupied with Magnetism in its connection with Electricity. But the general phenomena of both these sciences are described as
fully as the thorough comprehension of the relations existing between them appeared to require. It has been
the aim to give to the book a practical, rather than a
theoretical character, and to introduce hypotheses no
further than was essential to a clear explanation of the
phenomena described. Many important observations and
results connected with magnetism and electricity have
never been brought together in any scientific work; and
the present volume, though mainly intended as a companion to the apparatus manufactured by me, has, in
consequence, taken the form of a treatise on those
branches of science to which it relates, and may be
used as a text-book.
Since the appearance of the first edition of this
Manual, in 1842, some progress has been made in the
departments of science treated of, and many new and
1#




vi                  PREFACE.
improved instruments have been contrived'to illustrate
them. In preparing a new edition, alterations and additions have been made, to adapt it better to the purposes of a text-book for Colleges and High Schools, and
also as a companion to the apparatus.  The Manual
has for some years been in use as a text-book, for the
sake of its demonstrations, in the United States' Military
Academy at West Point.
The preparation of the present, as well as of the
former edition, has been principally under the charge
of Drs. John Bacon, Jr., and William F. Channing.
The arrangement which has been adopted, and which
differs from that of any previous work, is founded upon
a natural order; and the attempt has thus been made
to establish a more scientific relation between the facts
of magnetism.  Many of the instruments and experiments are original, and are now for the first time
described.  Among the new  articles of apparatus, a
considerable number have been contrived or improved
by Mr. E. B. Horn. Numerous wood cuts are intro.
duced to illustrate the subjects under consideration.
DANIEL DAVIS, JR.
B.OSTON, Auigust, 1S47.




CONT E NTS.
INTRODUCTION.
fta
DEFINITIONS AND EXPLANATIONS...................  1
PRODUCTION OF ELECTRICITY.
1. FRICTIONAL ELECTRICITY,.................... 7
2. GALVANIC ELECTRICITY,........................... 8
3. THERMO-ELECTRICITY.....................6...... 67
4. ANIMAL ELECTRICITY,.........82.....      82
MAGNETISM.
BOOK I.
DIRECTIVE TENDENCY OF THE MAGNET.
1. IN REFERENCE TO ANOTHER MAGNET...,...... 89
2. IN REFERENCE TO A CURRENT OF ELECTRICITY,. 94
3. IN REFERENCE TO THE EARTH,................... 130




Viii                      CONTENTS.
BOOK II.
INDUCTION OF MAGNETISM.
Page
1. BY  A  MAGNET,.................................... 139
2. BY A CURRENT  OF  ELECTRICITY,................  159
3. BY  THE  EARTH,.......................2.........  223
BOOK III.
INDUCTION OF ELECTRICITY.
1. BY  A  CURRENT  OF  ELECTRICITY...............                 227
2. BY A MAGNET,................. 64
3. BY THE  EARTH,................................... 




INTRODUCTION.
DEFINITIONS AND EXPLANATIONS.
1. MAGNETISM. — The term magnetism expresses
the peculiar properties of attraction, repulsion, &c.,
possessed, under certain circumstances, by iron and
some of its compounds, and in a somewhat inferior
degree by nickel, a closely-allied metal. Cobalt is
perhaps slightly magnetic.
ELECTRO-MAGNETISM. - That branch of science
which relates to the development of magnetism by
means of a current of electricity, is called electromagnetism. It will be treated  of in  Book I.
Chapter 2, and in Book II. Chapter 2.
MAGNETO-ELECTRICITY treats of the development
of electricity by the influence of magnetism, and
will form the subject of Book III. Chapter 2.
2. MAGNET. -The body which exhibits magnetic
properties is called a magnet. This name is confined
to the metallic substances mentioned above; but all
conductors of electricity are capable of exhibiting
similar attractions and repulsions while conveying
a current.




~2            I) A VX I  M;' S I  As A' r A  L.
NATURAL  MAGNETS. - Certain  ores of iron are
found to be possessed of the magnetic properties in
their natural state.  These are called natural magnets, or loadstones.
ARTIFICIAL MAGNETS.- Bodies belonging to the
magnetic class, in which magnetism  is artificially
induced, ar called artificial magnets.
3. INDUCTION OF MAGNETISM. - Whenever magnetic properties are developed in bodies not previously possessed of them, the process is termed the
induction of magnetism.  When this is effected by
the influence of a magnet, it is called magnetic induction; when by a current of electricity, electromagnetic induction.
INDUCTION OF ELECTRICITY. —  This term expresses the development of electricity by the influence
of other electricity in its neighborhood, or by the
influence of magnetism.  In order to distinguish
the' inductive action of' an electric current from the
static induction of electricity at rest, the former is
called electro-dynamic induction.   The development
of electricity by the influence of a magnet is termed
magneto-electric induction.
4. POLES. - The magnetic phenomena manifest
themselves principally at the two opposite extremities of the magnet; the force of the attractions and
repulsions diminishing rapidly as the distance from
them increases, until it becomes entirely insensible
at the middle point. These extremities are called
the poles of the magnet.




EXPLANATIONS.                   3
5. The earth itself is found to possess the properties of -a magnet, having magnetic poles corresponding nearly in their direction with the poles of
its diurnal rotation. Now, if a straight magnet be
suspended so as to allow of a free horizontal motion, it will be found to place itself in a direction
nearly north and south; as will be explained hereafter.  The end which turns towards the north is
called the north pole of the magnet, the other end
its south pole. Hence every magnet, whatever its
form, is said to have a north and a south pole. In
the figures to be hereafter described, the north pole
is indicated by the point of an arrow, and the south
pole by the feather; or by the letters N and S respectively. The poles of a galvanic battery will
be described  farther on, when treating of that
instrument.
6. PERMANENT MAGNETS. -It is found that pure
soft iron easily acquires magnetism when exposed
to any magnetic influence, but immediately loses
this magnetism when that influence is withdrawn.
But steel, which is a compound of iron with a small
quantity of carbon, and especially hardened caststeel, though it acquires the magnetic properties less
readily, retains them more or less permanently after
they are  acquired.  Hence a magnet formed of
hardened steel is called a permanent magnet.
7. BAR MAGNET. — An artificial permanent magnet,
in the form of a straight bar, is called a bar magnet.




4             DAVIS SS MANUAL.
Fig. 1.           Fig. 1 represents
=_=.-=_    a.. _: ja small case, containing  two  bar
_ ____...___'_ =__   magnets, with two' I --:-  -.... x'   short pieces of soft
A....:          __ iron  connecting
their poles: these
act as armatures (see ~ 9), and serve  to preserve
the power of the magnets.  The magnets, when not
in use, should be kept packed in the case, with their
opposite poles connected by the armatures, in the
manner shown in the cut.
COMPOUND BAR MAGNET. —A magnet composed
of several straight bars joined together, side by side,
with their similar poles in contact, for the purpose
of increasing the magnetic power, is called a compound bar magnet.
Fig. 2.
Such a magnet, composed of three simple magnets, fastened together, is represented in Fig. 2.
Fig. 3.
8. HORSESHOE OR U-MAGNET. -A magnet which is bent into such a form as to
bring the two opposite poles near together,
so that they can be connected by a short,
straight piece of iron, is called a horseshoe
or U-magnet.
Fig. 3 represents a steel magnet of this
description.




EXPLANATIONS.                  O
Fig 4.    COMPOUND HORSESHOE MAGNET. —A
magnet composed of several horseshoe
magnets joined together, side by side,
as in Fig. 4, for the purpose of increasing the power, is called a compound
o  horseshoe magnet, or magnetic battery.
These magnets are charged separately,
and are put together with all the similar
A    poles in the same direction.
9. ARMATURE. -A piece of soft iron, adapted to,
and intended to connect the poles of a magnet, is
called an armature, or keeper.  Horseshoe magnets
are usually provided with an armature, consisting
of a straight bar of iron, for the purpose of preserving their magnetic power: this should be kept
constantly applied to the poles of the magnet when
it is not in use; as shown in Fig. 4, where A is the
keeper. Armatures are employed in various experiments, and their forms vary with the purposes intended.
Fig 5.' 0. MAGNETIC NEEDLE.
- A light and slender
mnagnet, mounted upon a
centre of motion, so as
to allow it to traverse
freely in certain directions, is called a magnetic needle. It may be
so mounted as to move
only horizontally, as in
Fig. 5; or its motion
~~~




6             DAVIS5S MANUAL.
Fig. 6.   may be confined to a vertical direction, as shown in Fig. 6.  This last
form is called a dipping needle.  By
means of a universal joint, the needle
may be supported in such a manner as
to have freedom of motion in both a
horizontal and vertical direction.
11. The most obvious effects exhibited by magnets are their power to attract iron, and their
tendency, when freely suspended, to assume a determinate position in reference to the earth. For a
long time, these were the only properties which were
noticed, or at least which received particular attention. The attractive power of the loadstone over
small pieces of iron seems to have been known from
the remotest antiquity; but its polarity with regard
to the earth does not appear to have been observed,
at least in Europe, until the eleventh or twelfth
century of the Christian era.  It is, however, stated,
by a distinguished orientalist, M. Klaproth, that the
Chinese were acquainted with the polarity of the
magnet, and employed it as a guide to travellers
on land, at least as early as the second century;
also, that the use of the mariner's compass in navigation originated with them, and was communicated to the European nations through the Arabs.




FRICTIONAL  ELECTRICITY.               7
PRODUCTION  OF  ELECTRICITY.
12. As a current of electricity is requisite in
many of the experiments to be mentioned hereafter, it becomes necessary to describe the various
means by which it may be produced.
I. MECHANICAL OR FRICTIONAL ELECTRICITY.
13. The electricity developed by the electrical
machine is called mechanical or frictional, from the
mechanical force or friction by which it is obtained.
It possesses properties differing much in degree from
those exhibited by the galvanic arrangements described below, and is far inferior in producing magnetic effects, which require an electric current.  On
the other hand, it greatly surpasses galvanic electricity in exhibiting the phenomena of electricity at
rest, tunder its two forms of positive and negative.
14. The great development of electricity observed
during the escape of steam from high pressure boilers, may be mentioned here. This is collected for
purposes of experiment, by plunging into the steam,
escaping from the safety valve, a brass rod (Fig. 7)




8             DAVIS S  MANUAL.
furnished with a brush of points, P, at one end, to
collect the electricity, and held by means of an inFig. 7.
sulating handle attached to the other end.  A length
of six or eight feet is found' advantageous, in this
instrument, to convey and insulate the electricity,
which may be conveniently drawn from the lower
part of the rod. In the cut, the brass rod is represented as terminating in a brass ball, B, insulated from
the wooden handle, H, by a stout glass rod, G.
15. The electricity obtained in this way from
steam is of high intensity, affording sparks of an inch
or more in length, and charging the Leyden jar so
as to give strong shocks. It is almost always _positive, and is not obtained unless the steam is of high
pressure, so as to issue from the valve as a transparent vapor.
II. GALVANIC OR VOLTAIC ELECTRICITY.
16. These names are given to that form of electricity which is produced by chemical action. It
is found that, when two metals are placed in contact
with each other, and with some liquid capable of
acting upon one more than upon the other, electricity
of a peculiar character is developed. The peculiar
electrical relation of the metals employed, also exerts
an influence upon this result. The metals most




GALVANIC  ELECTRICITY.                 9
extensively used are zinc and copper, or zinc and
platinum; and the chemical agent is some liquid
containing an acid having a powerful affinity for
zinc. The language adopted in describing the resulting phenomena was founded originally on the
supposition that electricity is given out to the copper
from the zinc, which is corroded, through the liquid.ig. 8.    between them.  This is shown in
the adjoining cut (Fig. 8), which
represents a glass vessel, nearly
filled with the fluid, and containing a zinc plate, marked Z, and
one of copper, C.  Now, the supposed motion of the electric current within the vessel, is from Z
to C; and if the two metals are
connected by a wire without the
vessel, as in the cut, in order to
fulfil the condition of metallic
contact, the electricity is supposed
to pass around through the wires
from the copper to the zinc again, to restore the
equilibrium of the fluid. Thus the current is considered as passing from zinc to copper within the
series, and from  copper to zinc without it.  The
wire connected with C is called the positive pole of
the arrangement, and that with Z the negative pole.
17. The electricity proceeding from the positive
pole is the same in its relations as the electricity
from the prime conductor of the electrical machine,
which originally received the name of positive; while




10            D A V I S'S MANUAL.
that from the negative pole corresponds with the
electricity obtained from the rubber of the machine.
These terms are, however, to a certain extent, arbitrary. It is still an open question, whether there is
one fluid moving in a particular direction, or two
fluids moving in opposite directions, or no motion
of a fluid at all.  The fact which is sought to be
explained by these theories remains fixed. In the
above-described circulation of the electrical fluid,
technically called the galvanic circuit, there is an
electrical influence propagated in a certain unchanging direction; and as the control of the magnetic
and chemical reactions produced depends upon our
knowledge of this, it is necessary that the signification of the terms should be understood.
18. Professor Faraday has proposed a nomenclature of electricity, which has been adopted in some
scientific treatises. The poles are called by him
electrodes, from  the Greek ";xsxrpov and  6~aS; that is,
ways or paths of electricity; the positive pole the
anode, from the Greek atvoao&, an ascending or entering way, and the negative pole the cathode, from
the Greek x%&o&kc, a descending way or path of exit.
The terms positive and negative pole are, however,
still most frequently employed to designate these
extremities, and the wire without, when in connection with these poles, is spoken of as the channel
of a positive current passing from the former to
the latter.
19. Instead of using two metals to form the
galvanic circuit, one metal, in different states, may




QUANTITY  AND  INTENSITY.                11
be used on the same principle; the essential condition of this current being only that one part of a
conductor of electricity shall be more corroded by
some chemical agent than another part. Thus, if a
galvanic pair be made of the same metal, one part
of which shall be softer than another, as of cast and
rolled zinc, so as to be differently corroded, or if
a greater amount of surface be exposed to corrosion
on one side than on the other, or a more powerful
chemical agent be used on one side, a current will
be determined from the part most corroded through
the liquid to the part least corroded, whenever the
circuit of the poles is completed.
20. Galvanic electricity is capable of producing
the most extensive magnetic, chemical, and calorific
effects. In this respect, it has a far greater capacity
than mechanical electricity, though it is found that,
by the accumulation of this latter, the same effects
can be produced in proportion to the amount present. This has led to the natural inference, that,
in galvanic electricity, the quantity present is immense, while in mechanical, the quantity is small.
On the other hand, it is found that, in the latter
form, the electricity is much more energetic in its
physical reactions; that it appears to be condensed
upon insulated matter, and strives to obtain an
equilibrium  by diffusion in every direction.  It is,
therefore, said that mechanical electricity has more
intensity than galvanic, though it is difficult to assign
other than a general idea to this word. Owing to
this difference of intensity, the substances, such




1'2           D AVIS'S  MANUAL.
as glass, earthen ware, wood, ivory, which act as
non-conductors to galvanic electricity, are much
more numerous than the corresponding class, with
reference to mechanical electricity. A true comparison between these two forms of electricity, would
be made between the galvanic current and a current of mechanical electricity, freely circulating, as
through a wire connecting the prime conductor and
the rubber of an electrical machine. So compared,
the fluid would be found in both identical in its
effects, with scarcely greater difference in the conditions of quantity and intensity than we are able
to produce by different arrangements of galvanic
series.
21. There are two modes by which the peculiar
powers of a galvanic arrangement may be increased -
first, by increasing the size of the plates used; and,
secondly, by increasing their number. 1. The extension of the size of the plates.  If the size of the
plates, that is, the extent of the surfaces acted upon
by the chemical agent, is increased, some of the
resulting effects become more powerful in the same
ratio, while others do not. The power to develop
heat and magnetism is increased, while the power
to decompose chemical compounds and to affect
the animal system is very slightly, or not at all, augmented. Batteries constructed in this way, of large
plates, are sometimes called calorirnotors, from their
great power of producing heat; and they usually
consist of from one to eight pairs of plates. They
are made of various forms.  Sometimes the sheets




QUANTITY  AND  INTENSITY.              1i
of copper and zinc are coiled in concentric spirals,
sometimes placed side by side; and they may be
divided into a great number of small plates, provided
that all the zinc plates are connected together, and
all the copper plates together, and then, finally, tnat
the experiments are performed in a channel of electrical communication opened between the one congeries and the other; for it is immaterial whether
one large surface be used, or many small surfaces,
electrically connected together.  The effect of all
these arrangements, by which the metallic surface of
a single pair is augmented, is to increase the quantity
of the electricity produced.- 2. The extension of the
number of the plates consecutively; that is, by connecting the copper plate of each pair with the zinc
plate of the next pair.  By this arrangement, the
electricity is obliged to traverse a longer or shorter
series of pairs; each pair being separated from the
adjoining ones by a stratum of imperfectly conducting liquid, or by the walls of an insulating cell.
The result is, that the electricity acquires that additional impulse, which has already been referred to,
as intensity. It has greater power to pass through
imperfect conductors, or through intervals in the
circuit, to give shocks to the animal system, and
to decompose chemical compounds; and when the
number of consecutive pairs of plates is increased to
some thc usands, or even hundreds, the electricity
developed approaches very near in its character to
that produced by the electrical machine; it manifests
similar attractions and repulsions, and in fact the
2




14            DAVIS S MANUAL.
Leyden jar may be charged with it. With a very
extensive series excited by water only, and in which
each cell was carefully insulated, an Enlglish electrician has lately obtained an electrical discharge
between the poles, although separated to a considerable distance. The electricity from one pair of
plates has a very low intensity, As the number of
consecutive pairs is multiplied, the intensity increases, until at length it approximates to that of
frictional electricity, which is able to strike across a
considerable interval of air, and to fracture solid nonconductors interposed in its circuit.
22. In consequence of the low intensity of the
electricity required for electro-magnetic experiments,
it is very easy of insulation. This is a great advantage in regard to the practical construction of
magnetic apparatus. Where electricity exists in a
state of high intensity, it has a strong tendency to
pass off and dissipate itself through imperfect conductors; but where it exists only in great quantity,
it requires nearly perfect conductors to allow it a
passage. The electricity developed by a single pair
of plates, however much its power may be increased
by increasing the size of the plates, will scarcely pass
across the smallest interval of air; and a wire conveying the current may be perfectly insulated by a
covering of varnish. In working the electrical machine, )n the other hand, the electrified parts of the
apparatus must be kept at a distance from each other,
raised on glass supports, or suspended by silken lines;
and then, sinless the atmosphere is very dry, the




SMEE'S BATTERY.                  15
electricity will be rapidly dissipated.  But in the case
of currents of low intensity, however great what is
called the quantity may be, two wires may lie side by
side, with a coating of varnish or wax between them,
and convey different and opposite currents, without
any perceptible electrical intercommunication.
23. For the purposes of electro-magnetic experiments, electricity of a low intensity is required; the
power of the magnetic effects of a current of electricity depending upon an increase of its quantity,
mainly. Increasing the number of consecutive pairs
would only add to the intensity of the current, making
it more unmanageable in respect to insulation, wit>out adding much to its magnetic effects. Galvanic
batteries having many pairs of plates are therefore
unsuitable for these experiments. The maximum
magnetic effect is produced by a single galvanic combination, or at most by five or six. In some of the
reactions, however, of a magnet with a coil or electric current, as well as in experiments on decomposition and deflagration, batteries capable of producing
intense currents are required.  The most convenient
forms of single pairs and of series will therefore now
be described.
24. SMEE's BATTERY.- This form  will be first
Fig. 9.    spoken of, as furnishing all example of
the galvanic battery in its most simple
form, as it has been already illustrated
in Fig. 8.  It consists of a glass tumbler, or other receiving vessel, on which
a little frame rests, supporting the apparatus within.  The metals employed




16           D A V I S S MANUAL.
are platinum  and zinc.  The original battery of
Smee consisted of zinc, and silver covered with a film
of platinum; but the metal platinum itself is found
so much superior to the platinized silver, and the difference in expense so slight, that it has been substituted. The arrangement, also, as shown in the figure, has been modified from the original form. The
metal platinum is used in this case, as it is the least
oiidizable of the metals, and therefore capable of
producing a more powerful current with zinc than
any other.'On the frame will be seen two screwcups for the attachment of wires by which the
current may be conveyed in any direction. One
of these screw-cups communicates with a strip of
platinum foil, which is suspended between two zinc
plates, both of the surfaces of the platinum being
opposed to the zinc. The amount of galvanic action is generally in proportion to the metallic surfaces of different kinds opposed to each other, and
also in proportion to the nearness of those surfaces.
The other screw-cup is connected with both the
zinc plates, thus uniting them into a single element
of the pair. The screw-cup connected with the
platinum is insulated from the metallic frame which
supports it, by rosewood; and a thumb-screw, seen
at the left, confines or releases the zinc plates, so
that they can be renewed from time to time.
25. The liquid used to excite this battery is
sulphuric acid (oil of vitriol), diluted with ten or
twelve parts of water by measure. This acid acts on
common zinc when the galvanic circuit is not estab



SM EE'S BATTERY.                17
lished, and a great loss would therefore ensue from
the corrosion of the plates during the interval of experiments, even if the zinc was withdrawn from the
acid whenever practicable. To remedy this defect,
it has been found that zinc which has its surface
amalgamated  (or combined with  mercury) withstands the action of diluted sulphuric acid, unless t
is in galvanic connection with another metal; and
accordingly the zinc of this battery, and of other
batteries in which acid is used, is commonly amalgamated. It then remains almost uncorroded until the
galvanic circuit is completed by making contact
between the wires, when the zinc is immediately
attacked. The amalgamation of the zinc is easily
effected by rubbing it with a little mercury and
mnriatic acid at the same time. This battery, when
once in action, is very constant.  It does not, however, like the batteries hereafter to be described,
arrive instantly at its highest rate of action when the
circuit is completed, blt takes an appreciable time to
reach this point, and it is not therefore fitted for use
with apparatus where the circuit is rapidly broken
and renewed.  No adequate increase of power is
obtained by adding to the size of this battery.
26. In order to understand some of the phenomena which will be spoken of hereafter, it is
necessary to notice what takes place when the circuit of the battery is completed and the galvanic
current begins to flow. The first thing which is
observed is a rapid evolution of gas in bubbles from
the platinum  plate.  It was stated that electricity
2*




18           D AVIS' S MANUAL.
was supposed to pass through the liquid between the
plates in a direction from the zinc to the copper or
platinum.  This passage of electricity through fluids
which are not themselves elementary, is attended
always, according to Faraday, with decomposition,
wvhether the fluid is between the plates of the battery or interposed in the course of the wires or poles
leading from it. Thus, where a battery is excited
simply by water, that fluid is decomposed; its oxygen attacks the zinc, and its hydrogen is given off
in contact with the electro-negative metal.
27. In Smee's battery, the water of the acid
solution may also be considered as undergoing decomposition, one atom of its oxygen uniting with
one atom of zinc, in order to enable the sulphuric
acid to unite with the resulting oxide of zinc, and
the corresponding atom of hydrogen being given off
in contact with the platinum.  It will be observed
that the oxygen and hydrogen appear at the opposite
sides of the fluid undergoing decomposition. It is
not, however, believed that these elements travel
through the intervening distance, but that the two
atoms of water in contact with the plates are simultaneously decomposed, and that a chemical equilibrium is then established by a progressive exchange
of elements in all the intervening particles. The subject of electro-decomposition will be further spoken
of in connection with the electrotype.
28. The gas which is so plentifully disengaged
at the surface of the platinum in this battery is apt to
bring the strip into contact with the zinc, and thus




SULPHATE  OF COPPER BATTERIES. 19
cause a discharge between the metals within the battery, instead of through the poles without. To obviate this, the platinum is either confined in its proper
position by some fixture at the bottom, or a bead of
glass is attached to it, which prevents its swinging
against the zinc.
Fig. 10.              Fig. 10 represents a series of
three pairs of this
battery, in which
it will be observed
that the platinum
of one is connected with the zinc
of the next, and
that the terminal wires proceed, consequently, one
from a platinum  plate and the other from a zinc
plate, as in a single pair.
29. CYLINDRICAL SULPHATE OF COPPER BATTERY.
-This battery, a vertical section of which is represented  in Fig. 11,
consists of a double
N -~ -''~'"  cylinder of copper, C C,
with a bottom of the
same metal; which answers the purpose both
of a galvanic plate and
of a vessel to contain
the exciting  solution.
The space between the
two copper cylinders receives the solution.  There




20            D AV IS' S MANUAL.
is a movable cylinder of zinc, marked Z  ill the
sectional view, which is let down into this solution
whenever the battery is to be put inl action.  It is,
of course, intermediate in size, as well as in position,
to the two copper cylinders, and is made to rest,}pon the exterior one by means of three insulating
branches of wood or ivory, projecting from it outwardly.  Thus it hangs suspended in the solution,
and presents its two opposite surfaces to the action
of the liquid, and to the inner and outer cylinders
of copper respectively.  There is a screw-cup, N,
connected with the zinc cylinder, and also one
marked P, with the copper cylinder; and, according
to the principles heretofore explained, when a communication is made between these cups, the electricity developed by the action within the battery
will pass from one to the other.
Fig. 12.         Fig. 12 is a perspective
N  view  of the same battery,
in which the parts will be
understood vwithout  further
explanation.
30. The liquid employed
to put this battery in action
is a solution of sulphate of
copper (common blue vitriol) in water.  To prepare
it, a saturated solution of the
salt is first made, and to this solution is then added
as mInuch more water. It may be convenient to know,
that a pint of water, at the ordinary temperature of




SULPHATE  OF COPPER  BATTERIES. 21
the atmosphere, is capable of dissolving one fourth of a
pound of blue vitriol; so that the half-saturated solution employed will contain about 2 oz. of the salt
to the pint. The addition of a small portion of alcohol to this solution is sometimes of advantage, by
increasing the permanence of its action.
31. The action in this case is as follows: The
zinc is oxidized by the oxygen of the water; the
oxide combines with the acid of the salt, forming
sulphate of zinc, which remains in solution; while
the oxide of copper, which was previously combined
with the acid, being set free, partly adheres to the
surface of the zinc cylinder, or falls to the bottom of
the solution as a black powder, and partly is reduced
to metallic copper, which is precipitated on the surface of the copper cylinder, or falls to the bottom in
fine grains. This reduction of the oxide to the metallic state takes place in the following manner: The
water of the solution furnishes oxygen to the zinc,
and thus enables it to combine with the acid; while
the hydrogen, which is liberated, again forms water
with the oxygen of the oxide of copper with which
it comes in contact, leaving the metal free. Hence
but little gas is given off during the action of a
battery charged by sulphate of copper, as the hydrogen, which usually escapes, is in this case mostly
absorbed.
32. In this form of battery, there is greater intensity in proportion to the quantity of electricity
produced than in Smee's, from the fact that the final
decomposition falls on oxide of copper, and not on




22            DAVIS S  MANUAl,.
water, which is separated into its elements with
greater difficulty than the former. The force of a
galvanic battery is equal, other conditions being the
same, to the amount of corrosive force in the solution,
minus the force of chemical affinity which has to
be overcome in the decompositions which necessarily
attend.the process. The sulphate of copper battery is
found more convenient than any other in a large class
of experiments, and also generally in the hands of
persons to whom the use of acids would be inconvenient. The solution of sulphate of copper is entirely neutral, and does not injure the colors or texture
of organic substances.  Smee's battery is a more condensed form, and the action on an equal surface of
zinc more energetic.
33. The coating of oxide of copper should always
be removed from the zinc after using the battery.
For this purpose a card brush is provided. With this
the surface of the zinc should be thoroughly cleansed,
with the aid of plenty of water, whenever it has been
in use.  If this has been neglected, so that the zinc
has become covered, in whole or in part, with a hard
coating, it will be necessary to scrape or file it, to
obtain a clean metallic surface. The deposit of copper, also, which will gradually accumulate below,
must be removed from time to time.
34. Batteries excited by sulphate of copper will
keep in good action for twenty or thirty minutes at a
time. The zinc cylinder should always be taken out
of the solution when the battery is not in use, but the
solution itself may remain in the battery, as it has no




PROTECTED  BATTERIES.                 23
chemical action upon the copper, but tends to keep
its surface in good condition.  When the solution
has lost its power, as it will do, of course, after a
time, it is not best to attempt to renew its efficiency
by adding a fresh quantity of the salt.  It should be
thrown away, and a new solution be prepared, according to the foregoing directions.
35. Fig. 13 represents a PROTECTED SULPHATE OF
COPPER BATTERY. - It has been stated, in ~ 31, that
Fig. i]13.    the zinc soon becomes coated with
oxide of copper or )pulverulent metallic copper, and the action of the
battery consequently declines. Now,
the efficiency of sulphate of copper
in  providing oxide of copper to
undergo decomposition in place of.,;H   1iiItIt l water, is equally great in the final
IllJ iliilij  result when it forms only that part
iii! i 1'   of the solution next the copper plate.
Accordingly, by interposing a cell
of porous earthen ware between the zinc and copper, two cells are produced, in the outer one of which
sulphate of copper may be used, and in the inner one
a solution of Glauber's salt (sulphate of soda), or common salt.  These do not coat the zinc, though sufficient of the copper solution will eventually pass
through to make it necessary occasionally to clean
the zinc plate. Dilute sulphuric acid may also be
used in the inner cell; but in this case, it is necessary
or advisable to amalgamate the zinc. A lip will be
observed in the figure, which is used for adding




24             D AVI S S MANUAL.
crystals of sulphate of copper to the solution as its
strength declines.
36. Fig. 14 shows the internal construction of
this form. In the centre is seen a solid rod of zinc,
Fig. 14.   with the space for the saline or acid
solution surrounding it. Next comes
the porous cell; and without this
again the space containing the sulphate of copper solution.  It is necessary, after the porous cell has
beenjin use, to allow  it to soak in
several waters before drying it to put
away.  Otherwise the salts which
are contained in it will crystallize in
the interstices and disintegrate its substance.
g. 15.       37. Another form  of protected sulphate of copper battery is represented
in Fig. 15, the chief difference consisting in the porous cell, which is of
thick leather.  Except, however, where
batteries of a particular shape or size
are required, for which earthen ware
cells cannot readily be obtained, those
previously described are preferred.
Fig. 16.     38. Fig. 16 represents a protected
battery, in which the leather forms a
double cell, enclosing the zinc, so that
a double cylinder of copper can be
used, as in the cylinder battery (Fig.
11), and both surfaces of the zinc are
thus opposed to corresponding  surfaces of copper




PROTECTED  B ATTE RE S.             25
Other porous or membranous substances, such as
thick brown paper, or bladder, will answer the same
purpose as leather, in preventing the ready admixture
of the solutions, and allowing a free 9assage to the
electrical current.  When the partition is sufficiently
thin or permeable, the battery is as powerful as if
charged in the usual mode with a solution of blue
vitriol.
39. The action within this class of batteries is as
follows: The zinc is oxidized as usual, at the expense
of the water of the solution which surrounds it;
while the hydrogen, instead of being given off at the
surface of the negative plate, as in most batteries,
decomposes the sulphate of copper, forming water
with the oxygen of the oxide of copper, and liberating the sulphuric acid, which passes through the
porous partition into the other cell. A gradual and
steady supply of acid is thus furnished to dissolve
the constantly forming oxide of zinc.
40. The protected battery is also commonly called
constant, or sustaining, from the fact that it will maintain a nearly unvarying power for several days in
succession, if the solution of sulphate of copper is
kept saturated by occasionally adding a little of the
pulverized salt and stirring the liquid, to make it of
uniform strength. With weaker solutions, or a less
permeable partition, an action of sufficient energy for
many purposes may be sustained for a week or more.
and, when it declines, may be renewed by cleaning
the zinc plate, and removing any loose deposit from
the cells. This constancy of action renders the bat3




26            D AVISJ'S MANUAL.
tery of great value in the electrotype process, which
will be described hereafter. The deposition of metallic copper on the negative plate is the principa.
inconvenience attending it: this deposit sometimes
adheres so firmly as to be difficult of removal, which,
however, is only necessary when it interferes mechanically with the working of the battery.
41. GROVE'S BATTERY. —The sulphate of copper
batteries, which have been spoken of, have been sometimes called hydrogen-consuming batteries, because
they provide a chemical agent, oxide of copper, with
which the hydrogen produced in the first instance by
the decomposition of water, unites to form water
again, thus separating copper from its oxide in the
metallic form. The only chemical affinity which
has to be overcome, in this battery, is that of oxygen
for copper. Its force may be stated as equal to the
affinity of oxygen for zinc, minus the affinity of oxygen for copper. In Grove's battery, which is now
to be described, a further advance is made. The
electro-negative metal, as in Smee's, is platinum,
which, from the condensed form of the battery, can
be used without too great expense.  Instead of sulphate of copper, strong nitric acid is employed for
the purpose of furnishing oxygen to the hydrogen,
with the following advantage: Nitric acid consists
of five equivalents of oxygen and one of nitrogen.
The fifth equivalent of oxygen is held by a very
slight affinity, many chemical agents sufficing to
reduce the nitric acid to nitrous acid, which has one
equivalent less. The increase of power in Grove's




GROVE' S BATTERY.                27
battery over the sulphate of copper battery, for the
same amount of zinc dissolved, is equal to the difference of affinity between oxygen for nitrous acid and
oxygen for copper. The force of Grove's battery is,
therefore, equal to the affinity of oxygen for zinc,
minus the affinity of oxygen for nitrous acid.
42. It should also be remembered that the energy
of a galvanic arrangement depends, to some extent,
upon the difference in the affinity for oxygen of the
metals employed, which, in the case of platinum and
zinc, is at a maximum. The zinc plates are amalgamated, and the battery is worked by two fluids;
the one within the porous cell and in contact with
the platinum being strong nitric acid, and the otherr
which is in contact with the zinc, being sulphuric
acid, diluted with ten or twelve parts of water, as in
Smee's, ~ 25.  This battery is the most energetic
yet known. A Grove's battery, exposing a surface
of zinc equal to 20 square inches, was found, by its
magnetizing power, to afford a current of greater
quantity than a sulphate of copper battery exposing
210 inches of zinc. The intensity of the current in
Grove's is also considered three times as great as in
the latter, and perhaps four times as great as in Smee's.
The same platinum  and zinc of a Grove's battery,
being used as a Smee's, gave less than a fifth of the
current in magnetizing power.  Grove's battery is
also exceedingly constant.
43. The disadvantages of this battery for common
use are, first, the strength of the acids which are employed, making it disagreeable and unsafe to a careless




28             D A V IS SS MANUAL.
experimenter; and, secondly, the red fumes of nitrous
acid which it gives off abundantly while in action.
These are irrespirable and injurious to nice apparatus
which may be exposed to them. By placing a metallic covering (protected from the acid fumes) over
the battery, and allowing the gases to escape through
an orifice stuffed with cotton, wet with a little alcohol, these may be, to some extent, neutralized. It
should also be remarked that the quantity of electricity produced is, in all these batteries, proportional
to the quantity of zinc dissolved; and where a small
surface of zinc gives a large current, it is acted upon
with greater rapidity. The intensity of the current
produced in these forms depends upon the chemical
affinities which are concerned; and on this account
there is a gain in the sulphate of copper battery over
Smee's, and a still greater in Grove's.
44. The construction of Grove's Battery is shown
in Fig. 17. The containing vessel is glass. Withig. 17.  in  this is a thick cylinder of amalgamated zinc, standing on short legs,
and divided by a longitudinal opening
on one side, in order to allow the acid
to circulate freely.  Inside of this is a
porous cell of unglazed porcelain, containing the nitric acid and strip of platinum. The platinum is supported by a strip of brass,
fixed, by a thumb-screw and an insulating piece of
ivory, to the arm proceeding from the zinc cylinder.
The amalgamated zinc is not acted upon by the
dilute sulphuric acid until the circuit of the battery




GROVE S  R BTTERY.                   29
is completed; but a small portion of' the nitric acid
filters through the porous cell, by which the zinc
would be readily attacked.  It is therefore advisable
to remove it from the acid when the battery is out
of action for any length of time.
45. Fig. 18 represents a series of twelve pairs
of Grove's Battery.  In this compound battery, each
FKg. 18.
strip of platinum, except the terminal one, is soldered
directly to the arm projecting from the adjoining,zinc
plate. The terminal strip, with its screw-cup, is
supported by the first zinc plate, but is insulated from
it, as in Fig. 17.  Such a series is of very cotlsiderable practical importance, as it is now  extensively
used for purposes requiring a constant galvanic current of great quantity and intensity.  It is the form
which is in almost all cases resorted to for the magnetic telegraph.  For chemical and illustrative experiments requiring a current of great power, this has
now, to a considerable extent, superseded the older
form of consecutive plates, which will be described
immediately.   It was stated that Grove's battery
may be considered as having three times the inten3*




30            D AVIS' S MANUAL.
sity of the sulphate of copper battery. A series of
twelve pairs, therefore, would be as powerful as
thirty-six pairs of the latter; and more so than the
same number of pairs in which only sulphuric acid
is employed.
46. In Fig. 19, the TROUGH BATTERY, hitherto so
much used, is represented.  In the form in which it
is shown, the plates can be suddenly immersed or
withdrawn from  the acid at pleasure by means of
the windlass. The zinc plates are flat, and inclosed
ig- 19.          in copper cases, open
at top  and bottom.
There is no division
of the acid into separate cells.  A  represents the trough, B'. h i g h ~ the wooden case containing  the  plates.
The liquid used is sulphuric acid, diluted with forty
or fifty parts of water. If greater power is desired,
a little nitric acid may be added. E E are small
hand-vices, connected with the poles, for the purpose
of holding wires, &c. The battery represented in the
cut, consisting of twenty-five pairs of plates, should
be able to ignite a considerable length of wire, to
decompose acidulated water with rapidity, and to
give a brilliant light with charcoal points.
47. Fig. 20 represents a larger battery of a similar construction. There are two distinct series of
fifty pairs, each connected with two of the four cups
on the table above the battery. In this way the




TROUGH BATTERY.                 31
whole mray be used as a single series of one hundred
pairs, or as a battery of fifty pairs of double size, by
establishing proper connections between these cups.
Or only half the battery may be put in action, each
having a separate trough to contain the acid. The
plates are stationary, and the troughs are raised up to
them by means of two racks moved by the crank
and handle, H, which lift the platform on which the
Fig. 20.
troughs stand. Either trough may be removed from
the platform at pleasure, when it is wished to use
only half of the battery.
48. In the cut, the arrangement for producing the
arch of flame between charcoal points is shown. Two
pointed pieces of prepared boxwood charcoal are fixed
in the pincers at A, and, the battery being put in
action, are brought in contact. The spark passes,
and the points become ignited. They may then be




32            DAVIs' S MANUAL.
separated to a greater or less distance, in proportion
to the power of the battery; and the current will
continue to flow through the interval with the production of intense light and heat. This experiment,
as well as the deflagration or destructive combustion
of metals, is performed with facility by a series of
Grove's battery.
49. In the batteries just described, in which the
plates are fixed permanently in a frame, the solution
of sulphate of copper cannot be employed, on account
of the deposit which it forms. Hence diluted acid
is used; and the batteries will not maintain a good
action for more than a few minutes at a time; in
fact, their highest rate of action only continues for a
few seconds after immersion. The plates require to
be taken out of the acid occasionally during the experiments, and exposed to the air a minute or two.
50. PHYSIOLOGICAL  EFFECTS  OF GALVANISM.The nervous system of animals is very susceptible
to the galvanic influence; so much so, that the sensations and motions, thus excited, are among the most
Fig. 21. delicate tests of the existence of a galvanic
current.  It was by observing the contractions in the leg of a frog, when two dissimilar metals, happening to rest upon it, were
brought in contact with each other, that
Galvani laid the foundation of the science
which bears his name.  This primitive experiment is shown by the apparatus represented in Fig. 21, where a strip of silver and one of
zinc are attached -to a little block of wood or ivory.




PHYSIOLOGICAL  EFFECTS.               33
One of the metals is curved over above, so that it
may be brought in contact with the other by a very
slight pressure of the finger. The lower extremities
of the metals approach, and are made to grasp the
thigh of a grasshopper recently killed. On making
contact between the metals above, the limb will be
extended, and on removing the finger, it will again
contract. In this case, as in the frog, the exciting
fluid of the galvanic pair is the moisture of the skin
or the natural fluids of the animal, where the skin
is removed.
51. The same apparatus, of larger size, serves
to show the contractions produced in the leg of a
frog. In this case, a great amount of motion is produced by an almost insensible current; and the only
objection to the experiment is, the destruction of life
which it requires, and the difficulty of always obtaining the animal.  The legs are prepared, as in
Fig. 22.
Fig. 22, by removing a small portion of the lumbar vertebrme, with the extremities attached, from a subject




34            D A V I S'  S MAN  AL.
recently killed, and stripping off the skin. The legs
are shown in the figure in a contracted position,
in which they are placed before completing the circuit. The dotted lines exhibit the extended position
which they assume under the influence of the current
52. Many animals residing in the water are very
sensitive to the galvanic current, owing partly, withMg. 23.  out doubt, to the large surface they expose to the conducting medium, water.
Fig. 23 represents a similar galvanic pair
to that described above, introduced into a
glass vessel containing a leech; one metal
being on each side of the animal.  On
completing contact, thn leech is instantly
disturbed, and endeavors to escape from
its position in the course of the current.
A small fish, in a similar position. also
exhibits sensibility; and where a stronger
current is passed through the water, as will
be mentioned under the head of " Animal
Electricity," the creature exposed to it becomes rigid
and incapable of motion. A powerful current instantly deprives it of life.
53. If a leech, or fish-worm, be placed on a disc of
silver, such as a silver dollar, (Fig. 24,) which rests on
Fig. 24.   a moistened plate of zinc, the animal will
show no uneasiness while it remains on
one metal, but will instantly withdraw
on attempting to pass the limit which
separates one plate from the other, with
all the appearance of receiving a painful electrical
discharge.




CONDUCTION  OF GALVANISM..           35)
54. The earliest of all observations in galvanism
was undoubtedly the sensation produced by dissimilar metals, when placed above and below the tongue.
If the edges of a piece of silver and zinc, so situated,
are brought together, a metallic taste and stinging
sensation are instantly perceived, the tongue being
placed, with reference to the metals, just as the leg of
the grasshopper or frog, in Figs. 21 and 22. If one
of the metals be placed under the upper lip, instead
of on the tongue, the eye will apparently see a flash
of light when they are brought together.
55. CONDUCTION  OF GALVANISM.- Metals differ
very much in their power of conducting galvanic
electricity. The following appears to be the order
of conducting power in a few of the most useful
metals - silver, copper, brass, iron, platinum; the first
named being the best conductor.  For conducting
wires or poles, copper is generally used; for delicate
connections, silver. The difficulty of conduction of
course increases with the length of a metallic wire, and
this becomes an important consideration in the construction of galvanic apparatus. The wires employed
with batteries should always be of sufficient size, and
of pure and well-annealed metal. It will be found
that they lose much in conducting power by being
frequently bent.
56. Iron and platinum are used where it is an
object to employ the poorest conductors, as in igniting a wire. Galvanic ignition seems to result from
a law, that, in proportion to the resistance made by
the substance of a conductor to the passage of elec



36             DAVIS'S MANUAL.
tricity, heat is evolved. Thus, when a current of
electricity is passed through a metallic wire in greater
quantity thant it can readily transmit, the wire becomes more or less heated. If its length and thickness be proportioned to the power of the battery, it
may readily be melted. A single pair of plates would
be the most suitable arrangement for producing this
effect, were it not that an increase of intensity enables a greater quantity of electricity to traverse the
wire. Hence, for igniting a great length of wire, a
battery of a considerable number of pairs is necessary; but a much thicker wire may be ignited by a
few  pairs of large size.  When a very extensive
series of small plates is used, the current acquires so
high an intensity that its power of producing ignition
is diminished, as it becomes capable of traversing a
pretty fine wire without obstruction.
57. Either of the batteries of single pairs, which
have been described, have sufficient power to ignite
a fine wire of iron, or other metal, through which the
current is made to pass. This effect is most easily
produced in those metals which offer the greatest
resistance, not only to the passage of electricity, but
also to that of heat; hence a larger wire of platinum
may be ignited than of perhaps any other metal, as
it is a poor conductor both of electricity and of
heat.- A steel wire, when intensely heated in this
way, burns with beautiful scintillations. The shorter
and finer the wire, within certain limits, the greater
is the effect produced.
58. Fig. 25 represents an instrument, by means




CONDUCTION  OF GALVANISM.    37
of which the current may be made to pass through
various lengths of wire.  The sliding arms may be
/Fg. 25.           fixed in a variety of
positions,  so  that
their   extremities,
holding  the  wire,
may be at different
distances apart. A
fine wire, if not too
long, thus interposed
in the course of a
powerful current, is
C    immediately  fused
or deflagrated. This instrument has been used to
determine the conducting power of metals, by passing the current of a constant battery through an
instrument capable of measuring its magnetizing
power, and then also opening a collateral circuit to
the same current through a certain length of fine
wire, first of one metal, then of another. The instrument employed to measure the magnetizing power was the Magnetometer, which will be described
hereafter. In this instance, the current is divided:
a portion of it, equivalent to the conducting power
of the metal under experiment, passes through the
fine wire; and the measuring instrument, through
which the remainder of the current passes, indicates
a corresponding diminution.  When the whole current was made to pass through the instrument, the
magnetizing power in one experiment was 24,400
grs.; that is, the electro-magnet employed attracted
4




38            D A V IS'S MANUAL.
its armature with a force equivalent to this weight.
The following table will show the reduction of this
amount, when fine wires of various metals were employed in the way above described.
Magnctizing Power.  Reductlion.
Silver coin,.   7,350 grs.   17,050 grs.
Copper,..  8,180  ". 16,220  "
Pure silver,.  8,800 ". 15,600  "
Brass,... 12,046.12,354  "
Gold coin,. 12,820  ". 11,580'
Iron,..  16,600  ".  7,800  "
Platinum,.. 17,010 ".   7,390  "
The conducting power of the metals is, of course,
in proportion to their reduction of the magnetizing
power of the collateral current.  The alloy of silver
with a little copper used in coinage, is the best conductor of the metals tried, and platinum the poorest.
59. POWDER CUP.  In fig. 26 is represented a
little instrument designed to show the heating power
Fig. 26.
of the battery current. Two copper wires, wound
with cotton thread, except at their ends, are joined
by a short piece of fine platinum wire. These wires
pass through the bottom of a small glass cup, C, so
that the platinum  wire lies free in its cavity.  On




VOLTAIC GAS PIS'TOL.              39
putting a little gunpowder into the cup, C, and then
connecting the copper wires with the poles of the
battery, the platinum will become heated, in consequence of the flow of the current through it, so as to
inflame the powder. The recent discovery of guncotton has a very useful application to this instrument, and to the galvanic battery generally, as a
means of igniting explosive compounds. The temperature at which gun-cotton explodes being only
about 3600 F., requires a comparatively slight elevation of temperature in the wire.  For blasting
or submarine explosions, this will be an important
advantage.
60. THE VOLTAIC GAS PISTOL, represented in Fig.
27, is constructed on the same principle as the lastdescribed instrument.  The wires pass up through a
brass piece which screws into the barrel, they being
completely insulated from each other. A sectional
Fig. 27.          view of this part
is annexed.  The
fine platinum wire
stretches across be____  tween  the  two.
The  introduction
of a proper quantity of hydrogen
gas may be effected
in the following manner: Connect with a self-regulating reservoir of hydrogen a leaden or other tube, so
bent as to deliver the gas under the surface of water
in a jar. The pistol being uncorked, and the brass




40            D AVIS' S MANUAL.
piece unscrewed, immerse the muzzle in the jar to
such a depth that the water may fill one quarter of
the barrel. Then replace the brass piece, and, bringing the muzzle over the end of the tube, open the
stop-cock of the reservoir.  When the escape of
bubbles shows the pistol to be full of gas, withdraw
it, and insert the cork. In this way it will contain
one volume of hydrogen to three of air, which is
the best proportion. If too much hydrogen is introduced, no explosion will occur. It is not, however,
necessary to be very particular; and it will answer
the purpose almost equally well, without going
through the troublesome process above, if the pistol
is held for a few moments over a jet of the gas.
The explosion is louder and more certain to occur,
if it is filled with a mixture of oxygen and hydrogen, in the proportion of one volume of the former to two of the latter.
61. The pistol- being corked, connect one of the
wires with the zinc pole of the battery, and the other
with the copper pole.  The circuit will now be completed through the platinum wire; this will instantly
be ighited, setting fire to the gas, which will expel
the cork with a loud report. The removal of the
brass piece and wires allows the mixed gases to be
fired by the application of flame when desired.
62. GALVANO-DECOMPOSITION. - It has been stated, that the liquid between the plates of the battery  undergoes decomposition V  a condition of
the passage of the galvanic current. It was also
stated that this took place not only in the battery.




GALVANO-D EC O IP O SITION.          41
but whenever the current produced by it was made
to pass through a liquid consisting  of more than one
element. In this case, it is found that a certain class
of substances always appear at one pole of the battery, and a certain class always at the other pole.
Thus, when solutions of metallic or other salts are
decomposed, the metal, or base, always appears at
the zinc, or negative pole; while the electro-negative
element, previously in combination with the metal or
base, as chlorine, oxygen, cyanogen, appears at the
other, or positive pole. The decomposition is exactly
proportional to the quantity of electricity which passes;
and instruments have been contrived to measure
the current, by measuring the amount of matter decomposed by it. It is also found that, for a given
amount of electricity, a definite proportion of each
element is evolved; and these proportions are the
same as the chemical equivalents of those elements.
Substances differ very much in the facility with
which they are decomposed. Some form the most
delicate tests we possess of the passage of an electric
current. Others require the most powerful galvanic
apparatus to separate thenl into their elements.
63. If the poles of a battery of two or three pairs
are made to terminate in strips of platinum, and to
dip into water, a slender stream of gas will be seen
to arise from each wire, hydrogen being evolved from
one, oxygen from the other. If the wires are of iron
or copper, the oxygen will unite with the one connected with the positive pole to form oxide of iron
or of copper, and hydrogen alone will be given off.'
4*




42            D A AVIS'S MANUAL.
Platinum wires or strips are not attacked by the oxygen, and are therefore best for the decomposing surfaces. The decomposition of water is greatly facilitated by dissolving in it some salt, as, for instance,
Glauber's salt, or, what is still more effectual, by
the addition of one part of sulphuric acid to ten or
fifteen of the water. These substances increase its
conducting power. If, however, the power of conduction be increased beyond a certain limit, the
evolution of gas will be lessened.
64. Fig. 28 represents a Decomposing Cell.
mounted on a stand. Two platinum wires, conFig. 28.     nected with the cups A and B on
the stand, pass up into the cell,
o  H  C            which is of glass.  A glass tube,
G, may be inverted over these
wires, to collect any gas which
is evolved; it passes through a
cork, fitting the mouth of the cell
I  with sufficient tightness to allow
the tube to be filled with the
P   P            liquid by merely inverting the
instrument.  The cell, being partly filled with acidulated water,
and the tube, wholly full, connect the cups, A and B, with
1    those of the battery.  Immedi* ately bubbles of gas will be seen
to escape from  each wire, and to
rise into the tube, displacing the
liquid from it.  When the tube is full, it may be re



GALVAN O-DE C OM P OS ITION.         413
moved and the mixed gases exploded by holding
its mouth to a flame.
6'5. By having two glass tubes, O and H, passed
through a cork, so that one of them may be inverted
over each wire, as shown in the cut, where p p are the
platinum wires, the gases may be obtained separately;
oxygen only being collected in the tube placed over
the positive wire, and hydrogen alone in the other.
The volume of the latter gas is twice that of the former, as indicated in the figure by the relative height
of the liquid in the tubes O and H, occupied respectively by the oxygen and hydrogen. On removing
the tubes when full, the hydrogen will burn if a
flame be applied, and the oxygen will increase the
brilliancy of the combustion of any ignited body put
into it. When these tubes are graduated, they constitute the instrument called voltascope, or voltameter.   The  amount of gas liberated is exactly
proportioned to the electricity which passes, and
affords the most exact measure of the amount of a
current when it continues for any length of time.
66. A solution of iodide of potassium is perhaps
more easily decomposed than any other salt. In this
case, instead of the elements of water, the components of the salt itself appear at the poles; iodine at
one side, and potash, by a reaction between potassium
and water, at the other.  When iodine and starch
are brought together, an intensely blue precipitate
results. Melloni availed himself of this fact, to determine the heat of insects, in some of the most delicate
experiments ever made, by means of his thermo



44            D AV IS S  MANUAL.
electric pile.  A  little starch being added to the
solution of iodide of potassium, the passage of the
slightest electrical current is made apparent by the
blue precipitate at the positive pole.  The direction,
as well as the existence, of a current is thus shown.
The decomposing cell (Fig. 28) will answer for this
experiment; but a more delicate form is shown in
Fig. 29, where a glass tube is supported on a stand,
two platinum wires, each connected
with a small screw-cup, entering on
opposite sides.  These wires are insulated, except at the extremities,
which are flattened into little discs,
and are parallel to each other. When
a current of almost inappreciable
f —       quantity passes, as that from a galvanic pair composed of a zinc and a
silver wire, whose extremities are dipped into distilled water, or from two or three turns of a small
electrical machine, one of these discs will be seen,
when compared in the light with the other, to become of a purple hue, the effect constantly increasing
as the current is continued.  The disc recovers its
brightness by stirring the solution near it, or on being
touched with a little splinter of wood.
67. Most of the salts having alkaline bases are
decomposed into an acid and an alkali, which appear
at the opposite poles.  Some of the phenomena
occasioned in this -way are best observed by placing
a porous diaphragm across the decomposing cell, so
as to form separate apartments for the poles.  Fig.




GALVANO-D EC OMP  O SITION.           45
30 represents a tumbler divided vertically; with the
edges ground together, so as to form a tight joint,
Fig. 30.    when they include between them
a diaphragm  of unsized paper.
They are held together by rings
of India rubber. A little screwcup is clamped to the side of
each of these apartments, from
which  platinum  poles descend
into the vessel.
68. Let this apparatus be filled
with a solution of Glauber's salt, (sulphate of soda,) to
which has been added enough of the infusion of red
cabbage to give it a blue color. On the passage of the
current from a single pair, the blue color will soon be
changed to red in the cell containing the positive pole,
and to green in the other, sulphuric acid and soda
being respectively evolved. By reversing the current,
the original color will be first restored in each cell, and
then the opposite change will occur. If the solution
is colored blue by the infusion or tincture of litmus,
it will become red in the cell in which the acid is
developed, but will suffer no change in the other.
When the yellow infusion of turmeric is used, it is
turned brown by the alkali evolved in the negative
cell, but is not affected by the acid in the other.
Similar phenomena present themselves with a large
number of salts, but in some cases different effects
are produced.
69. If a solution of muriate of ammonia, colored
by some vegetable infusion, be employed, chlorine




46           DAVISI  S MANUAL.
gas will be given off from the positive pole. This
may be recognized by its peculiar odor, and by the
bleaching effect it produces upon the liquid in the
positive cell, which quickly becomes colorless. In
this case, ammonia and hydrogen are set free in the
negative cell, and muriatic acid and oxygen should
have been liberated in the other.  The chlorine
appears therefore to be a secondary product, set free
by the combination of the hydrogen of the muriatic
acid with oxygen, to form water. Or, according to
the view which regards sal-ammoniac as a chloride of
ammonium, chlorine is directly liberated at the positive pole; and at the negative, ammonium, which,
not being able to exist in an isolated state, is instantly resolved into its components, ammonia and
hydrogen.
70. In most cases, it is desirable to have the distance between the poles in the decomposing cell as
small as possible, as the current passes more easily
and the action is more rapid. Where, however,
different colors are to be exhibited, the instrument
ig. ~31.       represented in Fig. 31 may
be used with advantage.
This is a U-shaped tube, into which the poles enter at
the top. It may either form
a continuous cell, or it may
be divided by thrusting a
piece of loosely crumpled,
unsized paper, or of cotton
cloth, into the bend; or the




ELECTRO-METALLURGY.                47
tube may be in two parts, cemented together into
a brass support or stand, with a porous diaphragm between.  When the experiment with sulphate of soda,
~ 68, is tried with this instrument, three separate
zones of color, blending into eacii other, are perceived.
At the positive pole, it is red; at the negative, green;
and in the region between, the color remains blue.
As the experiment proceeds, the red and green solutions encroach on the blue, until a small space only
remains between them, if the tube is continuous,
where the acid and alkaline portions blend together.
71. ELECTRO-METALLURGY. -Solutions of metallic salts undergo decomposition in the same way as
the alkaline salts. The result is the deposition of
the metal itself at the negative pole. Even with the
alkalies, a battery of great power, in place of the alkaline oxide usually evolved, separates the metal which
is its ultimate base. The decomposition in alkaline
and metallic salts takes place, probably, in the base
of the salt, the metal being separated at one side, and
the oxygen, or electro-negative element, at the other.
In some cases, as with the alkalies, the elements thus
eliminated decompose water, and an acid and alkali
finally appear at the poles. The deposition of metal
has been spoken of in the sulphate of copper battery.
The observation of this important fact, in its connection with the mechanic arts, was made at about
the same time, in 1837 and 1838, by Jacobi, of St.
Petersburg, and Spencer, of Liverpool. The science
of The Electrotype, or Electro-metallurgy, thus had its
origin.




48           D AV I S S MANIUAL.
72. The great object in depositing metals is to
throw them down in a malleable, coherent state - the
reguline condition.  If different metallic solutions
should be put in one of the decomposing cells already
described, it will be found that copper will probably
be deposited on one of the poles with its proper color,
while silver and gold will be thrown down as a
black powder. A great difference, therefore, exists
with the different metals. Some practical directions
will be given for the precipitation of those most
important.
73. Metals tend to be thrown down in three different states —as a black deposit, as reguline metal,
and in a crystalline condition. These forms all depend upon the fact, that in the decomposing cell
there are two bodies present, water and the metallic
salt, either or both of which may be decomposed by
the galvanic current. Now, when water is decomposed, hydrogen is evolved at the negative pole; and
when this takes place at the same time that the metal
is evolved at the positive pole, the black deposit
occurs. Smee suggests that the hydrogen and metal
are both thrown down together, and, the mixture
being incompatible, the metal appears in an infinitely
divided state. As an illustration of this principle, if
a metallic solution is acidulated, the water will be
more easily decomposed as the acid increases its
conducting power.  There will, then, be a greater
tendency to the evolution of hydrogen. Where a
metallic salt is difficult of decomposition, acid is
sometimes, therefore, added; where a black deposit is




ELECTRO-METALLURGY.                49
teared, it is avoided.  In Smee's acid battery, we
have seen hydrogen evolved at the negative plate; in
tile sulphate of copper battery, we have seen copper
deposited on the negative plate, but in a hard and
crvstalline condition. Now, unite these by adding
sulphuric acid to the sulphate of copper solution,
until you arrive at the point when, copper being still
deposited, hydrogen is just about to appear, and the
metal will be precipitated in a soft and malleable
form. The crystalline state results from the deposition of the metal in obedience to its own molecular
attraction. In the reguline state, this is modified by
the chemical agency of the elements of water.
74. The rule prescribed by Smee is in all cases
to regulate the strength of the battery current according to the strength of the solution. The follow
ing formula for controlling the evolution of hydrogen,
and thereby regulating the quality of the metal,
is condensed from his work on electro-metallurgy.
The tendency to the evolution of hydrogen, and to
a black deposit of metal, is increased by any of the
following means: By increasing the intensity or
quantity of the battery; by increasing the size of the
positive pole; by decreasing the size of the negative
or depositing pole; by approximating the poles; by
increasing the heat of the solution; by weakening
the solution; by making it acid. The tendency to
evolution of hydrogen may be diminished, and that
to the crystalline form of deposit increased, by any
of the following means: By diminishing the intensity or quantity of the battery; by increasing the
5




50            D AVIS' S MANUAL.
negative pole; by decreasing the positive pole; by
separating the poles; by saturating the solution; by
making it neutral; by diminishing its heat. It will
frequently be found that one or more of these means
may be employed in a particular operation, while it
will be impossible to make an advantageous application of others.
75. The metal of the electrotype preserves the
precise form of the negative pole or mould upon
which it is deposited. A finger-mark, or slight tarnish on the mould, is faithfully copied by the surface
of the metal thrown down in contact with it.  The
chief applications of this art at present are, the reproduction or manufacture in copper of medals, dies,
engraved plates, plaster casts, ornamental raised work,
and generally of all designs in relief or intaglio.
Daguerreotype pictures are also reproduced on a copper surface by the same means. Another important
application is the coating of the more oxidizable
metals with films of the noble metals, as in gilding
and silvering. If a sheet of copper should be thrown
down on an object which it was desired to copy, and
then be separated from it, a reverse impression of the
original would be obtained. To obtain a fac-simile
of the original, it would be necessary to use this
reverse as a mould, and proceed to precipitate metal
upon it, as in the first instance. This is accordingly one of the processes used to obtain reverses
or moulds of objects to be copied.
76. For this purpose, a medal or engraved plate
is placed itself in the solution and copper deposited




COPP  RPLATES  COPIED.            51
upon it. The negative wire of the battery should
be connected with the rim of the medal, and in case
of an engraved plate, it may be soldered to the
corners. The deposit is apt to adhere very firmly,
sometimes so much so that its removal is impossible.
This may be avoided by slightly greasing or oiling
the mould, and then brushing it over with a little
dry copper bronze.
77. The mould thus obtained may have a wire
soldered to it, and be placed in the solution like the
original one.  In most cases, it will be considered
safer to take a mould of a valuable medal or plate in
soft wax, or by some of the other processes to be
described.
78. An engraving printed from an electrotype
plate obtained by this process, is given, as a specimen
of the art, in the frontispiece to this Manual, in
company with one from the original copperplate.
No difference can be detected between the impressions, except that arising from the greater or less
quantity of ink left in the work, as occurs in different engravings printed from a single plate. This is
an important application of the art, as in cases where
a large number of impressions are required, or where
it is desirable to print from a plate in different places,
two or more plates have been obliged to be engraved,
while now it is only necessary to engrave one, which
will not be injured in the slightest degree by taking
copies from it, in one or other of the modes. Steel
plates must not be placed in the copper solution
without a modification of the process which is given




52            D AVIS'  S MANUAL.
in ~ 96.  It was found that the electrotype plate
used in the first edition, wore better and worked
better than the original.  This is due to the great
purity and equality of the deposited copper and the
hardness which can be obtained by regulating the
current so that there shall be the slightest tendency
to the crystalline deposit.
79. One of the early processes for obtaining moulds
is that called abroad the clichele process, which consists in stamping the object to be copied on fusible
alloy just at the moment of hardening. This alloy
consists of three parts of tin, five of lead, and eight of
bismuth, and melts at about the temperature of boiling water. Rose recommends an alloy of one part
tin, one part lead, and two of bismuth, which fuses
at about 200~ F. When the alloy is melted, it is
skimmed with a card, and the medal or plate, attached by wax or otherwise to a handle, is suddenly
and forcibly pressed upon it. When of small size,
a mould may be made, by a few trials, presenting
an accurate reverse of the original. The frontispiece
to the first edition was copied in this way. A clean
copper wire, to be used as the negative pole of the
battery, is heated at the end with a little rosin upon
it, sufficiently to melt and solder itself to the edge
of the fusible mould, which is then ready for the
solution. All parts on which the metal is not to
be deposited are protected by a coating of varnish
or wax.
80. The moulds heretofore described have been
formed of galvanic conductors.  Means have also




E L PE C T It O-M-  E T A L.1 URG (.  53
been foundcl of usilng moulds of the important class of
non-condLuctors, as wax, sealing-wax, and plaster of
Paris.  A mould is easily made of a soft composition
wax by bringing it to an even surface, and then pressinrg the object to be copied upon it with sufficient force.
The surface of the original plate should be slightly
oiled and brushed with blacklead, to prevent its adhesion.  In this way, very perfect moullds, even
of large objects, are obtained, and it will be found
the simplest and best mode of operating in most
cases.  Medals, copperplates, wood cuts, pages of type,
may be thius copied.  The page in which these remarks occur, has been electrotyped by this process,
and may be compared with the others, whichl are
stereotyped.  Parts of wood cuts in this volume
which it was desirable to insert in more than one
place, have been multiplied in the same way. Melted
wax may also be poured on surfaces which are to be
copied; but this is more troublesome, and less successful than the mode' already described.
81. It is necessary to render the surface of the
wax mould a conductor of electricity. This is done
by giving it a coating of good blacklead, which
should be rubbed over its face with a soft brush,
until it acquires a shining black appearance; a very
thin film is sufficient.  A copper wire is then to be
bent, so as to conform  nearly to'the edge of the
mould in its whole circumference, and then to be
heated in a lamp, and pressed upon the wax, when it
will become imbedded.  Communication between
the wire and the face of the rriould is to be insured
A'2




54            DAVIS' S MANUAL.
by rubbing a little blacklead on the parts around
the wire.
82. The mould, when thus prepared, may be put
into the solution, care being taken to remove airbubbles.  The deposit commences upon the wire,
and extends gradually over the blackleaded portions.
The mould may be employed again, if a new coating
of blacklead is given to it. The metallic moulds can
also be used any number of times, if uninjured.
83. 3MIoulds are obtained from  plaster medallions
by pressing them upon a matrix of soft wax, after
their pores have been saturated with water. The
earlier process consisted in soaking them with hot
water, and then pouring on wax and allowing them
to cool before separating it.  Reverses of plaster
casts are obtained by soaking them in hot stearine,
tallow, or wax, as long as any bubbles of air escape,
and then removing all excess of these substances
from the surface, and blackleading as before. There
is, however, a serious objection to placing plaster
casts in a solution, from the fact, that, if the smallest
quantity of sulphate of lime is dissolved, it affects
very injuriously the deposited metal.
84. Where it is not necessary to get a precisely
accurate copy of a surface, it can be made a good
conductor of electricity by giving it a coat of copperbronze varnish. Copper spreads at once over this
when placed in the solution. Fruit may thus be
easily covered with a metallic crust, preserving its
form and characteristic appearance.
S5  Impressions of seals may be copied by a very




BATTERIES EMPLOYED.                55
simple process. They are to be covered with a thin
film of blacklead, rubbed on with a soft brush. If
this does not adhere readily, the brush may be very
slightly moistened with alcohol, care being taken not
to roughen the surface of the seal. A wire is then
melted into the sealing-wax, and the seal placed in
the solution. The operation is similar in all respects to that required for the wax moulds.  The
original seal or stamp may be copied by taking an
impression in soft wax.
86. Where the mould is of copper, a single cell
has been sometimes made to do the work of both
battery and depositing apparatus.  The mould is
placed on one side of a porous partition in the copper
solution, and a wire connects it with a piece of zinc
on the other side, in a solution of sulphate of soda.
The action in this case cannot be nearly so well
controlled as where a separate battery is used, and
the deposited metal is not as good.
87. To copy small objects, a single Smee's battery is sufficient. Where a large surface is to be
deposited on, it requires the superior energy of the
sustaining sulphate of copper battery. The leather
cell here has an important application, as it can be
adapted in shape to masses of spelter or commercial
zinc, which is the most economical form in which
that metal can be employed in the large way.
88. With the copper plate of the battery is connected a piece of copper which is to be immersed
with the mould in an acidulated solution of blue
vitriol, contained in a glass, or well-glazed earthen



56           DAVIS'S MANUAL.
ware vessel, or in a wooden trough. The piece of
copper and the mould should both be connected with
the battery, and the copper placed in the solution,
before the mould is introduced; in this way all local
action is prevented; and the deposition of copper
commences immediately.  Any air-bubbles which
adhere to the mould must be dispersed. The solution is prepared by adding 2 oz. of sulphate of copper
and 1 fluid oz. of sulphuric acid to every 15 fluid
oz. of water. - As the copper is deposited on the
mould, an equal quantity is dissolved off from the
immersed plate, so that the original strength of the
solution is maintained except for the loss of water by
evaporation.
89. Fig. 32 represents a Smee's battery,3, of the
mercurial, or "odds and ends" form, a new-and imFg. 32.          proved variety, connected with a depositing apparatus.  A
B   little mercury is placed in the bottom  of
the glass vessel forming the battery.  A
platinum plate is seen
j 1  1             i~suspended-in the centre, beneath one of
the screw-cups. One
or more zinc plates rest against the side of the vessel,
and are in contact with the mercury below. A curved
wire is seen descending through the liquid, insulated
by a glass tube, to the mercury, which it connects, in




COPPER  DEPOSITED.               57
common with the zinc, with the plate on the top aid
the other screw-cup. This battery will operate if
simple scraps of zinc are placed in the mercury. The
mould in the depositing cell will be seen to be connected with the zinc or negative pole of the battery.
90. During the solution of the positive plate, a
considerable quantity of black matter is left, mostly
carbon, which would injure the copy if allowed to
fall on the mould. It is, therefore, best to place both
in a vertical position, the face of the mould being
opposite the piece of copper. The solution may be
stirred occasionally, to keep its upper and lower parts
of equal strength.
91. When the process is going on well, the deposited metal will be of a very light copper color.
The rapidity of the deposition depends greatly upon
the temperature; the process proceeds much faster in
warm weather than in cold, and still more so if the
solution be kept hot. A thickness of one tenth of an
inch may require from three days to a week for its
formation, when artificial heat is not used. When a
sufficient thickness has been attained, the copy may
generally be removed from the mould without difficulty, care being taken to cut away any copper
which embraces the mould at the edges.
92. Every ounce of copper deposited requires the
solution of somewhat more than an ounce of zinc
from the zinc plate of the battery. Five or six electrotypes may be made at once, without increasing
this expense, by arranging in succession several vessels, each containing a mould and a copper plate con



58            DAVISY S MANUAL.
nected by a wire with the mould in the next one
9he plates of copper and the moulds should all be
nearly of the same size, and the solution should contain less blue vitriol and more sulphuric acid than
directed in ~ 88, particularly if the series extend
beyond two or three. When the moulds are small,
glass tumblers form the most convenient vessels.
Fig. 33 illustrates this arrangement, in which one
battery is seen connected with three depositing cells.
Fig. 33.
The moulds will be observed to occupy the same
position in relation to the current in each of the cells.
The copper plates opposed to the moulds are dissolved in proportion to the amount of copper deposited, so that the solutions retain their strength
by the recomposition of sulphate of copper, exactly
equal to the decomposition at the surface of the
moulds. The increase of the number of cells requires, therefore, no increase of electrical energy,
except for the greater distance through the solutions
which the galvanic current has to pass. In this way
several ounces of copper are obtained, with but a




ELECTRO-ETCHING.                  59
slight increase in the quantity of acid or blue vitriol
required for working the battery, and a little more
corrosion of the zinc plate.
93. The surface of the electrotype copy is usually
of a bright copper color; but sometimes it presents a
dull surface. If it is discolored, it may be cleansed
by immersion for a few moments in dilute nitric
acid, and then washed with water.  It may be
bronzed by brushing it over with blacklead immediately upon its removal from the solution, and
having heated it moderately over a clear fire, by
rubbing it smartly with a brush, the slightest moisture being used at the same time, in order to remove
the black lead. It may also be gilt or silvered.
94. If the copper plate which is opposed to the
mould in the solution is coated with wax, in which
lines are drawn reaching the metal, it will be etched
by the acid, and may afterwards be printed from
like a plate etched in the usual way by nitric acid.
There is the peculiar advantage in this process, that
the action is perfectly regular over the whole plate,
or that a gradation of action may be produced by approaching the negative pole to the parts where the
shades require to be bitten in deepest.  There are also
no nitrous fumes evolved, as with nitric acid. A plate
of electrotype copper is preferable to any other for
etching, as its purity insures an equal result. The
negative pole should be small, and at some distance
from the plate to be etched, when it is intended
that it shall act uniformly.
95. A process of surface-printing, called Glycog



60           D AVIS'S MANUAL
raphy has recently been devised, by a reverse operation to that of galvanic etching. A design is drawn
through an etching ground upon a polished plate,
as before; the whole is then blackleaded, and a
plate of copper thrown down, in which the lines of
the drawing are of course raised like those of a
wood cut. This plate is printed from, like an engraving on wood, in the same page with common
type; which cannot be done with a copper plate or
steel plate engraving.
96. It is sometimes desirable to precipitate copper
on iron or steel, to prepare it for receiving the noble
metals, or for acting as a mould in the common sulphate of copper solution.  The solution used for
this purpose is the cyanide of copper, which is not
decomposed by iron, and which is prepared by dissolving oxide of copper, or sulphate of copper, in a
solution of cyanide of potassium. This salt is only
useful for throwing down a film of copper upon iron
or other oxidizable metals. The surface of the iron
must be perfectly clean.
97. Copper is the only metal which can be used
with advantage for the purposes which have been
described.  The precipitation of the noble metals,
silver, gold, and platinum, is of great importance for
purposes of ornament and protection to other metals.
Their salts give water a high conducting power, and
are themselves easy of decomposition. There is,
therefore, a great tendency to the evolution of hydrogen, and this evolution is peculiarly liable to
reduce the metal as a black powder. It is as yet




ELECTRO —GILDING AND SILVERING. 61
almost impracticable tbo reduce platinum in other
than the divided state, which has, however, an important application in the coating of platinum plates
for batteries, where it prevents the adhesion of
hydrogen. The solution used for this purpose is the
chloride obtained by dissolving platinum in nitromuriatic acid, and partially evaporating to expel any
great excess of acid. The solution may be subsequently diluted to any required amount.
98. The battery used for gilding and silvering
consists of from three to six or eight pairs, according
to the extent of surface to be covered, and the resistance offered by the solution. As the solutions grow
older, they require less power to decompose them;
but there is then a greater tendency to the black
deposit.  The positive pole usually employed is a
platinum wire, or pointed strip of platinum foil.
This is not dissolved by the solution, and consequently the electro-negative element of the salt has
Fig. 34.
to be evolved, instead of entering into a new combination.  It is on this account that several pairs
have no more efficiency in decomposing the solution than one pair where the positive pole is dissolved. The object of the present arrangement is
6




62           DAVIS' S MANUAL.
to bring the whole action more exclusively under
the control of the battery, and to avoid local action.
For ordinary purposes, a Smee's battery, weakly
charged, is preferable. Fig. 34 represents an odds
and ends battery of this kind, connected with the
silvering apparatus, differing only from that described
in ~ 89 by being uncovered. It will be observed
that the positive pole, in the decomposing cell, is
placed at a distance from the object to be silvered.
99. Another form of Smee's battery, connected
with a depositing apparatus for silvering daguerreotype plates, is exhibited in Fig. 35.  This is the
Fig. 35.
same form with Grove's battery, the porous cylinders
only being removed. The platinum plates are attached to little arches of wire, which enter small
holes in the zinc, into which they may be wedged
or from which they may be removed at pleasure.
In the depositing cell, a daguerreotype plate, D, is
seen attached by its edge to a screw-cup and clamp
prepared for that purpose, and thence connected with
the terminal zinc cylinder of the series, Z. Opposed
to the daguerreotype plate, is a plate of silver, S, connected with the platinum pole, P, of the battery, and




ELECTRO-GILDING AND  SILVERING. 63
which is dissolved by the solution in proportion as
silver is deposited on the daguerreotype plate. A
very pure coating of silver is thus given to a plate
from which the best pictures are obtained.
This fig. shows the porous cell belonging to
the battery when used as a Grove's. The
use of this cell, and of nitric acid within it,
constitutes the difference between these forms.
100. Fig. 36 represents a series of the sulphate
of copper battery applied to gilding and silvering.
Fig. 36.
This may be used where a very large surface is to be
deposited upoh. The figure is intended to illustrate
chiefly the deposition of alloys by the galvanic current. If two metals are contained in a solution; the
general law is, that the one most easily reduced by
the electrical process will be deposited first, and in a
state almost absolutely pure.  If the energy of the
current, however, is very much increased, all the
metals present will go down in variable proportion.
Thus, if there is a little silver in the gold electrotype
solution, a feeble current will throw down the silver
first; if there is copper present, and no silver, a




64            D A VIS' S MANUAL.
feeble current will throw down a pure yellow deposit
of gold, while a stronger one will throw down a
reddish metal resembling the gold of jewellers and
of the mint.
101. The salt of silver, used for precipitation, is
the ferrocyanide.  A  silver dollar is dissolved in
nitric acid, and the silver precipitated in the state of
chloride, by muriatic acid or common salt. The
precipitate is washed, and then added to a sufficient
quantity of a hot saturated solution of ferrocyanide
of potassium (yellow prussiate of potash), to dissolve
it. Sufficient water is then added to make two
quarts.  The solution may be poured off from the
sediment which remains, or it may be used at once.
It must be remembered that the salt employed contains cyanogen, the active principle of prussic acid.
Prussic acid itself and oxygen are evolved at the
positive pole during the decomposition.  A solution
of cyanide of silver may also be used, which is obtained by dissolving the precipitate of chloride of
silver, mentioned above, in cyanide of potassium,
instead of the ferrocyanide. In this case, the solution of cyanide of potassium need not be so strong,
or be raised above the common temperature. Only
sufficient of the cyanide of potassium need be added
to take up all the chloride of silver.
102. The solution of gold employed is the cyanide. This is made by dissolving a five dollar piece
in nitromuriatic acid (a mixture of one' measure of
nitric acid with four of muriatic), and evaporating the
salt very carefully, in order to drive off the excess of




ELECTRO-GILDING AND SILVERING. 65
acid.  This is then redissolved with a sufficiency
of cyanide of potassium to give a clear solution, it
requiring somewhat less than an ounce of the cyanide for this purpose. Enough water is added to
make two quarts.  As a general rule, it requires
more battery power for gold than for silver. The
addition of a small quantity of cyanide of potassium
to a solution of gold diminishes the tendency to
black deposit where this is exhibited, and where the
fault is referable to the solution.
103. It is necessary that the surface on which a
metal is deposited should be perfectly clean, and free
from a film of air, that it may adhere; and the metal
should be thrown down with sufficient energy to
prevent any local action with the mould. The article to be silvered or gilded may in some cases be
cleaned by a solution of potash; but the deposit is
more certain to adhere if the surface is rubbed with
a little rottenstone when first placed in the metallic
solution, and connected with the battery. Whenever, during the process, the deposit becomes discolored or rough, the negative plate should be taken
out, and brushed with a little whiting and soap and
water. The thickness of the coating of gold and
silver is proportional to the time occupied by the deposition and the amount of electricity which passes.
For the same amount of electricity, 32 grains of
copper, 108 grains of silver, and 99 grains of gold,
are deposited, these being the chemical equivalents
of those metals.
104. If a small object, such as a dial of a watch,
6*




6b           DAVIS'S MANUAL.
be placed in a solution of cyanide of silver or gold in
a capsule, the metals will be thrown down upon it
Fig. 37.   in a very satisfactory manner, if a
small slip of zinc be placed in contact with it, as represented by Z in
Fig. 37, and the whole be heated
by a spirit-lamp.  It is necessary
that there should be rather more
cyanide of potassium in the solution
than in that for the ordinary process.
The dial of the watch itself becomes
here one plate of the battery, and the
slip of zinc the other. The zinc is
dissolved by the solution; but by the law regulating
the deposit of alloys, only gold is thrown down by
the feeble current which is excited.
105. When a solution of acetate of lead is decomposed, a very beautiful result ensues at the positive pole. To exhibit this, a large polished metallic
surface is connected with the positive wire in the solution, and a fine negative pole is brought near to it.
Metallic lead is precipitated on the negative wire,
and at the same time a precipitate of deutoxide of
lead takes place on the positive plate, exhibiting
concentric rings of prismatic colors, owing to the
different thickness of the coating of oxide. These
depositions have received the name of metallic chromes.
In this instance, the oxygen about to be evolved at
the positive pole unites with a portion of the protoxide of lead in solution, and the resulting deutoxide
appears on the surface of the plate.  Instead of a




THERMO —ELECTRICITY.                 67
fine negative wire, a spherical body connected with
it may be approached to the positive plate, the different distances of its parts producing a beautiful
gradation of oxide.
III. THERMO-ELECTRICITY.
106. The term thermo-electricity expresses a form
of electricity developed by the agency of heat. It
was discovered by Professor Seebeck, of Berlin, in
1822, that if the junction of two dissimilar metals
was heated, an electrical current would flow from
one to the other. Thus, if the ends of two wires,
or strips of German silver and brass, are made to
touch each other, or are brazed together, and the
junction heated, a current will flow from the German
silver to the brass, if the free extremities of the wires
are connected by any conductor of electricity, and
an electrical circuit will be established, as the galvanic circuit is established by connecting the poles
of the battery.  In the cut, Fig. 38, G represents
Fig. 38.
the German silver, and B the brass; the direction of
the current being indicated by the arrows.
107. In thermo-electricity, as in galvanism, instead of two metals, one metal, in different con



68            DAVIS S MANUAL.
altions, can be used to excite a current.  Thus
merely twisting the middle of an iron or platinum
wire, and heating it on one side of the twisted portion, will produce a current, flowing, at the heated
part, from the untwisted to the twisted portion,
when the extremities are connected.
108. A current may also be excited with two
wires of the same metal, by heating the end of one,
and bringing it in contact with the other. It is
difficult to succeed in this experiment when metals
are used whose conducting power for heat is great.
Thus copper or silver wires produce a very feeble
current; but iron or platinum an energetic one,
especially when the ends, which are brought in contact, are twisted into a spiral. The direction of the
current at the junction is from the cold to the hot
wire; and it ceases as soon as an equilibrium of
temperature is established between the two. A considerable current is also produced by heating the
junction of two platinum wires of different diameters.
The current flows from the fine to the coarse wire,
whether the heat is applied at the point of junction
or to either wire at a little distance from it. In large
arrangements, plates or strips of dissimilar metals
are generally used.
109. The thermo-electric current, thus excited
between two metals, is generally referred to the difference in their conducting power for heat, and to
the different orders of crystallization to which their
particles belong, the laws of crystallization being
supposed to result from the electrical character of the




THERMO —ELECTRICITY.                69
particles. Where the same metal, in different conditions, is used, the production of electricity is referred to the unequal propagation of heat on each
side of the heated point, caused in the single wire by
the obstruction occasioned by the twist, and in the
case of two wires, by the contact of the cold wire, or
where they are connected together, by the difference
in their diameters. The causes, however, have not
yet been fully investigated, and many points are involved in great obscurity.
110. Metals differ greatly in their power to excite
a current, when associated together in thermo-electric
pairs.  Some of the peculiarities in the combinations of the more usefull metals will be stated.  It is
necessary, however, to say a few words with regard
to the galvanometer, an instrument to indicate or
measure electrical currents, and which will be more
fully described in Book I. Chapter 2. A curreint
of electricity, passing through a wire, is found to deflect a magnetic needle in its neighborhood.  By an
Fig. 39.
arrangement such as Fig. 39, where G is the galvanometer, consisting of a magnetic needle in close




70            DAVIS' S MANUAL.
proximity to a coil of wire, above which is fixed a
graduated circle, the direction of an electrical current
made to pass through the wire is indicated by the
deflection of the needle from the north and south
line, in one direction or the other, and its strength
is measured by the number of degrees to which it
is deflected. The deflection of the needle will be
frequently referred to hereafter. When using this
instrument, the zero points of the graduated circle
must be placed north and south, so as to lie in the
same direction as the magnetic needle above them
does when influenced by the earth's magnetism alone.
In the figure, a thermo-electric pair, of bismuth and
antimony, heated by a spirit-lamp, is shown in
connection with the galvanometer. The arrows indicate the course of the current from the antimony,
A, to the bismuth, B, in the exterior circuit; its
direction being of course the reverse of that at the
junction, where it flows from B to A.
111. The character of the juncture between the
plates or wires has an important influence on the
amount of the current with the same metals. Frequently, when the elements of the pair are merely
made to touch each other, the current is greater than
when they are brazed or soldered together. Generally, the slighter the connections are, the better.
They must be sufficient to conduct all the electricity
generated, but no more; for if they are unnecessarily
large, they allow the electricity to return to the
metal whence it proceeded, without passing through
the exterior circuit.




THERMO-ELECTRIC  PAIRS.              71
112. The metal from which the current proceeds
through the heated junction is analogous in situation
to the zinc, or positive plate, in the galvanic pair, from
which the current proceeds through the liquid of the
battery, as described in ~ 16. The metal to which
the current proceeds, through the junction, is analogous to the copper, or negative plate. The positive
pole of the thermo-electric pair is the extremity of
the negative metal, as the copper pole is the positive
pole of the battery. The negative thermo-electric
pole is the extremity of the positive metal. In the
observations and table which follow, the positive
element of the pair, answering to the zinc in a
galvanic pair, will always be named first.
113. GERMAN SILVER AND ANTIMONY.  The current excited by these is greater than that from bismuth
and antimony at the same temperature. Their junctions being put into hot oil, of a fixed temperature,
and the free ends of the plates connected with the
galvanometer used in these experiments, the bismuth
and antimony occasioned a constant deflection of the
needle of 75~; the German silver and antimony, a
deflection of 85~: the heat being increased with the
bismuth and antimony to the melting point of bismuth, the deflection was 82~, while the German
silver and antimony, heated in a spirit-lamp, gave
a deflection of 88~.
114. BISMUTH AND ANTIMONY. -Plates of these
metals have been heretofore generally used in large
thermo-electric arrangements. The current excited
by heating their junctions is greater than from many




r72           D A VDAVIS S MANUAL.
other metals, when a feeble heat is used; but from
the fusibility of bismuth, the heat can never be raised
very high.
115. GERMAN SILVER AND CARBON.- A current of
considerable energy is produced by this combination.
In this and in the succeeding experiments, where the
use of carbon is mentioned, the kind employed was
the gas-carbon deposited from carbureted hydrogen
in the retorts of the gas-works. This substance lines
the upper part of the interior of the retorts in compact layers, and is entirely different from coke, which
is the residue from the coal, after its gas has been
expelled. It is nearly or quite pure carbon, and is a
better conductor, both of heat and electricity, than
ordinary charcoal.
116. German silver is an alloy of nickel with
copper and zinc, containing in 100 parts, 50 of
copper, 30 of zinc, and 20 of nickel. It is used in
many of the thermo-electric instruments, which will
be hereafter described. German silver is positive to
all the metals that have been tried, even to nickel
itself; with the exception of bismuth, to which it
is negative.
CARBON AND SILVER, OR IRON. —In these combinations, and also with antimony, the carbon is positive, the current being rather feeble.
117. Tile deflections given in the following table
admit of comparison with each other to a considerable extent, though not so strictly as if wires of the
same size had been employed in all the experiments.
It must be remembered, too, that as the needle




THERMO-ELECTRIC  P A IR.                    73
approaches the extreme angle of deflection, 900, a
much greater increase of the current is required to
carry it a few degrees farther towards 900 than
when it is near the zero.   Hence, a deflection of
40~ does not indicate a current of half the power
of one of 800, but considerably less. Nor can momentary deflections be compared with permanent
ones, in estimating the power of the current; as a
current which, by its first impulse, causes the needle
to  traverse a large arc, may not be able to main —
tain more than a few degrees of steady deflection.
CURRENT FLOWS THROUGH HEATED JUNCTION.  DEFLECTION OF
From positive         To negative.     THE NEEDLE.
German Silver,.....   Antimony,.88~
German Silver,..... Silver,.......           850
German Silver,.....    Brass,.........     850
German Silver,.....    Iron,...........       850
German Silver,.....    Palladium,......       850
German Silver,.....   Copper,.                 850
German Silver,.....  Cadmium,......           85~
German Silver.....    Zinc,..........  84
German Silver,.....  Platinum,........  810
German Silver,.....  Carbon,.........         820
Silver..........   Antimony.......  88~
Bismuth,.........  Antimony.......  82~
Bismuth,.........  Silver,..........  78~
Bismuth,.........  Palladium,.......  85~
Bismuth,.........  Carbon.........  85~
Bismuth,......... German Silver,          83~
Platinum........   Carbon,........  78~
Carbon,.........  Antimony,.......  750
118. The actual, though not the relative, amount
of the deflections, will vary with the galvanometer
7




74            DAV S'S MANUAL.
employed.  In accurate experiments, the arrange.
ment should be such that the deflections shall not
exceed 200. Within this limit, the deviation of an
astatic needle from the zero point is strictly proportional to the quantity of electricity circulating.
119. The wires were not soldered together, but
their ends were brought in contact before the application of the heat, and kept so to the end of the
experiment.  With the more, fusible metals, the
greatest heat was employed which was consistent
with their fusibility. The object was to produce
the greatest current that could easily be obtained
from each' combination. It will be found that there
is an entire difference between the series of positive and negative metals for thermo-electricity and
for galvanism.
120. In some cases, the direction of the current
is reversed, either by raising the heat at the junction
to a high degree, or by heating one metal more than
the other. The following are instances of this kind.
The metal of each combination, which is positive
at low temperatures, is named first. Increasing the
temperature of the negative metal generally increases
the amount of deflection produced by heating the
junction; while, if the higher heat is applied to the
metal which is positive at moderate temperatures,
a current in the opposite direction is established.
The direction of the current in these combinations
is, however, often uncertain, and thd few experiments which have been made afford no explanation of the cause of the changes.




THERMO-ELECTRIC  CURRENT.    75
t21. IRON AND PLATINUM.- -When heat is applied
th the junction, or to the platinum a little one side
ol it, a deflection of about 50~ is obtained; when
to the iron near the junction, or when the junction
itself is raised to a red heat, the direction of the
current is immediately reversed, it now flowing from
the platinum to the iron, and the needle is deflected 60~ or 700 in the opposite direction.
122. COPPER AND IRON.- With finte wires the current is feeble, with large ones tolerably powerful.
The deflection is increased by heating the iron near
the junction. When the junction is raised to a red
heat, the current is reversed, and still more readily
when the heat is applied to the copper near it.
SILVER AND IRON. -Deflection considerable. On
heating the silver, an energetic current ensues in the
opposite direction; also, in a less degree, by raising
the junction to a red heat.
BRASS AND IRON. - Current moderate; reversed at
a red heat, and still more effectually by heating
the brass.
ZINC AND IRON. -Current moderate, and on heating the zinc near the junction to its melting point,
changes its direction.
123. PLATINUM  AND  SILVER. - Deflection  70~.
On heating the platinum, a strong current flows in
the opposite direction.
BRASS AND SILVER.- The current is reversed at a
red heat, or by applying the heat to the brass, near
the junction.
124. In quantity, the thermo-electric current much




76            D A V l S)S MANUAL.
resembles a feeble galvanic current. In intensity, it
is somewhat less.  In a single galvanic pair, electricity is set in motion in a certain direction, and
cannot return in the same path to the zinc, from
which it proceeded, without passing through the
fluid between the plates, which is a poor conductor.
It is, therefore, partially, though very imperfectly,
insulated. In a thermo-electric pair, the electricity
is set in motion from one of the metals to the
other, through the metallic junction.  Here there is
no insulation. The current flows through a good
conductor, and can only be the excess of the force
which sets the electricity in motion over its constant
effort to return to equilibrium. It is probably for
this reason that the intensity of thermo-electricity
is less than that of galvanism.
125. A single galvanic and thermo-electric pair
were taken, each of which deflected the needle 750,
permanently. The galvanic current was then made
to flow through a hundred feet of fine steel wire
1-150 of an inch in diameter. From the poor conduction of the wire, the deflection was reduced to
600. With the thermo-electric pair, it was found
that the needle was deflected 60~, when the current
passed through only fourteen feet of the wire. As
the conducting power of a wire is in proportion to
the intensity of the current, the intensity of the
thermo-electric current in this instance was equal
to one seventh of that of the galvanic current.
126. In soldering the wires or plates together,
they are not usually connected in a straight line,




THERIMiO-ELECTRIC  SERIES.              77
but at an acute angle with each other. If several
of these single pairs are associated together consecutively, that is, by connecting the German silver of
the one to the brass of the next, or the bismuth of
-one to the antimony of the next, and so on: we have
a thermo-electric battery, in which the power of the
current is exalted as by the successive pairs of plates
in a galvanic battery.  It will le understood that in
these cases there is German silver and brass alternately, or bismuth and antimony alternately, &c.,
throughout the whole series. For the sake of compactness, the wires or plates are laid side by side,~nd
soldered by their alternate ends, while they are insulated or separated from each other by paper or
pasteboard, which prevents any passage of electricity
from one to the other, except through the junctions.
127. A thermo-electric series of considerable power may be constructed of strips of German silver and
brass. It will bear contact with red-hot iron, and is
very compact.  Strips of pasteboard are interposed
between the adjacent metals. Fig. 40 represents
such a series, consisting of ten pairs. When several
Fig. 40.
i- -  mn.e
pairs are connected in this manner, it is necessary
that the junctions should be somewhat larger than
in the case of a single pair. Then the slighter
7*




78            DAVIS' S MANUAL.
the connection the better; but as the current has
to flow through all the junctions in a series of pairs,
the electricity generated would scarcely be conducted
through them at all, were they all imperfect. By
heating the junctions of the strips at one end of the
series with a spirit-lamp, a current is produced which
increases or diminishes as the heat is applied, depending altogether for its existence on the difference of temperature in the opposite junctions. On
grasping the junctions, at one end, in the fingers,
even the warmth of the hand produces a sensible
effkt.  It is evident that, if the junctions at both
ends of the series were equally heated, currents would
be produced in opposite directions, which would
neutralize each other.
128. THERMO-ELECTRIC  BATTERY.- In Fig. 41
is represented a thermo-electric battery, composed of
Fig. 41.
sixty pairs of plates of bismuth and antimony, each
three inches long, three fourths of an inch broad, and
one fourth of an inch thick. They are arranged side
by side, in a metallic case, B, so that one series of
junctions, underneath the battery, may be heated by
the radiation of a hot iron plate, E, the edge of which




TIHERMO-ELECTRIC  BATTERY.    79
Is seen underneath the battery in the cut; while the
opposite junctions, seen at A, are kept cool by water
or ice placed in the receiver, which forms the upper
part of the battery. A still greater depression of
temperature is produced by a mixture of snow or
pounded ice, with half its weight of common salt.
In order to make the receiver sufficiently water-tight,
as well as to insulate the plates from the case, they are
cemented into it with plaster. Refrigeration at one
end of the bars, as would be anticipated, is found to
produce a current in the same direction, and equal to
that which would be produced by a similar excess of
heat at the other end; difference of heat at the different ends, however produced, being the occasion of the
current. By associating both of these causes in this
battery, there is a corresponding increase of power.
As the metals employed in the battery are fusible,
the radiant heat of the iron ought never to exceed
3000 Fahrenheit. Thea iron plate being laid upon
a large tile, the battery is placed over it, the iron
being pretty near the ends of the bars, but not in
contact with them.
129. The terminal plates of the battery are con.
nected with two screw-cups, passing through the
metallic case, and insulated from it. In the cut,
the battery is seen in connection with an apparatus,
which will hereafter be described, by which the
magnetizing power of the current is shown.  The
ends of the coil of insulated wire, C, being fixed in
the cups, the current is obliged to traverse the coil,
and the two semicircular bars of soft iron, seen at




80            DAVIS I S  MANUAL.
D, are held together, by the magnetism thus induced,
with so much force as to require a weight of forty
or fifty pounds to separate them. This battery-has
sufficient power to give shocks and sparks, and produce various magnetic phenomena, with the appropriate apparatus, which will be described hereafter,
when the principles on which those effects depend
have been explained.
130. By forming a compound thermo-electric battery of a number of slender bars of antimony and
bismuth, soldered together by their alternate ends,
an instrument is obtained far more susceptible to
slight differences of temperature than the mercurial
or air thermometer.  Such is the construction of
Melloni's Thermo-Multiplier.  It consists of about
fifty little bars of the two metals, enclosed in a brass
cylinder, one half or one third of an inch in diameter,
and two and a half inches long. The terminal bars
or poles of this little battery are connected by wires
with a delicate galvanometer. When the ends of
the bars at one extremity of the cylinder are exposed
to any source of heat, while the temperature of the
other ends remains unchanged, an electric current
is excited which deflects the needle of the galvanometer more or less in proportion to the heat. The
radiant heat from the hand brought near the battery
is sufficient to move the needle several degrees.
This instrument was employed by Melloni in his
important researches on the transmission of radiant
heat from different sources through various transparent and translucent bodies.




PRODUCTION  OF COLD.              81
131  In thermo-electricity, an electrical current is
produced by heating unequally the opposite ends of
metallic plates, associated in a thermo-electric series.
The converse of this is found true.  If a galvanic
current is made to pass through the same series, the
opposite junctions will acquire heat on the one side
and lose it on the other.
132. Fig. 42 represents an instrument for showing
the simultaneous production of heat and cold by the
Fig. 42.        galvanic current.  It
consists of three bars,
A= i~./    two of bismuth and
one of antimony, arT      T         ranged as seen in the
l  figure, where the antimony is shown at A,
i   _.....   ~        and the two bars of
bismuth at B and B'.
The bars are soldered together under the bulbs of
two air thermometers, T and T', a little cavity being
made to receive the bulb of each thermometer; a
drop of water is put in each cavity, in order to facilitate the conduction of heat from the metals to the thermometers. The galvanic current being sent through
the metals, in the direction indicated by the arrows,
from the bismuth B', through the antimony, to the
other bar of bismuth, and thence back to the battery,
cold is produced at the junction of A with B', as will
be indicated by the thermometer T', and heat at the'junction between A and B, as the thermometer T
will show. By reversing the direction of the battery




82            D AVIS' S MANUAL.
current, the effect on the two thermometers will be
reversed.
133. The elevation of temperature produced is
always greater than the depression; this difference
is probably due to the low conducting power of
the metals for electricity, which causes them to become slightly heated by the current - a phenomenon
altogether distinct from the heating of the junction
by it. It will be observed in the figure that the current has the same direction as that which would be
produced, were the battery removed, by the application of heat at the junction of A with B', or of
cold to that between B and A; the current which
produces heat flowing in the opposite direction to
the current which would be produced by it.
IV. ANIMAL ELECTRICITY.
134. This name has been applied to the electricity
developed in the organs of certain fishes. These
organs are found, by anatomical examination, to consist of numerous cells, arranged in columns, and
divided by septa or small discs, so as to suggest at
once the idea of a galvanic arrangement. Further
observations show that they are supplied with large
branches from the nervous system, so much so, that,
in the torpedo and gymnotus, the ganglia and nerves
belonging to the electrical apparatus exceed in extent all the rest of the nervous system of the animal.
It is also found that this manifestation of electrical




ANIMAL  ELECTRICITY.              83
power is subject to the nervous system, and controlled by the will. An actual connection, therefore,
exists here between the vital and electrical phenomena. Several cases have recently been authenticated, in which electricity has been evolved by the
human subject in a state of nervous disorder. The
animals naturally endowed with this power are
surrounded by a conducting medium, water, which
enables them  to paralyze other creatures in their
neighborhood, which thus become their prey.
135. There is a remarkable analogy between this
production of electrical force and the production of
muscular force in the ordinary contraction of the
muscles. In both cases, the power which is developed is inherent in the organ where it is manifested,
and in both the power is controlled and exercised by
the agency of the will or influence of the nervous
system. Liebig has suggested a theory of muscular
action, which supposes that the contractile force of
the muscles is due to a principle set free by the
oxidizement of the animal tissues by the blood, in
the same way that electricity is set free by the oxidizement of zinc. He supposes that, when a muscular contraction takes place, the nerves, supplying
the part, withdraw their vital protection, and oxidation under the chemical laws, and the consequent
development of force, result.  An evidence relied
upon in support of this theory is the waste of animal substance which occurs proportional to muscular
action. A further application of this theory would
extend it to the electrical organs of fishes, where,




84           D A V I S S MANUAL.
by oxidizement of tissues, suitably arranged, electricity itself, instead of muscular force, would be
eliminated, when the protecting agency of the nervous system was withdrawn. An illustration of the
supposed active and passive states of the animal
organs under the nervous control may be readily found
in Smee's battery, where the plates remain opposed
to each other without any action until the circuit is
completed, when an electrical discharge at once takes
place. However hypothetical the views of Liebig
may be, they may be useful as connecting together
some phenomena belonging to a very dark and unexplored field of science. The analogy of the action
of the will or nervous system in producing, in the
one case, muscular, and in the other electrical action,
is at any rate an important one.  In the diseased
human subjects which have been mentioned, the
development of muscular force seems to have been
changed into a development of electricity. We have,
as yet, no knowledge of the relation between these
two principles. Professor Renwick has, however,
remarked, "In these electric fish we behold nervous
power converted into electric force; it cannot be
doubted that the converse of this is possible."
136. The GYMNOTUS is found in the fresh waters
of South America. It is interesting as having been
the subject of a series of experiments by Faraday, in
which the phenomena of magnetism, electrical decomposition, the production of heat; the shock and
spark were all obtained. Fig. 43 represents an individual of this species, which was preserved alive for




Y M NOT ITS                85
some time in this city.  The upper figure gives a
lateral view, the lower one a view from above, with
the hand in a favorable position to receive a shock,
passing from the anterior to the posterior part of
the animal, through the surrounding water. It will
readily be comprehended, that, even when the animal
is in a straight position, the finger placed near its
8




86(           D AV IS7S MANUAL.
side will become the medium of the passage of electricity, as it is a better conductor than water, and the
current between the extremities extends to some
distance from  the animal.  The electricity passes
through the finger in this case, only from one side to
the other, but still, by acting on the local nerves, produces the sensation of a considerable shock. When
a finger of one hand is placed near the head of the
animal, and a finger of the other near the tail, the
discharge passes through the arms and body, and a
very powerfill shock is experienced.  When two
fingers are thus placed in water, in the course of a
discharge of ordinary electricity, a shock is felt in the
same way; but in proportion as the water is made a
good conductor by adding salt, the effect decreases.
Thus, in the experiment (~ 52) with a leech or fishworm, if the water be made a good conductor, the
animal ceases to be troubled when the galvanic
circle is completed. When a small fish was placed
in the water with the gymnotus, the latter was
observed to curve himself so as to bring the fish
between the extremities of his body, and then to
make a discharge, which was either fatal or produced
entire numbness and rigidity.
137. The anterior region of the body was found
by Faraday to be positive, and the posterior negative, at the moment of discharge. The electrical
organs of the animal occupy almost its whole length,
with the exception of the anterior region. It is provided with a beautiful fringe-like ventral fin, which
gives it great ease of motion. It is found to suffer




TO 1P E 1).                87
great exhaustion from the frequent exercise of its
electrical power, showing thus another link between
the nervous and electrical systems.
138. The TORPEDO, or ELECTRICAL RAY, is found
on the sea-coast in different parts of Europe. A new
Fig. 44.
species Torpedo occidentalis, Storer, has lately been
taken at Wellfleet, Massachusetts. It is represented
in the above figure.  They are described as varying in weight from 20 to 200 lbs., and the shocks
which they give, according to the testimony of fishermen, are sufficienlp instantly to prostrate a man.
These shocks are also felt, in a less degree, by one
holding a harpoon or rope, to which the animal is
attached.  The electrical organs are situated on the
anterior and lateral portion of the animal.  The
electrical lobes of the brain are larger than the whole




88            DA} IS'  S MANUAL.
remainder of that organ, and the volume of the electrical nerves greater than those supplying all other
portions of the animal.
139. The SILURUs ELECTRICUS, the third of the
electrical fishes, is found in the rivers of Africa.
In shape, it is elongated like the majority of fishes,
and is provided with cirri, or feelers, about the
mouth.  It is believed that the fishes of this class
remain concealed near the bottom, and decoy their
prey by waving these cirri, so as to counterfeit the
appearance of worms. This would give to the electrical Silurus a great advantage in bringing other
animals within the scope of the powerful agency
with which it is endowed.




MAGNETISM.
I.
DIRECTIVE TENDENCY OF THE MAGNET.
I. IN REFERENCE TO ANOTHER MAGNET.
140. ATTRACTIONS AND REPULSIONS. - The effects
produced by the opposite poles of a magnet, though
in some respects similar, are in others contrary to
each other; the one attracting what the other repels.
Poles of different magnets, of the same name, that is,
both north or both south, are found to repel, while
those of an opposite name attract each other. This
is shown by the following experiment.
Let the south pole of a magnet, N S, be brought
near to the north pole of a
T                dipping needle, mounted
upon a stand, as represented in Fig. 45. The
needle, previously nearly
~By,'....'~:   vertical, will move into
the position a a, its north
pole (indicated by the
head of the arrow in the cut) being attracted by
8*




90            DAVI I S  MANUAL.
the south pole of the magnet. If, now, the magnet
be reversed, and its north pole be made to approach
the corresponding pole of the needle, the latter will
assume the position r r, its north pole being repelled
by that of the magnet.  The south pole of the
needle, on the contrary, will be repelled by the south
pole of the magnet, and attracted by its north pole.
141. In place of the dipping needle, a compass
needle, or any horizontal magnetic needle, can be
employed. A very fine and perfectly dry sewingneedle, being previously magnetized, and then laid
carefully upon the surface of water, will float, and,
being thus at liberty to move freely in any direction, may be conveniently used to show the attractions and repulsions described above.  A  larger
needle will answer equally well, if passed through
a small piece of cork, to enable it to float.
142. The intensity of the attraction or repulsion
exerted between two magnetic poles varies in the
inverse ratio of the square of their distance; that is,
if the distance of the poles is doubled, the force with
which they attract or repel each other is reduced to
one quarter of its previous amount; if their distance
is trebled, to one ninth; and so on. These attractions and repulsions are not affected by the interposition of glass or metal, or any substance whatever
between the two magnets; unless the interposed
body is itself susceptible of magnetism.
143. In consequence of the mutual attractions and
repulsions exerted between the poles, a magnetic
needle, brought near to a fixed magnet, assumes cer



DIRE CTIVE TEND ENCY OF MAGNET. 91
tain determinate directions depending upon the relative positions of the two. Thus, in Fig. 46, N S
Fig. 46.         represents a stationary
bar magnet, and the
three arrows indicate
the directions taken
sy  -    = DnT]  by a magnetic needle
placed  in  different
positions.  The two
needles near the south pole of the bar direct their
north poles towards it, the north pole of the stationary
magnet being too remote to influence them to any
extent. But the needle placed over the centre of the
bar has its direction determined by both poles, and
assumes a position parallel to the bar, but with its
poles in a reverse direction.
144. In Fig. 47 are represented various directions
assumed by a small movable magnet brought near
Fig. 47.
IN    s         o
to a stationary one. These may be readily observed
by carrying a small bar magnet, suspended by a
string, or, better, a magnetic needle, mounted upon
its stand, slowly round a stationary bar magnet.




92            DAVISS  MANUAL.
145. The arrangement of iron filings around the
poles of a U-magnet, as rfepresented in Fig. 48,
/g. 48.      depends upon the directive tendency communicated to the
minute particles.  As will be
explained hereafter, each  particle of the filings becomes a
temporary magnet, with a north
and a  south  pole, while  attached to the steel magnet. The
filings are held together by their
mutual attractions, and, having
thus great freedom  of motion,..': readily obey the directive influence exerted upon them.
146. Spread a thin covering of iron filings over a
sheet of paper, or thin pasteboard, and place a powerful U-magnet vertically beneath it, with the poles
close to the paper.  The dotted lines in the cut (Fig.
49) show the arrangement which the particles of iron
FKg 49.         will assume.  Each one
~,,:,,,,....~;;;  >   becomes a magnet with
\,:,',,;I;',5...',,',,    j  two poles, and connects
-.'.,',',:.-:_-.<2,,:,  itself with  those  ad-: —5~;N~  ----- &:41> joining it, so as to form
curved lines of a peculiar
z///.,.,\, —— "','','',' character.  This experi\',    "'  ment may be performed
n a still more satisfactory manner, by supporting the
paper, with the magnet in contact with its under
surface, and then showering down iron sand or iron




MAGNETIC  CURVES.               93
filings from a sand-box held some inches above.
The particles of iron, as they strike the paper, can
thus more readily assume the positions to which
they tend under the magnetic influence.
147. The lines formed by the filings afford a good
experimental illustration of what are called magnetic
curves; that is, the curves into which an infinite
number of very minute magnetic needles suspended
freely would arrange themselves, if placed in all
possible positions about a magnet. When the particles are very small, the attractive force exerted upon
them by the magnet, being the difference of its
action upon the two poles of each particle, is slight;
while the directive force is very considerable. The
direction assumed by each particle, and consequently
the form of the magnetic curve, connecting any point
on one half of the magnet with the corresponding
point of the other half, is deducible, on mathematical
principles, from the laws of magnetic attraction and
repulsion. The curvature of the lines is due to the
combined action of the two poles of the magnet. If
only one pole acted on the minute particles, they
would arrange themselves in straight lines, diverging
in all directions from the pole, like radii from the
centre of a sphere. This may be partially shown,
by placing a bar magnet perpendicularly under the
paper which is strewed with filings, with its upper
pole close to the sheet.




94           DAVIS'S  MANUAL.
II. IN REFERENCE TO A CURRENT OF ELECTRICITY.
148. It was discovered by Professor (Ersted, of
Copenhagen, in the year 1819, that a magnet, freely
suspended, tends to assume a position at right angles
to the direction of a current of electricity passing
near it.  This may be shown by the following
experiment: -
149. Let N S (Fig. 50) be a magnetic needle
poised upon a pivot so as to allow of a free horiFig. 50.       zontal motion, and W  R
w        I --          R a wire passing directly
-?x.', over and parallel to it.
S  - -N   Of course, the direction
of the wire must be north
and south, as the needle
will necessarily assume
that direction, by the influence of the earth. If,
now, the extremities of
the wire are put in connection with the poles of a galvanic battery, in such
a manner as to cause a current of electricity to pass
through it, the needle, N S, will be deflected, and
will turn towards the position a b or c d, according
to the direction of the current of positive electricity,
whether from W to R, or from R to W. If the wire
is placed in the same direction below the needle, the
deflections will be the reverse of those produced by




DE  F  ECTION  OF NEEDLE.             95
the same current when flowing above. If the positive current is passing from south to north in the
wire, as shown by the arrow in the cut, the north
pole of the needle will turn to the'West if it be
below the wire, and to the east if above it.
150. In these cases, the needle will not be deflected so far as to assume a position exactly at right
angles with the wire, on account of the influence of
the earth, which still acts upon the magnet, and
tends to draw it back to its original position. It
will accordingly come to rest in a state of equilibrium between the two forces, in a direction intermediate between a line at right angles to the wire
and that of the needle when controlled by the magnetism of the earth alone.
151. The same experiment may be performed
with the dipping needle, the wire being placed parallel with the needle. By thus varying the positions
of the wire and the needle, it will be found that in all
cases the needle tends to place itself at right angles
with the wire, and to turn its north pole in a determinate direction with regard to the wire.
152. The action of a conducting wire upon a
magnet exhibits) in one respect, a remarkable peculiarity. All other known forces exerted between
two points act in the direction of a line joining these
points; such is the case with the electric and magnetic actions separately considered. But the electric
current exerts its magnetic influence laterally, at right
angles to its own course. Nor does the magnetic
pole move either directly towards or directly from




96            DAVIS' S MANUAL.
the conducting wire, but tends to revolve around it
without changing its distance.  Hence the force
must be considered as acting in the direction of a
tangent to the circle in which the magnetic pole
would move. It is true -that, in many positions of
the magnet with regard to the wire, apparent attractions and repulsions occur; but they are all referable
to a force acting tangentially upon the magnetic
poles, and in a plane perpendicular to the direction
of the current. This peculiar action may be better
understood by means of a figure.
153. Thus, let p n (Fig. 51) be a wire, placed in
a vertical position, and conveying a current downwards (p being connected with the positive pole of
Fig. 51.       the battery.)  Now, suppose
the north pole of a magnet,
N, to be brought near the
wire, and to be perpendicular._~..... —..... ~to any point C.  If free to.around C as a centre, in the
direction indicated by the arrows in the cut; that is, in the
same direction as that of the
t             hands of a watch, when its
face is upwards.  The plane of the circle which the
pole describes is horizontal. On causing the current
to ascend in the wire, the pole will rotate in the opposite direction. If the wire is placed in a horizontal
position, the plane in which the pole revolves will,
of course, be vertical. The actions of either a de



ASTATIC NEEDLE.                  97
scending or an ascending current upon the south
pole are exactly the reverse of those exerted on the
north pole.
154. In the experiment given in ~ 149, no revolution occurs, because the current, acting at once on
both poles, tends to give them motion in opposite
directions; so that the magnet comes to rest in a
position of equilibrium between these two forces,
across the wire. It will be shown hereafter that a
continued rotation may be produced by confining
the action to one pole. If the wire is movable and
the magnet fixed, the former will revolve around the
latter in a similar manner, and in the same directions.
Thus a wire conveying a descending current tends
to rotate round the north pole of a magnet, in the
direction of the hands of a watch.
155. The two following articles of apparatus are
used to show the directive tendency of a magnet
in reference to an electric current.
156. ASTATIC NEEDLE. —A magnetic needle, so
contrived that its directive tendency in respect to the
Fig. 52.     earth is neutralized, so that it
As shall remain at rest in any poll  Nq sition, is called an astatic needle.
~s                 It is constructed as represented
in Fig. 52, consisting essentially
of two needles, one above the
other, placed in positions the
reverse of each other in respect
to their poles.  Such a system
will not be affected  by the
9




98            DAVI S'S  MANUAL.
magnetic influence of the earth, as whatever forces
may be exerted upon the upper needle will be
counteracted by-equal forces exerted in reverse directions upon the lower. It would be the same,
indeed, with the influence exerted by the current of
electricity, if the wire could be placed in such a
positi6n as to act equally on both needles. But by
placing the wire parallel to and above the upper
needle, the influence of the wire will be far more
powerful upon the upper than upon the lower one,
and, the action of terrestrial magnetism being neutralized, the needle will assume a position exactly
at right angles with the conducting wire. If the
wire be placed as nearly as possible between the
needles, and parallel to them, the influence of the
upper side of the wire will deflect the upper needle
in the same direction as the lower needle will be
deflected by the action of the lower side of the wire,
causing a more powerful effect.
157. MAGNETIC NEEDLE, HALF BRASS. —— In this
instrument the steel needle is wholly upon one side
of the point of support, and is counterpoised by a
brass weight on the other side. By this arrangement, the action of a current upon the pole which
is situated at the centre of motion can have no influence in turning the magnet in any particular direction; and its motion will be determined solely by
the action upon the other pole; no rotation, however, can be obtained.  The object of the instrument is to show the directive tendency of a single
pole with reference to the electrical current.




GALVANOMETERS.                    99
158. GALVANOMETERS. -- If the wire transmitting
the electrical current, after passing over the needle, is
bent and returned under it, as in Fig. 53, it might
be supposed that,
A~~B: ls B               as the electricity
which flows from
C  to A  in  the
upper part of the
wire must pass in
a contrary direction, in returning
from A to B, below (the cup C
being connected
with the positive
pole of the battery, and B with the negative), the influence of the
one part of the wire would neutralize that of the
other; for it has already been stated that the needle
is deflected to one side or the other, according to the
direction of the electrical current.  And this would
in fact be the case, if the returning part of the wire
were upon the same side of the needle with the other
part, and at an equal distance from it. But a wire
transmitting an electrical current, when passing below
the needle, will produce an effect the reverse of that
produced by one passing above, if the current in both
cases flows in the same direction.  Hence, if the
direction of the electric current is reversed in the
wire which passes below, it will exert a force auxiliary, and not antagonist, to that of the wire passing




100          DAVIS'S MANUAL.
above. This is the case with the arrangement here
represented. The two portions of the wire are not
allowed to touch each other where they cross, but
are insulated at that point by some non-conductor of
electricity, as by being wound with thread. Instruments of a variety of forms are constructed on the
above principle, and are called galvanoscopes or galvanometers, as they serve to indicate the presence of a
current of electricity, and in some degree to measure
its quantity.
159. The vertical portions of the wire also aid
in deflecting the needle; as may be shown by connecting both the cups B and C with one pole of the
battery by two wires of equal length and thickness,
and the cup A with the other pole (say the positive).
The current will then be divided into two portions
very nearly equal, both flowing in the same direction,
and at the same distance from the magnet, M, but
one below and the other above it. Now, if the
horizontal portions of the wire alone acted on the
needle, it would remain unaffected; but it will be
found to be deflected to a considerable extent by the
current which is descending in the vertical portion
of the wire near A, and ascending in that below B,
as these conspire in their influence.
160. HORIZONTAL GALVANOMETER. - If the wire
is carried many times around the needle, as in Fig.
54, the power of the instrument is much increased,
as each turn of the wire adds its influence; provided
the wire is not so long, or of so small a size, as to be
unable to convey the whole of the current.  The




UPRIGHT  GALVANOMETER.               101
instrument thus becomes a delicate test of the presence of a current of electricity.  The coil of wire is
FT. 54.         Us
supported on a tripod stand, with levelling screws;
the ends, C and D, of the wires being connected with
the screw cups, A and B.
Fig. 55.     161. UPRIGHT GALVANOMETER.
In this instrument, represented in
Fig. 55, both the coil of wire and
the needle are placed in a vertical
position, the north pole being made
a little heavier, in order to keep
the magnet perpendicular.  When
a current is passed through the
coil, the deflection is towards a
horizontal position.  The needle is
made of large size, for the purpose
of exhibiting the deflections before
an audience.




102           L A V I S S MANUAL.
162. GiALVANOMETER WITH ASTATIC NEEDLE.
This instrument is represented in Fig. 39, in connection with a thermo-electric pair. It is constructed on
a small scale, in order to be delicate;. and the needle
is nearly astatic.  The slight degree of directive
tendency which the needle is allowed to retain
becomes the measure of the force of the electric
current, as the angle of deflection from the north and
south line shows how far this resistance is overcome.
This instrument may be made so extremely delicate
in its indications, that, if two fine wires, one of copper and one of zinc, are connected with it, and their
ends slightly immersed in diluted acid, or even in
water, it will be very perceptibly affected.  Before
proceeding to experiment with any galvanometer,
it should be so placed that the direction of the coil
may coincide with that of the needle, as this is
the position of greatest sensibility.
163. The galvanometer is a measurer of what is
called the quantity of electricity, but takes no cognizance of intensity. Mechanical electricity, which
possesses great intensity and but little quantity, very
slightly deflects the needle of the galvanometer.
The current from one galvanic pair influences the
needle powerfully, the quantity being very great, and
the intensity small. If a hundred pairs be connected
together in a single series, the intensity is increased
a hundred fold; but the quantity remains the same,
and the needle is but little more deflected than by
one pair. The reason that there is any difference
in this respect is, that, when the electricity is of




ROTATION  OF  MAGNET.              103
high tension, the wire of the galvanometer obstructs
the current less, and more actually passes through it.
164. In thermo-electricity, with a single pair, the
intensity is less in proportion to the quantity than
with a single galvanic pair, and the current is strongly
indicated by the galvanometer.  The amount of
decomposing power in a current of electricity is
always exactly as its quantity.  The galvanometer
indicates, therefore, the electro-magnetic and the decomposing capacity of a current of electricity. An
intense electrical current decomposes more easily than
one of little intensity; but the amount of matter
decomposed is proportional merely to the quantity
of the current. Besides the galvanometers in which
a magnetic needle is used, other instruments for
measuring the quantity of an electric current will be
described farther on.
165. When a wire conveying a current of electricity is brought near to a magnetic pole, the pole
tends to revolve around it, as has been explained in
~ 153. If the current acts equally upon both poles, no
rotation occurs, because they tend to move in opposite directions; and the magnet rests across the wire
in a position of equilibrium between the two forces.
But if the actioh of the current is limited to one
pole (which was first effected by Professor Faraday),
a continued revolution is produced. If the magnet
has liberty of motion, it will revolve around the
wire; if the wire only is free to move, it will rotate
around the pole. When both the wire and the magnet are at liberty to move, they will revolve in the




104          DAVIS I S  MANUAL..
same direction round a common centre of motion.
A number of instruments have been contrived for
exhibiting these movements.
166. MAGNET  REVOLVING ROUND A CONDUCTING
WIRE. -In the instrument represented in Fig. 56,
Fig. 56.     the magnet, N S, has a double
A           bend in the middle, so that this
part is horizontal, while the
extremities are vertical.  To
its north pole, N, is attached a
piece of brass at a right angle,
bearing a pivot, which rests in
an agate cup fixed on the stand.
A wire loop, attached to the
B upper pole, S, encircles a vertical wire fixed inl the axis of
motion, and thus keeps the
magnet upright. The galvanic
current is conveyed by this
vertical wire: it is surmounted
by a brass cup, A, and its lower
end dips into a small mercury
cup on the horizontal portion
of the magnet. From this part projects a bent wire,
which dips into a circular cistern of mercury, open
in the centre, to allow the magnet to pass through,
and supported independently of it. A wire, terminated by a brass cup, B, for connection with the
battery, proceeds outwardly from the cistern. This
arrangement allows the current to flow down by the
side of the upper pole of the magnet, until it reaches




REVOLVING  MAGNET.             105
its middle, whence it is conveyed off in such a direction as not to act upon the lower pole. On making
connection with the battery, the magnet will revolve
rapidly around the wire; the direction of the rotation
depending upon that of the current.
167. MAGNET REVOLVING ON ITS AXIs. - The instrument represented in Fig. 57 is designed to show
Fig. 57.         that the action between the current
and  the  magnet
takes place equally well when the
magnet itself forms
the  conductor  of
B     the electricity. The
lower end, N, of the
magnet, being pointC /ed, is supported on
an agate at the bottom of a brass cup,
connected under the
base-board with the screw-cup C., The upper end,
S, is hollowed out to receive the end of the wire
fixed to the cup A; the brass arm  supporting this
cup is insulated from the brass pillar by some nonconductor of electricity.  To the middle of the
magnet is fixed a small ivory cistern, for containing
mercury, into which dips the end of a wire, connected
vrith the cup B. Thus the magnet is supported with
its north' pole downwards, and is free to rotate round
its vertical axis. A little mercury should be put into
the cavity at S, and into the brass cup at N, and the




106          D AV IS S  MANUAL.
ivory cistern be filled sufficiently to establish a connection between the magnet and the wire attached
to B. When the cups A and B are connected with
the battery, the current will flow through the upper
half of the magnet, causing it to rotate rapidly. If
the cups B and C form the connection, the current
will traverse the,lower half, equally producing revolution of the magnet. Now, connect A and C with
the battery, and no motion will result; because the
electricity passes through the whole length of the magnet in such a manner that the tendency of one pole
to rotate is counteracted by that of the other to move
in the opposite direction. Connect B with one pole
of the battery, and A and C both with the other pole.
The magnet will now revolve; since the current ascends in one half of its length and descends in the other.
Fig. 58.         168.  REVOLVING  WIRE:B   |    C      FRAME. - The revolution of
a conductor round a magnet
is shown by the instrument
represented in Fig. 58. Two
l  R  light frames of copper wire,
R R, are supported by pivots
resting on the poles, N and
n S, of a steel magnet of the
U form; a small cavity being drilled in each pole to
receive an agate for the bearing of the pivot. The lower
extremities of the wires dip
into mercury contained in
two circular cisterns sliding




REVO L V I NG C Y  I N D E R.    107
on the arms of the magnet. Bent wires, passing from
the interior of the cells, support the cups A and D;
and the cisterns themselves are fixed at any required
height by means of binding screws attached to them.
Each of the wire frames is surmounted by a mercury
cup; into these dip the wires projecting downwards
from the cups B and C.
169. The cisterns being partly filled with mercury,
fix them at such a height that the lower extremities
of the wire frames may just touch its surface. The
cups surmounting the frames should also contain a
little mercury.  When the cups A and B are connected with the battery, the left-hand frame will revolve, in consequence of the action of the north pole
of the magnet upon the current flowing in the vertical
portions of the frame. By uniting C and D with the
battery, the other frame will rotate. On transmitting
the current from A to D, it will ascend in one frame,
and, passing along the brass arm which supports B and
C, will descend in the other, causing them both to
revolve in the same direction. Instead of the frame,
a single wire may be employed, having the form of a
loose helix surrounding the pole, its convolutions
being a quarter of an inch or more apart.
170. REVOLVING CYLINDER. -This instrument is
on the same principle as that last described, and the
motion takes place in the samle manner; the only
difference being that two light copper cylinders, c c,
Fig. 59, are substituted for the wire frames. These
cylinders are serrated at their lower edge, as shown
in the figure, to lessen the friction which they expe



108          D A V I SS MANUAL.
rience in moving through the mercury. The cups
for battery connections are lettered in correspondence
Fig. 59*     with those in the preceding
il i   c  cut. (Fig. 58.)
171. In the case of a conducting wire revolving round
a magnet, the circumstance
of the two being joined together does not affect the result, the wire moving with
sufficient power to cause the
magnet to turn on its axis
with considerable rapidity,
when delicately supported: a
bar magnet is of course employed.  A figure and description of an instrument designed
to show this revolution will
be found in Silliman's Journal, Vol. XL. p. 111.
172. The current passing within the voltaic battery itself exhibits the same electro-magnetic properties that it does while flowing along a conducting
wire connecting the poles. Hence the battery, if
made small and light, will revolve by the influence
of a magnet.  This is effected in the following
manner.
173. ROTATING BATTERY. - A small double cylinder of copper, closed at the bottom, is supported upon
the pole of a magnet, by means of an arch of copper
passing across the inner cylinder, and having a pivot
projecting downwards from its under surface, which




ROTATING  BATTERY.              109
rests in an agate cup on the pole. The inner cylinder of course has no bottom. A cylinder of zinc is
Fg. 6 0.   supported by a pivot in a similar
manner upon the copper arch,
& L s   and, being intermediate in size
between the two copper cylinders, hangs freely in the cell.
This arrangement allows each
plate to revolve independently of
the other. In Fig. 60 two batteries are represented, one on each
pole of a U-magnet, the one on
the south pole being shown in
section; in this the zinc plate
z is seen suspended within the
copper vessel.
]~]/]!IlL    174. On introducing  diluted
acid into the copper vessel, an electric current immediately begins to circulate, which passes from the
zinc to the copper through the acid, and, ascending
from the copper through the arch, descends again to
the zinc. Hence the zinc plate is in the condition of
a conductor conveying a stream of electricity downwards, and will consequently revolve under the infl1ience of the pole which it surrounds.  The copper
cylinder, on the contrary, is in the situation of a
conductor conveying a current upwards, and will
rotate in the opposite direction. When there is a
battery on each pole of a U-magnet, the two copper vessels will be seen to revolve in contrary directions, and the two zinc cylinders in directions
10




110           DAVIS' S   MANUAL.
opposite to these, and of course also contrary to
each other.
170. VIBRATING WIRE. —A copper wire, seen at
W, in Fig. 61, is suspended over a small basin for conFig. 61.         taining mercury excavated in the stand,
by means of a brass
* \\                arm supporting a mercury cup, in which
i  the upper end of the;  a                   wire rests: this mode
of suspension allows
- it to vibrate freely,
if its upper end is
properly bent.  Two
cups for connection
with the battery communicate, one with the mercury in the excavation,
the other with the cup which sustains the wire.
176. The basin being supplied with a sufficient
quantity of mercury to cover the lower end of the
suspended wire, lay a horseshoe magnet in a horizontal position on the stand, with one of its legs on
each side of the wire. When communication is
established with the battery, the poles of the magnet
will conspire in urging the wire either backwards oi
forwards between them, according to the direction in
which the current flows through it, and the position
of the magnetic poles. In either case, the motion
will carry it out of the mercury into the position
shown by the dotted lines in the cut; and the circuit




VIBRATING  WIRE.              l11
being thus broken, the wire wil' fall back by its own
weight; when, the current being reistablished, it
will again quit the mercury, as before, and a rapid
vibration will be produced. The vibration may be
made somewhat more active by raising the magnet a
little from the stand, and nearly to the height of the
middle of the wire.
177. The instrument represented in Fig. 62 differs
from the one last described in the mode of suspenFig. 62.    sion of the wire and the vertical position of the magnet. The
wire is fixed to a little cylinder,
[ h      moving freely on a horizontal
axis, which is supported by a
brass pillar, secured upon one
of the magnetic poles. A small
mercury trough, supported be/ I~HIMHI   tween the legs of the magnet,
is connected, under the stand,
with' one of the screw-cups.
Thus the current is conveyed
to the mercury. The other cup
is connected with the magnet,
and, through the pillar, with the wire, W.
178. A single magnetic pole, placed either in a
horizontal or vertical position, by the side of a wire
suspended as in the last two figures, will cause it to
vibrate; but its motion is less active than with two.
The tendency of the wire is to revolve round the
pole presented to it, as has been explained in ~ 154;
and, when suspended between a north and south
pole, simultaneously around both




1512         DAVIS' S MANUAL.
179. GOLD LEAF GALVANOSCOPE. - In this instriument, represented in Fig. 63, a narrow slip of gol&
Fig. 63.    leaf, G, is suspended between the
poles of a U-magnet.  The legs
of the magnet are brought near
to each other, in order to render
their action on the current conveyed by the gold leaf more
powerful, and also, to allow of the
whole arrangement being enclosT            ed in a wide glass tube, T.  This
prevents the interference of currents of air, and secures the gold
leaf from injury. The upper end
of the slip is in metallic connection with the screw cup surmounting the tube.  Its lower
end  communicates  with  the
screw cup on the stand. When
-  i,   a very feeble current of electricity is transmitted through the gold leaf, it becomes curved forwards or backwards, according to
the course of the current; in either case, tending to
move away from between the magnetic poles in a
lateral direction. The motion depends on the same
cause as that of the wire in the instrument described
in ~ 177. This instrument does not indicate the
quantity of the electrical current, but is an exceedingly delicate test of its existence and direction. A
powerful current would of course destroy the gold
leaf.
180. REVOLVING SPUR-WHEEL. - The reciprocat



REVOLVING  SPUR-WH EEL.            113
ing movement in the instrument described in ~ 177
may be converted into one of rotation, by making
Fig. 64.     use of a copper wheel, the
circumference of which is
cut into rays, instead of the
wire.  The points of the
wheel, W (Fig. 64,) dip into mercury contained in a
small trough, T, supported
between the legs of a Umagnet, fixed in an upright
position. The axis of the
wheel is supported by two
brass pillars upon the poles,
N and S, of the magnet.
\\ bOne of the screw-cups on
the stand is connected with
the magnet, and through'that with the'wheel. The mercury trough, which
is insulated from the magnet, is connected with the
other cup.
181. When connection is made with the battery,
the current passes from the axis of the wheel to the
trough through any one of the points which happens
to touch the mercury. Under these circumstances,
the ray through which- the current is flowing passes
forward between the poles of the magnet, like the
vibrating wire in Fig. 62, until it rises out of the
mercury. At this moment the next succeeding ray
enters it, and goes through the same process; and
so on.
10*




114           DAVIS' S MANUAL.
182. If the quantity of mercury is so adjusted that
one ray shall quit its surface before the succeeding one
touches it, a spark will be seen at each rupture of
contact. When the machine is set in motion in the
dark, so that it may be illuminated by the rapid succession of these sparks, the revolving wheel will
appear to be nearly at rest; exhibiting only a quick
vibratory movement, in consequence of the sparks
not succeeding each other precisely at the same point.
This optical illusion arises from the fact, that the
electric light is so extremely transient in its duration,
that the wheel has not time to move to any appreciable extent during the electrical discharge; and
therefore each spark shows it in an apparently stationary position. If the sparks occur at one place
more frequently than at the rate of eight in a second
of time, the eye cannot appreciate them separately,
and the impression of a continuous light is received.
For this reason, the wheel is seen constantly, as if it
were illuminated by a steady light, instead of an
intermitting one
183. A more rapid revolution will be obtained if
a small electro-magnet be substituted for the steel
magnet. The electro-magnet is included in the circuit with the spur-wheel, so that the current flows
through them irt succession. Here the direction of
the rotation will not be changed by reversing that of
the current, since the polarity of the electro-magnet
is also reversed.
184. REVOLVING DIsc. - It is not essential to
divide the wheel into rays, in order to obtain rota



DE  LA RIVE' S RING.              115
tion. A circular metallic disc will revolve equally
well between the poles of a magnet. In this case,
the electric circuit remains uninterrupted during the
entire revolution, and no sparks appear, as with
the spur-wheel.
185. DE LA RIVE'S RING. -A coil of wire, while
transmitting the electric current, exhibits the same
reactions with a magnet as the straight wire in Figs.
Fig. 65.         61 and 62; but the
circular  direction
in which the curCI~Z\ ~    rent flows gives to
the coil an apparent magnetic polarity.  This fact
may be shown by
means of the apparatus figured in the
adjoining cut. One
end  of the wire
forming the coil,
or helix, C, is soldered to a very small plate of copper, c, and the other
to a similar plate of zinc, z. These plates are fastened to a small piece of wood, in order to keep them
apart. and placed in a little glass cup, D. To put
the instrument in action, a sufficient quantity of
water, acidulated by a few drops of sulphuric or
nitric acid, is poured into the glass cup to cover the
plates, and the whole apparatus is floated in a basin
of water. The coil will now be found to place itself
with its axis north and south, being influenced by




116           D A V I S S MANUAL.
the polarity of the earth, like a compass needle. The
arrow indicates the course of the galvanic current in
the coil from the copper to the zinc.
186. Take a, bar magnet, M, and, holding it horizontally, bring its north pole near to the south pole
of the ring. The ring will move towards the magnet, and pass over it until it reaches its middle, where
it will rest in a state of equilibrium; returning to this
position, if moved towards either pole and then left
at liberty. Now, holding the ring in its position,
withdraw the magnet, and pass it again half way
through the coil, but with its poles reversed. The
ring, when set at liberty, will, unless placed exactly
at the centre, move towards the pole which is nearest, and, passing on till clear of the magnet, will
turn round and present its other face. It will then
be attracted, and pass again over the pole till it rests
in equilibrium at the middle of the magnet.
Fig. 66.                  187.  VIBRATING COIL
ENGINE.- In
the   instrument  represented    in
Fig. 66, two
heliacal coils
are  so  suspended from
an arm attached to the
tall brass pillar as to al



VIBRATING  COIL ENGINE.             117
low of their moving to some distance over the poles,
N and S, of a steel magnet. When uninfluenced by
the electric current, the coils hang vertically from
their points of support.  There are two cranks on
the axis of the balance-wheel above the magnet, which
convert the vibration of the coils into revolution.
On opposite sides of the axis are two wires, tipped
with silver springs; each of these is connected under
the base-board with one end of the wire forming each
coil. At the point where the springs press, a part of
the axis is cut away, leaving a third or a quarter of
the cylindrical surface prominent; over this is soldered a thin slip of silver. The effect of this arrangement, which is called a break-piece, is, that each
spring, alternately transmits the electric current from
the axis to its coil during a part of the revolution of
the wheel, while pressing upon the cylindrical portion of the axis. The screw-cups are connected, one
with the axis of the wheel, the other with one end
of the wire of both coils.
188. Connection being made with a galvanic battery, the current traverses one of the coils, which
then moves over the pole which it surrounds, towards the middle of the magnet, in the same manner
as De la Rive's ring. When it has passed some
distance, the motion produced in the balance-wheel
interrupts the current by breaking the contact between the cylindrical part of the axis and the spring,
and the coil falls back to its first position. As the
wheel moves on, the other spring comes in contact
with the axis, causing the other coil to be charged,
and to go through the same movements. The coils




118         DAVIS'S MANUAL.
thus vibrate alternately, producing a rapid rotation
of the balance-wheel.
189. RECIPROCATING COIL ENGINE.- Fig. 67 represents an instrument in which two coils, C C, mountFIg. 67.
--
ed upon wheels, pass over the legs of a steel magnet
fixed in a horizontal position, the wheels moving
upon inclined rails. When the instrument is connected with the battery, the coils become charged,
and move along the poles towards the bend of the
magnet. The battery connections must be so made
that the current shall pass in the proper direction, or
the coils will tend to move away from the magnet
rather than over it. When the coils reach the bend
of the magnet, the platform upon which they are
secured comes in contact with the wire-spring, A,
and, by the movement produced in that, breaks the
contact at B.  The current being interrupted, the
coils slide down the rails by the force of gravity,
until the platform reaches a pin on the rod near B,
whose movement renews the contact.
190. RECIPROCATING MAGNET ENGINE. - Fig. 68




REVOLVING COIL.                 119
represents an instrument on the same principle as the
last, except that the coils are stationary and the magFIg. 68.
net moves within them. It is supported by wheels
moving upon the inclined rails. When the bend of
the magnet reaches the coils, the axle of the wheels near
B strikes a spring, and breaks contact at B, by pushing
along a little rod below the wheels. As the magnet
falls back by gravity, the axle touches another spring
on the same rod, drawing it along until a pin upon the
rod strikes the pillar at B, and renews the circuit.
fig. 69.      191. REVOLVING  COIL.- This
c,          instrument consists of a U-magnet
I  s    fixed upon a stand, in a vertical
position, and a circular coil of insulated copper wire, C (Fig. 69), so
arranged as to revolve on a vertical
axis between the magnetic poles.
The rotation is effected in a different manner from  any previously
mentioned.  The polarity of the
coil is reversed twice in each revo.
lution, by means of the pole-changer,
invented by Dr. Page, which is
employed in many of the instruments to be hereafter described.




120           DAVI S S MANUAL.
The pole-changer attached to the coil is seen at P,
and a horizontal section of it is shown in Fig. 70.
Fig. 70.    It consists of two small semi-cylindrical pieces of silver, s s, fixed on
opposite sides of the axis of motion,
A, but insulated from that and from
each other; to each of these segments is soldered one
end of the wire composing the coil. The battery
current is conveyed to the coil by means of two
wires terminated by horizontal portions of flattened
silver wire, W W, which press slightly on opposite
sides of the pole-changer, and must be so arranged
that the direction of the current in the coil may be
reversed at the moment when its axis is passing between the poles of the magnet.
192. The coil being placed with its axis at right
angles to the plane of the poles, and connection made
with a battery, one extremity of the axis, or, in other
words, one face of the coil, acquires north polarity,
and the other south polarity, in the same manner as
De la Rive's ring. The action of the magnet now
causes it to move round a quarter of a circle in one
direction or the other, according to the course of the
current, so as to bring its poles between those of the
magnet. In this position it would remain, were it
not that, as soon as it reaches it, the pole-changer,
which is carried round with it, presents each of its
segments to that stationary silver spring which was
before in contact with the opposite segment. By
this movement, the current in the coil is first interrupted for a moment, and, as the coil passes on, is
immediately renewed in the contrary direction, thus




REVOLVING  RECTANGLE.               121
reversing the polarity. Each end of the axis being
now repelled by the magnetic pole which previously
attracted it, the coil turns half way round, so as to
present its opposite faces to the poles. At this point,
the direction of the current is again reversed, causing
the motion to be continued in the same direction;
thus producing a rapid revolution. Instead of a coil
of large diameter, the, wire may be coiled into a long
helix of small diameter, which will rotate in the
same manner.
193. REVOLVING  RECTANGLE. - This instrument
is similar in principle to the last described, a rectFig. 71.       angular coil of wire, C
(Fig. 71), being substitut{ A Hi      ed for the ring. The rotation is much more rapid in
SF- / f( ]S IN l s  consequence of the proximity of the rectangle to
the magnet, not only near
its poles, but throughout
the greater part of its
length.  By this means,
the great speed of eight or
ten thousand revolutions
in a minute may be attained.  In the cut, the
two silver springs, which
press on the pole-changer,
are seen at a, each of them
attached to a stout brass
wire, proceeding from one of the cups for battery
11




122          DAVIS' S   MANUAL.
connection; these wires pass through the brass arch
surmounting the U-magnet, but are insulated from it.
194. REVOLVING GALVANOMETER NEEDLE. - Fig.
72 represents an instrument of similar construction
to the Upright Galvanometer
Fig. 72.
(Fig. 55), except that a continued revolution of the magnetic needle is obtained by the
following arrangement: One end
of the wire composing the coil
connects with one of the screwcups on the stand, and its other
end with the bearings of the
shaft on which the needle re-.volves.  On the shaft is a breakpiece, seen at B, in the front
view of the needle, given separately in the cut, upon which a
wire, W, connected with the other screw-cup, presses
during half of the revolution. The instrument may
be used as a galvanometer, by removing W from the
break-piece, and bringing it into contact with a silver
pin, not shown in the cut, which connects it directly
with the coil, without the intervention of the breakpiece. If the direction of the battery current is reversed, the needle revolves in the opposite direction.
195. REVOLVING COIL AND MAGNET.-In this instrument, represented in Fig. 73, a circular coil of
wire, C, is fitted to revolve, on a vertical axis, between
the poles of two steel magnets. So far it resembles
in principle the Revolving Coil (Fig. 69); but in




REVOLVING  COIL  AND  MAGNET.  123
this instance the magnets rotate also. For this purpose, they are made of thin steel, and bent into a semiRFg. 73.        circular form.  The two
are connected by strips
of brass, so as to form a
circle, the similar poles
being near each other,
as indicated by the let-' N    l l C   ) j ters in the cut.  This
circle is a little larger
than the coil, and revolves freely round it
on a vertical axis.  A
peculiar arrangement is
required, in  order  to
transmit the battery current to the pole-changer belonging to the coil. The springs which press upon
it are connected with two small cylinders of silver,
surrounding the shaft of the magnet and insulated
from it, one being a little below the other; or the
cylindrical shaft itself may answer for one of them.
The wires proceeding from the screw-cups on the
stand press upon these cylinders. In this manner
the current is conveyed to the springs bearing upon
the pole-changer in a constant direction, notwithstanding that they are carried round with the magnet
in its revolutions.  When connection is made with
the battery, the mutual action between the coil and
the magnet causes them both to revolve, but in contrary directions; on the well-known mechanical principle, that action and reaction are always equal and




124           DAVIS'S MANUAL.
opposite to each other. On changing the direction
of the galvanic current, both the wire coil and the
magnets revolve in the opposite direction.
196. THERMO-ELECTRIC REVOLVING ARCH. - It
fig. 74.      has been shown that when
a  galvanic  current flows
through a helix, such as De
la Rive's ring, ~ 185, its faces
acquire polarity, and, if free
WA, _L~~4     Ie Eto move, arrange themselves
north and south. In Fig.
74 there is a stand, supporting an upright brass pillar
with an agate cup at the
top.  On this is balanced
by a pivot, at A, an arch of
brass wire, the two ends of
which are connected by a German silver wire encircling the pillar.
197. If the stand be arranged according to the
points of the compass, and one of the junctions of the
brass and German silver be heated by a spirit lamp
on the east side of the stand at E, a thermo-electric
current will be set in motion from the German silver,
through the heated junction, to the brass, and back,
through the arch, to the German silver. The current,
thus established, gives polarity to the faces of the
arch, as if it were a coil or helix; circulating in
such a direction, that the face, which is turned towards the north, exhibits south polarity. Since the
magnetic pole of the earth there situated is itself a




THERMO-ELECTRIC ROTATIONS. 125
south pole, similar poles are presented towards each
other, and the arch is obliged to make a semi-revolution on its axis, in order to present its northern face
to this pole. This movement brings the other junction into the flame, and a current is produced opposite
to the former one, which changes the polarity of the
arch, and obliges it to move on through another semirevolution.  Thus the currents are reversed, and
slow rotation ensues.  This is probably the most
delicate reaction between the magnetism of the earth
and a current of electricity which has been observed.
198. If the lamp be put to the south of east, the
heated junction of the arch will move round by the
south; if it be put to the north of east, the heated
junction will move round by the north; just as a
compass needle, if its north pole is made to point
south, returns to its natural position either by the
east or west, if it is inclined to the one or the other.
If the spirit lamp be placed exactly west, or at W in
the figure, the current which is excited tends to keep
the arch stationary, by causing the face which exhibits north polarity to be directed towards the south
magnetic pole of the earth.
199. THERMO-ELECTRIC REVOLVING ARCH ON UMAGNET. - If a thermo-electric arch A B (Fig. 75),
similar to the one just described, be balanced on one
of the poles of a U-magnet, the reaction between the
polarity induced in it, by heating one of its junctions,
and the magnetism of the opposite pole of the magnet, will be much more energetic than in the former
case with the earth.  It resembles, in principle, the
11*




126          DAVIS' S MANUAL.
Revolving Coil, ~ 191, except that it is attracted and
repelled by a single pole instead of two, the pole on
Fig. 75.    which it is supported having no
influence upon it. In this and
J    \    s  other instruments of the same
kind, the upper part of the arch
may, with equal advantage, be
of silver instead of brass.
_B       Ad_%  alhl 1 200. The most favorable position for the lamp is not that
represented in the figure, but at
a right angle with the line connecting the two poles, and in a
line with the pole on which the
frame is mounted; or in a situation analogous to the east side
of the stand of the last-described instrument. By varying the
lamp to one side or the other of
this position, the arch will revolve in either direction, as before. On the opposite side of the pole, the
lamp would have no tendency to produce revolution;
though, if the arch were mounted on the south pole,
the lamp should be on the farther side of the magnet, and in a line with that pole, in order to cause
rotation.
201. THERMO-ELECTRIC REVOLVING WIRE FRAMES.
— This instrument, represented in Fig. 76, consists
of two frames mounted upon the poles of a U-magnet.
These frames are formed of two arches, or rather
rectangles, similar in construction to that in the




THERMO-ELECTRIC ARCH.                127
instrument last described, crossing each other at right
angles; and they act on the same principle as that,
Fig. 76.    the second rectangle only contributing to the rotation produced by
N1/I rf ~; s s the first. In each individual rectJ LIU   Iangle the current is reversed every
G(I;'F1i' \t  G half revolution.  These are generally made of silver and platinum;
but German silver, in combination
with  brass or silver, occasions a
stronger current. The lower horizontal portions of the frames, marked G G in the cut, are composed
of German silver, and the other
parts, s s, of silver. A  frame
is usually mounted on each pole;
the attractions and repulsions of
each frame proceeding altogether from the opposite
pole. In order to heat the junctions of both frames
at once, the lamp is placed between the two poles,
by which there is a loss of attraction and repulsion
to each frame through the distance of 900, in which
the heat would act, if two lamps were employed at
right angles to the line of junction of the poles.
202. THERMO-ELECTRIC ARCH ROTATING BETWEEN
THE POLES OF A U-MAGNET. — Fig. 77 represents a
thermo-electric arch, mounted upon a brass pillar,
between the poles of a horseshoe magnet; the circular part is of German silver, and the upper part of
silver. In this case, both poles conspire in producing
revolution, the motion of the arch depending,lpon




128           DAVIS' S MANU AL.
the sante principle as that of the Revolving Coil.
Yet the different mode of reversing the current
Fig. 7 7.   in this instrument occasions the
arch to rotate in either direction
N        S    when the lamp is in front of the
magnet, and  to remain at rest
when the lamp is on the other
side. A stand to support the lamp
slides on the brass pillar, and is
fixed at any required height by
means of a binding screw.  The
lamp should be placed in the position represented in the cut, in
front of the magnet, its north pole
being on the left.
203. When either of the junctions is in the flame, a current flows
from  the German silver to the
silver, ascending by the heated
side of the arch, and descending by the other. That
face which is presented towards the north pole acquires north polarity, and the other face south polarity. The influence of the magnet now causes the
arch to turn half way round, so as to present its
southern face to the north pole.  This movement
brings the other junction into the flame; the polarity of the arch is reversed, and it moves on as
before.
204. If the lamp be placed in the corresponding
position on the other side of-the magnet, the direction of the current will be such that the southern




DOUBLE  REVOLVING  ARCH.    129
face of the arch will be presented towards the north
pole.  In this position the arch tends to remain,
returning to it when moved to either side; and
consequently no revolution can be obtained. Care
should be taken not to allow the junction to remain
so long in the flame as to melt the solder.
205. DOUBLE THERMO-ELECTRIC REVOLVING ARCH.
- In this instrument, two arches, a and b (Fig. 78),
Fig. 78.. are so mounted as to
a ftBob                 revolve between the
_ l~h   ~poles of a U-magnet
fixed in a horizontal
position.  The hori.
zontal portion of the
arch a is of German
W silver, and the upper
part of silver; while in b, the lower portion is of
silver, and the upper part of German silver. A
single lamp is so placed as to heat both arches; the
current excited in each ascends on its right side
and descends on its left side, because the heat is
applied to the right junction of a and to the left of
b. Each of them now presents a north pole towards
the north pole of the magnet, the currents circulating
in the opposite direction to that of the hands of a
watch. They consequently both revolve, either in
the same or in opposite directions. If the arches be
transposed, so that b occupies the place of a, neither
of them will move so long as the lamp is in the
position represented in the cut.




130           DAVIS'S MANUAL.
III. IN REFERENCE TO THE EARTH.
206. A magnetic needle so suspended as to move
only horizontally, assumes, when left to itself, a determinate direction with respect to the earth. Its poles
place themselves nearly in a plane passing through
the geographical poles of the earth; this direction
consequently corresponds nearly with the meridian
of the place where the observation is made. When
displaced from this position, the needle returns to it
after a series of oscillations.  This directive tendency, and its probable discovery by the Chinese,
have already been mentioned in ~ 11.
207. Fig. 79 represents a magnetic needle, so
arranged as to move freely in a horizontal direction
f. 79.           only.  The mode of
suspension is the same
as  in  the  compass
needle; the latter, however, is fixed to a circular card which of
course moves with it,
and on which the cardinal points are marked.
208. If the needle
be suspended so as to
have freedom  of motion in a vertical direction, it is found not to maintain
a horizontal position, but one of its poles (in this
hemisphere the north) inclines downwards towards




DIPPING  NEEDLE.              131
the earth. At the magnetic poles of the earth, the
direction of the needle would be vertical; but the
inclination diminishes as we recede from the poles
towards the equator, and at the magnetic equator,
which does not vary greatly from the geographical
one, the needle becomes horizontal. A needle properly prepared for exhibiting this inclination, is called
a DIPPING NEEDLE.
209. Fig. 80 represents a dipping needle whose
mode of suspension allows of its turning freely in
Fig. 80.   any direction. It is fixed, by means
of a universal joint, to a brass cap
containing an agate, which rests upon
the pivot. The usual arrangement
allows only of motion in a vertical
plane, the needle having an axis
passing through its middle at right
angles to its length, which axis is
supported horizontally. This form
is represented in Fig. 6. The small
needles in Fig. 81 are mounted in
the same manner.  Sometimes a
vertical graduated circle is added, to
measure the angle which the needle
iNX  makes with the horizon. In using a
needle whose motion is confined to
a single plane, it must be so placed
that this plane may be directed north and south,
coinciding with the plane of the magnetic meridian.
A dipping needle, before being magnetized, should
be as equally balanced as possible, so as to emain at




132          DAVIS' S MANUAL.
rest in any direction in which it may be placed; a
high degree of accuracy is, however, difficult of
attainment.
210. The dipping needle assumes, in various latitudes, the directions exhibited in the annexed diaFig. 81.         gram (Fig. 81), where
the point of the arrow
S K           indicates  the  north
pole, and the feather
the south pole of the
needles placed around
the globe.  The angle which the needle
makes with the horizon at any place is
called the dip, at that
place. The tendency
of the needle to dip is counteracted in the mariner's
and surveyor's compasses, by making the south ends
of needles intended to be used in northern latitudes
somewhat heavier than the north ends. A needle
well balanced in this latitude remains sufficiently
horizontal for use in any part of the northern temperate zone, except for delicate experiments. Compass needles intended to be employed on voyages or
travels, where great variatiorns in latitude may be expected, are provided with a small weight sliding upon
the magnet.  By a proper adjustment of this, the
greater or less tendency to dip is counterbalanced.
In passing from the northern to the southern hemisphere, the weight must be transferred from the south
to the north pole of the needle.




TERRESTRIAL  MAGNETISM.    133
211. In Fig. 81, the North American magnetic
pole is represented near S, the north geographical pole
of the earth. The line L V is nearly the present line
of no variation (see ~ 215), and the curved line near
the geographical equator is the magnetic equator,
where the dip is at zero, and the direction of the
dipping needle the same as that of the horizontal
needle.
212. By comparing the directions assumed by the
needle in its various positions with regard to the earth,
as represented in Fig. 81, with those assumed by a
magnet in reference to another magnet, as illustrated
in Fig. 47, a great similarity is found to exist between them. This led to the opinion, which was
for a long time entertained, that the earth was
itself a magnet, or that it contained within it large
magnetic bodies, under the influence of which the
magnetic needle assumed these various directions;'
as a small needle does when placed in various positions near to a bar magnet.
213. But there is another mode of accounting for
the directive tendency of the magnet in respect to
the earth; and that is, by supposing, instead of magnetized bodies within the earth, lying parallel to the
direction of the needle, currents of electricity passing
around the earth, within it, but near the surface, at
right angles with that direction. This would identify
the directive power of the needle in respect to the
earth, with its directive tendency in regard to a current of electricity, as described under the last head.
And this is, in fact, the view at present adopted.
12




134          DAVIS S MANUAL.
214. The direction of the needle in respect to the
earth is not fixed. Its variation, that is, its deviation
from the true geographical meridian, is subject to
several changes, more or less regular.  So also is the
intensity of the action exerted on it by the earth, as
shown by the number of oscillations made by it in a
given time. When examined by means of apparatus
constructed with great delicacy, the needle is found
to be seldom at rest.
215. The instrument represented in Fig. 82 is
intended to illustrate the magnetism of the earth, on
the supposition stated in ~ 212. The compound bar
Fig. 82.        magnet, n s, is placed
Ir             in the magnetic axis of
the earth, not coinciding
exactly with the axis of
rotation, N. S. A small
magnetic needle, placed
at B, on the magnetic
B  meridian, will point both
to the magnetic pole s,
and to the north pole, N,
both being in the same
line. But, if the needle
be placed at A, or any where except on the magnetic
meridian, it will point to the magnetic pole alone,
the two poles not being in the same direction. The
several magnets represented at n s are not fastened
together, but only fixed on one axis. This allows
their poles to be separated a little, to imitate more
closely the distribution of terrestrial magnetism; the




TERRESTRIAL MAGNETISM.              135
earth really having four magnetic poles, two strong
and two weak: the strongest north pole is in America, the weakest in Asia. The line of no variation
on the earth's surface differs considerably from the
magnetic meridian, and the lines of equal variation
and equal dip are not exactly meridians and parallels
of latitude to the magnetic pole. The action of the
magnetism of the earth at its surface is, therefore,
irregular.
216. The variation of the needle at any place is
found by observing the magnetic bearing of any
heavenly body whose true position at the time is
known.  It is immediately obtained by comparing
the direction of the needle with the north star when
it crosses the meridian, or by calculation when the
north star is at its greatest elongation. An observation of the sun, however, is usually preferred. The
Fig  83.       latitude of a place,
Mr           A (Fig. 83), bel~ ~ing known, the
exact bearing of.. - -- -    A-'\        the sun, S, easf
or west, can be
\  obtained by calculation,* for any
given moment of
time at that place.
If the needle at
A points to M, instead of N, the true north, the
* See Bowditch's Navigator.




136           DAVIS'S  MANUAL.
angle M A  S will be the magnetic bearing of the
sun west. Suppose this angle to be observed by the
surveyor's compass, and found equal to 760, the time
being exactly noted. The angle N A S, the true
bearing of the sun at the time, is then calculated.
Suppose it equal to 85~ 30'. The difference between
the magnetic bearing and the true bearing, represented by the angle M A. N, is the variation of the
needle, and equals 90 30(.*
217. Fig. 84 represents an instrument intended
to illustrate the theory which ascribes the magnetism
of the earth to electrical currents circulating around
it at right angles to its axis. N S is merely a wooden
axis to the globe. When a galvanic current is sent
ig. 84.
through the coil of wire about the equatorial region,
small magnetic needles, placed in different situations,
arrange themselves as they would in similar terrestrial latitudes. By comparing this figure with Fig.
* The variation at Boston, in May, 1847, was 9 deg. 30 min.
west. The dip, at the same time, was 74 deg. 20 min.




TER R ESTRIAL  MAGNETISM.           137
82, representing the globe with the included magnet,
a comparison may be made between the two theories
of magnetism. The needles arrange themselves siimilarly on both globes. With a small dipping needle,
the resemblance between its positions on either and
those assumed by it on the earth's surface is very
striking.
218. It will be observed that, in Fig. 82, the
south pole of the included magnet is represented
near the north geographical pole of the earth.  So,
also, in Fig. 84, the rod, N S, passed through the
axis of the globe, shows the direction of the polarity
induced by the current to be contrary to that of the
geographical poles.  The reason of this may be
easily understood.  The northern magnetic pole is
the one which attracts the north pole of a magnet,
and therefore must itself possess south polarity, and
not north, as its name might seem to indicate. In
Fig. 84, the battery current is considered as flowing round the globe in the same direction as the
supposed currents in the earth; that is to say, from
east to west, in the opposite direction to that of the
earth's rotation.  The apparent magnetic polarity
induced in a coil of wire by the electric current,
which occasions the needle to assume these directions, will be described under a subsequent head.
219. The aurora borealis is found to affect a
delicately-suspended magnetic needle, causing it to
vibrate constantly, but irregularly, during its continuance, and especially when the auroral beams rise to
the zenith; if the aurora is near the horizon, the
12*




138          D A V I S  MANUAL.
disturbance of the needle is very slight. When the
beams unite to form a corona, its centre is generally
in or near the magnetic meridian.'220. Within a few years, a considerable number
of magnetic observatories have been established in
various parts of the world, for the purpose of making
systematic and corresponding observations in relation
to terrestrial magnetism. At these stations the variation of the needle, and the intensity of the earth's
action upon it, are observed and recorded almost
hourly, and on stated days at intervals of a few
minutes only. These observations, made by means
of excellent instruments, and at the same time in
widely remote regions, admit of comparison with
each other, and can hardly fail to throw light on
many parts of this important and intricate subject.




MAGNETISM.
II.
INDUCTION OF MAGNETISM.
I. BY THE INFLUENCE OF A MAGNET.
221. A piece of iron, of any form, brought near to
a steel magnet, becomes itself magnetic by induction. Thus, let M (Fig. 85) be a bar magnet of
RFg. 85.         steel, and B an
iron rod brought:~  ~) ~ near to it. By the
influence of the
magnet the rod will become magnetized; the end
nearest the south pole will become north, and the
end remote from it, south. The magnetic induction
is stronger when the bar is brought in contact with
the pole of the magnet; a decided effect, however, is
produced by the mere proximity of the magnet to
the iron. That the iron bar, while under the influence of the magnet, actually possesses magnetic properties, may be shown by presenting to it some iron
filings or small nails, which will adhere to each extremity; and also by bringing near to it a small




140          DAVIS'S MANUAL.
magnetic needle balanced on a pivot, the north pole
of which will be repelled by the end of the bar nearest
to the magnet, M, and attracted by the end farthest
from M. This induced magnetism immediately disappears when the iron is removed from the vicinity
of the magnet. If a small bar of steel be substituted
for the iron bar, it acquires magnetism much less
readily, but retains it after removal; becoming, in
fact, a permanent magnet.
222. It was formerly supposed that the attractive
force of the loadstone, or any other magnet, was
exerted upon iron simply as iron; but it is now
known to be the attraction of one pole of a magnet
for the opposite pole of another magnet.  In all
cases, when a magnet is brought near to or in contact with any magnetizable bodies, as pieces of iron,
iron filings, or ferruginous sand, all such bodies,
whether large or small, become magnetized; the
part which is nearest to either pole of the magnet
acquiring a polarity opposite to that of this pole,
while the remote extremity becomes a pole of the
same name.
223. The north pole of a bar magnet, M  (Fig.
86), placed on the centre of a circular plate of iron,
induces south polarity in the part immediately beneath it, and a weak north polarity in the whole
circumference, so that it will sustain iron filings, as
shown in the cut. If an iron plate is cut into the
form of a star, as in Fig. 87, each point acquires a
stronger north polarity than the edge of the round
plate in Fig. 86, and is able to lift several iron screws




INDUCTION  OF MAGNETISM.             141
or nails; the letters in the cut indicate the position
of the poles.
ig. 86                 Fig. 87.
M
N
224. Let the north pole of a steel magnet, N S
(Fig. 88), be placed on the middle of a bar of iron.
Fig. 88.     The part of the bar in contact
with the magnet becomes a
south pole, while north polarity
is distributed along both halves
of the bar.
225. If several pieces of iron
wire, of the same length, be
Wf suspended from a magnetic pole,
the lower ends diverge from
each other, in consequence of
their all receiving the same polarity by inductioni,




142          DAVIS S MANUAL.
while the upper ends are retained in their places by
the attraction of the magnet. Let two short pieces
of iron wire be suspended by threads of equal length,
fastened to one end of each piece, so that the wires
may hang in contact. If, now, the south pole of a
magnet be placed below the wires, the lower ends
of both will become north poles, and their upper ends
south poles; and the wires will recede from each
other. This divergence increases as the magnet is
brought nearer, until it reaches a certain limit, when
the attraction of the magnet for the lower poles overpowers their mutual repulsion, and causes them to
approach each other; while the repulsion of the
upper ends remains as before.
226. The mutual repulsion of two pieces of iron
magnetized by induction may also be shown by the
Fig. 89.  arrangement represented in Fig. 89.
There are two short pieces of iron wire,
upon the ends of which are secured
little brass rings, which answer the purpose of rollers. The two wires being
placed together, as represented by the
dotted lines at A, let a U-magnet, held
in a vertical position over them, be gradB B-A   ually brought near, keeping the poles in
the direction of the length of the wires.
As the magnet approaches, the wires become charged,
the similar poles lying in the same direction. When
the magnet is brought so near as to occasion sufficient
repulsion between the induced poles, the wires recede from each other, as shown at B B, in the cut.




ATTRACTION  OF ARMATURE.   143
227. Y-ARMATURE. -This consists of a piece of
soft iron, in the shape of the letter Y. If one of the
Fig. 90.   branches of the fork be applied to the
north pole of a horseshoe magnet, as
seen in Fig. 90, the lower end of the
armature. and also the other branch
of the fork, acquire north polarity,
Dl llll   and will sustain small pieces of iron.
If both branches of the fork be applied, one to each pole of the magnet,
as shown by the dotted lines in the
cut, the polarity of the lower end immediately disappears.  This is be-'.Io. cause the two poles tend to induce
opposite polarities of equal intensity in
the extremity of the armature, which
of course neutralize each other. If the branches of
the fork are applied to the similar poles of two magnets, their influence will conspire in inducing the
same polarity in the lower end, and a greater weight
will be supported by it, than when one branch is
applied to a single pole.
228. When the two extremities of an iron bar are
acted upon at once by both poles of a magnet, there
is a double induction, and the effect is greatly increased. Thus, let N S (Fig. 91) be a compound
U-magnet, and A an iron armature, of such a length
that, while one end is applied to the north pole of the
magnet, the other extremity may be applied to the
south. In this case, both poles of the magnet act,
each inducing a polarity opposite to its own in that




144          D A v Is' S MANUAL.
extremity of the armature which is under its influence. The force with which the armature adheres
Fig. 91. is consequently greatly increased, there
being a strong attraction between each
pole of the magnet and the corresponding extremity of the armature, that is,
corresponding in position; for the polarity of the parts in contact will evidently
be of opposite denominations. If a bar
of iron is placed between the north poles
_N~ E aof two magnets, both extremities become south poles, while a north pole is
developed at the middle of the bar.
229. ROLLING ARMATURE.-This
n-g. 92.
apparatus consists of a horseshoe
magnet and an iron wire or rod,
whose length is a little greater than
the breadth of the magnet.  To
the middle of the wire a small flywheel is attached.  This armature
is placed across the magnet, at some
distance from the poles, and the
magnet held in such a position,
with the poles downward, that the
armature may roll towards them.
When it reaches the poles, the
magnetic attraction of the iron axis
prevents its falling off, while the
momentum acquired by the fly-wheel carries it forward, causing it to roll some distance up the other
side of the magnet.




M AGNET AND  ROLLER.             145
230. MAGNET AND ROLLER.- Some of the phenomena of magnetic attraction and repulsion may
be shown by the arrangement represented
in Fig. 93. The poles
of the steel magnet, N
S, are raised by a little
wooden block, and a
iece of iron, A, is placed across, near the bend. A
short iron wire, K, has a brass ring encircling each
end, which prevents actual contact between the iron
and the magnet, and thus allows the wire to roll
freely. When K is placed in contact with A, it
immediately rolls up the magnet, in opposition to
gravity, until it nearly reaches the poles.  The
actual poles or points of greatest magnetic intensity,
illn a steel magnet, are not exactly at the ends, but
a little within them. They are so near that it is
usual to give the name of poles to the extremities
themselves. Upon bringing up the armature, R, to
the poles, the wire, K, is repelled by it (see ~ 226);
and from this cause, and the diminished action of the magnet upon it, the wire rolls
back towards A. Being repelled by A, it takes
a position of equilibrium near it; and when
R is removed, again rolls up the magnet.
231. With a small magnet, whose poles
are brought near together, as represented in
Fig. 94, the attractive force is sufficient to
raise the wire up to the poles, even when
the magnlet is placed in a vertical position.
13




146           DAVIS'S MANUAL
232. Fig,. 95 illustrates the successive development of magnetism in several bars of iron. The
M    Fig. 95. a    b  c  bar a being placed
so. ~s-  o Gus —     N N la    NtS   No near to or in contact
with the north pole of a magnet, M, becomes itself
temporarily magnetic, and is able to induce magnet-.sm in a second bar, b, this again in c, and so on,
each succeeding bar being less and less strongly
magnetized. The same thing occurs with the iron
nrails represented in Fig. 87, hanging from the
points of the star. If the magnet, M, be removed
from the bar a, the magnetism of the whole series
disappears.
Fig. 96.       233. When a bar magnet is
i ^       placed in a heap of iron filings,
and then lifted up, the filings
attach themselves to the poles
in clusters.  Each particle becoming a little magnet, they
adhere together in lines, whose
length depends upon the power of the  steel magnet.  A
U-magnet, plunged into a mass
>oM  1   of tacks, sustains long lines of
them, as represented in Fig.
96.  The chains of tacks suspended from the two poles attract each other, and, becoming
c'onnected, form  curved lines,
rudely representing the magnetic curves spoken of in ~ 147.




LOADSTONE.                    147
2'34. LOADSTONE. —The  arrangement of small
brads around the poles of a loadstone, is representgig. 97         ed in Fig. 97.  In
natural magnets the
poles are not usually
-;   so distinctly marked
as in artificial ones,
the  magnetic  forces
being more irregularly
distributed.
235.  The  attraction of a magnetic pole
for a piece of iron
is not exerted upon
the whole mass, since
both a north and a
south pole are necessarily developed in the
iron. Hence only that
part of the piece which is nearest the magnet can be
attracted, while the remote end must be repelled.
If the piece of iron has any considerable length, the
end which is repelled will be at such a distance from
the influence of the magnet, that its repulsion will be
overpowered by the attraction of the extremity which
is near it. If, however, the piece is very short, so
that the repelled pole is brought near to the magnet,
the repulsion is proportionally stronger, neutralizing
the attraction to a considerable extent; and, finally,
if the iron is of such a form as to bringthe two opposite poles as near together as possible, so as to expose




148          DAVI  SS MANUAL.
them nearly equally to the influence of the magnet,
the attraction becomes scarcely perceptible. - Thus,
let M (Fig. 98) be the south pole of a bar or horseFig. 98.    shoe magnet, and A a piece of sheet
iron somewhat smaller than the
end of the magnet. When this
_I   > iron plate is placed in the position
represented in the upper figure, the
/a    /   surface next the pole of the magnet, s / acquires north polarity, while the
opposite surface becomes south,
and, the iron being thin, the two
surfaces are both so near to the
magnet that one is repelled nearly as much as the
other is attracted. The thin plate will be found to
adhere to the pole with a very slight force, and will
tend to slip down into the position represented in
the lower figure. In this position it is much more
strongly attracted; for the two ends, instead of the
two surfaces, become the poles, and the end in contact is attracted, and the remote end repelled. The
same effect will be produced if the plate is applied
to the pole of the magnet by its edge, instead of
by one of its surfaces; by this means, the repelled
pole of the plate is removed to a distance from the
magnet.
236. The inductive action of a magnet is not impeded by the interposition of any unmagnetizable
body whatever. If a plate of glass be placed between the magnet and a piece of iron, the iron is
attracted as strongly as it would be at the same




ATTRACTIVE  FORCE.                 149
distance with no glass interposed. Fig. 99 represents a piece of iron wire, sealed up hermetically
Fig. 99.  in a glass tube.  When this is applied
to a U-magnet, the iron is attracted with
sufficient force to hold the tube in contact with the poles.  The polarity induced in the iron is precisely as great
as when only air is interposed.  The
INT    s   result is the same when the iron is enclosed in a tube of brass, or any metal
not susceptible of magnetism.
237. Fig. 100 represents an instrument designed
to measure the force of attraction between a Umagnet and its armature at different distances.  It
Fig. 100.
is on the principle of the steelyard.  The beam,
which is of brass, is supported by steel knife edges,
resting in curved supports of the same metal. To
the shorter arm of the beam is secured the armature,
consisting of a small cylindrical bar of iron.  The
force with which this is held down;s measured by
13*




150          DAV ISS MANUAL.
weights hung upon the graduated longer arm. The
magnet is held firmly against a vertical brass plate,
by means of a clamp of the same metal, which allows
the poles to be fixed at any required distance from
the armature.  The distance was determined in the
following way: Sheet brass of 240 of an inch in
thickness was taken, and several series of different
thickness formed by pressing strips of it firmly together and soldering them at the edges. By interposing these strips between the magnet and armature,
they were maintained at any desired distance, from a
single thickness, or 240 of an inch, up to ten thicknesses, or -  of an inch
238. The following tables were constructed from
experiments with two steel magnets, of different
lengths, and a long electro-magnet. The first magnet employed was a very short one, formed from a
steel bar, 2 inch square. It was 1- inches long, and
12 inches between the poles. The first column in
the table gives the number of brass strips interposed,
the magnet and armature being at first in contact.
The second column gives the weight sustained, or
the force of attraction, in grains; and the third, the
product obtained by multiplying the weight in each
case by the number of interposed strips. There is
a very slight spring in the brass, which diminishes
as the number of strips is increased.  In consequence of this, the armature is removed slightly
beyond the calculated distance, and the weights in
the first part of the table fall a little below the true
proportion.




ATTRACTIVE  FORCE.              151
TABLE I.
Distance.  Weight in Grains.   Product.
0...... 21,200
1......  4,000...   4,000
2......  2,170...... 4,340
3......   1,500....  4,500
4......  1,220....  4,880
5.....    970.... 4,850
6.....    810....   4,860
7......    670....  4,620
8......   595....   4,760
9......   500...    4,500
10......    445...... 4,450
239. The second steel magnet was of the same
thickness and width, but 11 inches long. The results obtained with this are given in Table II.
TABLE II.
Distance.  Weight in Grains.   Product
0....... 62,500
1.....32,000.... 32,000
2...... 17,300....   34,600
3.....  13,100...... 39,300
4....      9,600......38,400
S......  7,800..... 39,000
6...... 6,800... 40,800
7......  5,600... 39,200
8.....  4,700......  37,600
9......  4,050....        36,450
10......  3,600......  36,000




152           DAVIS' S MANUAL.
240. The third series of experiments was made
with an electro-magnet, 91 inches long, formed from
a round bar, 1 inch thick. Its poles were 11 inches
apart.  The results, with one pair of Smee's battery,
are given in the following table: —
TABLE III.
Distance.  Weight in Grains.    Product.
0.....   82,000
1.... 35,000......  35,000
2.....   25,000.... 50,000
3......20,000...... 60,000
4......  15,500...... 62,000......  12,100...... 60,500
6......   11,300...... 67,800
7......  9,300......  65,100
8......  7,400......    59,200
9......  6,500......  58,500
10...... 5,500......55,000
241. It will be seen that the numbers in the third
column of Tables I. and II. agree sufficiently well to
give the following practical rule for finding the relative attractive force at different distances; that, with
steel magnets of any length, the force with which
the armature is attracted varies in the inverse ratio
of the distance. The third column in Table III.
shows that the electro-magnet does not follow this
proportion. If the points of strongest polarity are
regarded as situated a little within the ends of the
magnet, for instance, at jl of an inch, or the thick



ATTRACTIVE  FORCE.              153
ness of five strips of brass, in the above tables, the
distances calculated from these will give very nearly
the true law, as stated in ~ 142. A still closer
approximation might probably be made.
242. Table IV. gives the comparative attractive
force of the long steel magnet, with the armature in
contact, and when separated by a single thickness of
brass, the charge of the magnet being progressively
reduced. In the first column are given the weights
sustained in grains, the armature being in contact
with the poles; in the second, the corresponding
weights when their distance was,~0 of an inch.
TABLE IV.
In Contact.         Distance, 1
62,500......... 32,000
52,500......... 28,000
33,500......... 10,900
6,60......  2,280
243. The same instrument was used to measure
the force of attraction between the opposite poles of
two bar magnets. These were made from the same
square bar of steel, and were each 6 inches long and': of an inch thick. One of them being suspended
vertically from the shorter arm of the brass beam,
the other was secured to the brass plate perpendicularly below the first. The poles were placed in
contact in the first experiment, and afterwards separated to different distances from 2~~ to a of an inch,
by strips of brass, the lower magnet being moved




154            DAVIS   S MANUAL.
sufficiently to keep the beam horizontal.  Beyond
- of an inch, the instrument was not delicate enough
to give the weight with sufficient accuracy.
244. In Table V. are given the results of this
series of experiments.
TABLE V.'Distance.              Weight in Grains
0.............6,000
1............          3,100
2............. 2,200
3.1........1,900
4........... 1,600
5............. 1,300
6.............1,200
7............   1,100
8.980
9............  890
10.............  770
In the first experiment in the table, the poles
touched by their whole surface.  By moving the
lower magnet laterally until the edges only of the
poles were in contact, the force of attraction was
considerably increased, being equal to 10,000 grs.
245. By considering the points of strongest polarity, or the true poles, as situated at a distance of four
and a half strips of brass within the ends of each
magnet, the results given in Table V. are made to
agree closely with the law of the inverse ratio of the
square of the distance.




ATTRACTIVE  FORCE.               155
246. The following table gives the attractive force
between one of the bar magnets and a straight bar
of iron, 6 inches long —
TABLE VI.
Distance.              Weight in Grains.
0.5-6... 5,600
1.............,480
2............  870
3............  670
4............  520
-............. 410
6..~........    360
7..........-.  300
8.......   270
9............  240
t0...........           200
These numbers may be brought under the law
stated in ~ 142, by calculating the distances from a
point is of an inch within the steel magnet. In the
iron bar, as in the armatures of the U-magnets, the
poles are probably situated at the surface.
247. A close analogy exists between the phenomena of magnetism and of statical electricity in
many important points. But in some respects it
fails. Electricity, whether positive or negative, can
be transferred from one body to another, so that a body
may be charged with an excess of electricity of either
kind. It is not so with magnetism. Every magnet
possesses both polarities to an equal degree, though
they may be diffused through portions of its mass of




156          DAVIS S MANUAL.
different extent. A long conductor, exposed to the
inductive influence of an electrified body, has opposite electricities developed at its ends. If, now, it be
divided in the middle, we obtain the two electricities
separate; one half of the conductor possessing an
excess of positive, the other of negative electricity.
The condition of a magnet in regard to the distribution of its polarities appears to be exactly analogous
to that of the conductor; the north polarity seeming
to be collected in one half of its length, and the south
in the other. We might, therefore, expect that, by
breaking the magnet in halves, we should obtain the
two polarities separate, one in each portion of the
bar. But such is not the case; each half at once
becomes a perfect magnet. In Fig. 101, which represents a fractured magnet, the original north pole
Fig. 101.
still remains north, but the broken end of the piece
has acquired a south pole. The converse occurs
with respect to the other portion in which the south
pole was situated. These halves may be again broken
with the same result; and, in fact, into however small
fragments a magnet may be subdivided, each will
possess a north and a south pole.
248. Suspend a piece of iron from one pole of a
magnet, and bring up to this pole the opposite pole
of another magnet.  The iron immediately falls;
the poles, when in contact, neutralizing each other,




ELEMENTARY  MAGNETS.              157
and corresponding to the middle or neutral portion of
a magnet. If the piece of iron is nearly as heavy as
the pole can sustain, it falls on the mere approach of
the other magnet to the pole, and before it touches it.
249. In order to explain the distribution of the
magnetic forces and the results of fracture, it is necessary to regard a magnet as made up of minute,
indivisible particles of steel, each of which possesses
the properties of a separate magnet. Fig. 102 is inFig. 102.   tended to give an idea of the mode
of arrangement of these elementary
magnets; they are, of course, so
minute as to be beyond the reach
of the most powerful microscopes.
All their poles lie in the same
direction, as indicated by the light
and shaded portions in the cut; the. light half of each little magnet representing its north pole, and the
shaded half its south pole. These
s        N   poles are distributed through the
whole length of the bar; but in consequence of their
mutual reactions, the resultant polarities are strongest
near the ends of the bar, becoming more and more
completely neutralized towards the middle.
250. Artificial magnets were formerly made by
induction from strong magnets previously prepared;
the original source of the power being natural magnets, or the magnetism  of the earth.  When this
was the case, it became important to ascertain what
arrangements, and what modes of applying a magnet
14




158          DAVIS'S MANUAL.
to a bar or needle, were most efficacious in communicating or developing the magnetic power; and,
accordingly, various and complicated arrangements
and manipulations for this purpose, are detailed in
old treatises on the science. Recently, other and far
more powerful means have been discovered for magnetizing bars of iron or steel, by the aid of electromagnetism; so that the old methods have been, in a
great measure, superseded. The induction of magnetism by means of steel magnets is now only
employed for needles or small bars.
251. It may, however, be convenient to know a
good process for magnetizing (or touching, as it is
technically called) with steel magnets. One of the
simplest and best for a small straight bar is the following: Place the middle of the bar on one of the
poles of either a straight or a U-magnet, and draw
one end of it over the pole a number of times; the
direction of the motion being always from the middle
to the end. Then turn the bar in the hand, and pass
the other half over the other pole of the magnet in
the same way. If the bar is thick, the process may
be repeated with its different sides. The end which
has been drawn over the south pole of the magnet
will now possess north polarity, and the other extremity south polarity. A U-magnet is more readily
charged by drawing over it the poles of a steel Umagnet, of corresponding width, in the same manner
as in the process, which will be described farther on,
for touching with electro-magnets.
252. The magnet which is used to induce mag



ELECTRO-MAGNETIC  INDUCTION. 159
netism loses none of its own power in the process, but
often receives a permanent increase by the reaction
of the polarities it has induced upon its own.  A
magnet, whose armature is kept constantly applied,
not only retains its power without loss, but acquires
a further increase up to a certain limit. That a
magnet possesses greater power while exerting its
inductive action, may be shown by suspending from
one pole of a bar magnet as much iron as it can
hold. If, now, a bar of iron be applied to the other
pole, the first will be found capable of sustaining
a greater weight than before.
II. BY THE INFLUENCE OF A CURRENT OF
ELECTRICITY.
253. It has already been stated, under the head
of the directive tendency of a magnet in reference to
a current of electricity, that a magnetized body,
freely suspended within the influence of such a current, tends to assume such a position that the line
joining its poles may be at right angles to it. It is
also found, that, if any magnetizable body be placed
in the vicinity of an electrical current, it acquires
magnetism  by its influence.  This phenomenon is
termed electro-magnetic induction. The present chapter, and the one referred to above, form together
the department of electro-magnetism.
254. In Fig. 103 is given a sectional view of a
series of four pairs of Grove's battery, whose poles
are connected by a copper wire, in which the current




160           D AVIS' V I S  MANUAL.
is flowing in the direction of the arrow. A small
iron rod, or short piece of iron wire, placed vertically
in front of the horizontal portion of the copper wire,
becomes instantly a magnet, with its north pole
below the wire. The direction of the induced po.
larity is reversed by transferring the iron rod to the
other side of the wire. The rod being placed in a
horizontal position above the wire, its north pole is
turned towards the observer; if below the wire, it is
directed from him. By carrying the rod around the
F-. 103.
wire, keeping it transverse, it will be seen that the
induced poles retain their relative position with regard to the current.  As the rod is carried round,
the end which was a north pole in the position
shown in the cut, remains north in every part of the
revolution.  The rod attracts iron filings, and acts
upon a delicate- magnetic needle, brought near it,
like a steel magnet; but its polarity instantly disappears, when it is removed from the influence of
the current. The induction diminishes as the rod is
removed farther from the wire; but it is not interfered with by -the interposition of any bodies not
susceptible of magnetism.




DIRECTION  OF  POLES.             161
255. A sewing needle, placed transversely across
the wire, becomes magnetic in the same way, and
retains a portion of its power when removed. If
placed parallel to the wire, it acquires feeble polarity
on its opposite sides, instead of in the direction of its
length, and probably will not retain it after removal;
it being very difficult to maintain this transverse
distribution of magnetism in magnets whose length
considerably exceeds their diameter.
256. Though the relation between the electric
current and the direction of the polarity which it
induces is fixed and determinate, yet it is very difficult to express.  The same law which has been
stated, in ~ 153, as governing the motion of a magnetic pole brought near the current, regulates the
direction of the induced magnetism.  It is convenient to reduce this to a rule, by which the position of the poles may be readily found in all cases.
257. The following mode of fixing the rule in
the memory, is perhaps the best that has been contrived.  First, it is more natural to fix our attention
on the current of positive than of negative electricity.
Secondly, in a vertical wire, a descending current
will occur to us more readily than an ascending one;
or, if we imagine ourselves borne along by the current, it would be more natural to conceive ourselves
moving with our feet foremost; but if, on the contrary, we suppose ourselves to be at rest, we should
conceive the current to be passing from our head
to our feet. Our face would, of course, be turned
towards the body to be magnetized; we should
14*




162           DAVIS' S MANUAL.
attend to the north pole in preference to the south;
and to our right hand rather than to our left. Combining these conditions, then, we may always recollect, that, if we conceive ourselves lying in the
direction of the current, the stream of positive electricity flowing through our head towards our feet,
with the bar to be magnetized before us, the north
pole of that bar will always be towards our right
hand. If any one of these conditions be reversed,
the result is reversed likewise.
258. Though a single magnetic pole is neither
attracted nor repelled by a conducting wire (~ 152),
the tangential action of the current upon both poles
of a needle, situated as in Fig. 103, causes it to ap
proach the wire. This result may be shown by
placing the conducting wire in a vertical position,
and presenting to it a needle suspended in a horizontal position by a thread.  When the needle is removed to some distance, or when very short, the
attraction becomes insensible. From the feeble magnetizing power of the wire, this experiment is best
performed with a needle previously charged.  If
the needle is brought up to the wir6 with its poles
in the reverse direction to those of the iron rod in
Fig. 104, or if, without changing their direction, it is
carried to the other side of the wire, it is repelled by
the combined tangential forces.
259. Fig. 104 illustrates the action of the forces
in producing attraction and repulsion.  W  represents a horizontal section of the conducting wire, in
which the current is ascending; and N S a magnetic




TANGENTIAL  FORCES.               163
needle, whose poles are at equal distances from the
wire. From W, as a centre, a circle is drawn,
Fig. 104.       passing through the poles.
The forces which move
the magnet are tangents
to this circle, and their
directions are indicated by
ow           the arrows.  It will be
seen that the resultant of
the forces, acting on each
_N'            f,   pole, urges the centre of
the magnet towards the
wire. If the magnet be transferred to the position
N' S', on the other side of the wire, its centre is urged
away from it. The force increases in proportion as
the magnet is nearer to the wire.
260. When the poles are at different distances
from the wire, the resultant of the tangential forces
Fig. 105.          moves the magnet
OW'                   obliquely  towards
the wire, until its
/ centre comes into
contact with it.  If
~-    _f -          there are two cur\ rents moving in
opposite directions,
one on each side of
the magnetized bar, and at equal distances from it,
their combined action urges the bar forward until
its centre comes into the same line with the wires.
In.Fig. 105, let W be a horizontal section of a wire,




164              A VIS' S   MANU AL.
in which the current is ascending, and W' one in
which it is descending.  The four small arrows
indicate the tangents to the circles, drawn as in Fig
104, from W and W' as centres. At the south pole
of the bar, the direction of the forces is nearly opposite, and they neutralize each other in a great degree:
at the north pole, they approach to parallelism, and
that pole is urged forward by a force nearly equal to
the sum of the two.  As this force is opposed only
by the feeble one at the south pole, the bar moves in
the direction indicated by the arrow below. When
the magnet is not equidistant from the wires, it no
longer moves in the direction of its length, as in the
cut, but its centre is drawn towards the nearest wire,
with a force which is accelerated as it approaches.
261. A short copper wire, connecting the poles of
a battery, will sustain iron filings, as represented in
Fig. 106. The lines of filings have not that brisFz1g. 106.      tling, divergent arrangement
which they exhibit under
*    the influence of a steel magnet, but adhere equally ali
around the circumference of
the wire; forming circular
bands, the particles of which
mutually cohere in consequence of each particle becoming a magnet with its
poles transverse to the wire.
The influence is equal at
every part of the length of the wire; hence these




HELIX, ON  STAND.             165
transverse bands, lying in contact with each other,
present the appearance of a closely-compacted layer.
Whatever form the metal conducting the electricity
may have, the filings will always arrange themselves
in lines encircling it at right angles to the course of
the current. The iron filings fall off when the current ceases to flow; but if steel filings be employed,
part of them remain attached, in consequence of the
adhesion of the magnetized particles among themselves. A short wire is better for this experiment
than one of considerable length; and a battery of
some power, such as a series of a few pairs of
Grove's, may be used with advantage.
262. HELIX, ON STAND. - The magnetizing power
of the wire is very greatly increased by coiling it in
Fg. 107.
the manner of'a corkscrew, so as to form a hollow
cylinder, into which the body to be magnetized can
be inserted.  Such a coil is denominated a helix,




166          DAV I SS MANUAL.
and is represented in Fig. 107, mounted upon a
stand. A single circular turn is more efficient than
the straight wire, and each turn adds to the power
within a certain limit, whether the whole forms a
single layer, or whether each successive turn encloses
the previous one in the manner of a spiral. When a
helix of great power is required, it is composed of
several layers of wire. The wire forming the coil
is insulated by being wound with cotton, to prevent
any lateral passage of the current.
263. Place a bar of soft iron within the coil, and
connect it with the battery by means of the two
cups attached to the stand. The two extremities of
the bar instantly become strongly magnetic, as will
be seen by bringing a key, or other piece of iron, in
contact with them. On separating one of the wires
communicating with the battery, the magnetic power
of the iron bar will be immediately destroyed, and
the key will drop. If iron filings, or small nails, are
held near one of the extremities of the iron, they are
taken up and dropped alternately, as the connection
with the battery is made or broken. By bringing a
magnetic needle near the two extremities of the bar,
in succession, one of them will be found to have
north and the other south polarity, and they will
attract and repel the poles of the needle accordingly.
A bar, temporarily magnetized by an electric current, is called an electro-magnet.
264. The following rule indicates the extremity
at which the north pole will be found. If the helix
l'e placed before the observer with one of its ends




ACTION  OF  HELIX.               167
towards him, and the current of electricity, in passing
from the positive to the negative pole of the battery,
circulates in the coil in a direction similar to that of
the hands of a watch, or the threads of a common
screw, then the north pole will be from the observer
and the south pole towards him. If it passes round
in the contrary direction, the poles will be reversed.
Or the formula may be stated thus: The north pole
will be at the farthest end of the heli.  when the
current circulates in the direction of the hands of a
watch.  By comparing this rule with that given in
~ 257, for a straight wire, it will be seen that it is
directly deducible from it.
265. Place a steel bar, instead of an iron one,
within the helix. It acquires polarity somewhat less
readily, but the polarity will continue after the connection with the battery is broken. Any small rods
or bars of steel, needles, &c., may be made permanent magnets in this way.  Bars of iron, or steel,
brought near the outside of the helix, obtain polarity in a much feebler degree. An iron tube does
not become perceptibly magnetic when a current is
passed through a helix placed within it, though it
becomes strongly so when enclosed in a helix sufficiently large to admit it.  If two soft iron bars are
inserted in the helix, at the opposite ends, in such a
manner as to have their extremities in contact in the
middle of the helix, they adhere with more force than
when one is within and the other not.
266. Place the helix with its axis vertical, and a
small rod of iron or steel within it.  If it be now




168           DAVIS S MANUAL.
connected with the battery, it may be raised from
the table without the bar falling out; the tendency
of the helix to keep the bar within it overpowering
its gravitation. A small steel bar, merely allowed to
fall through the helix, while the current is flowing,
acquires a considerable degree of magnetism.
267. If a needle, or a small bar of steel, previously
magnetized, is placed in the helix, in such a position
as to bring its north pole within the south pole of
the helix, as indicated by the rule in ~ 264, its poles
will be reversed, the end which was previously
north becoming a south pole. When of very hard
steel, and too large in proportion to the power of the
coil, its magnetism may merely be diminished.
268. A small magnetic needle, suspended by a
thread near the helix, will be drawn within it, its
north pole entering the south end of the helix, or its
south pole the north end. If the magnet be placed
within the helix, in a contrary direction, its north
pole entering the north end, it will be repelled, and
then, revolving without the helix, will return and
enter by the other pole. This effect will take place,
unless the electro-magnetic power of the coil is sufficient to reverse the poles. When the needle has
entered with its poles corresponding in direction with
those of the helix' the action of the coil tends to
keep it in the middle of its length, though not in
the line of its axis.
269. A magnetized needle, placed exactly in the
axis of a vertical helix, would retain this situation,
if undisturbed, being acted upon equally by the




HELIACAL  RING.                 169
forces around it. But it is a position of unstable
equilibrium, and if displaced from it in the slightest
degree, the needle is drawn towards that side of the
coil which happens to be nearest.
270. Let a short helix, such as is described in
~ 271, be laid on a table, with its axis vertical, and a
short and strongly magnetized needle of very hard
steel be introduced into it with its poles in the reverse direction to those of the coil.  If its polarity is
not reversed by the power of the current, it will be
found to stand erect in the axis of the helix, being
repelled equally from every side, instead of being
attracted to every side, as is stated in ~ 269 to occur
when its poles correspond in direction with those
of the coil.
271. HELIACAL RING. - This is a short helix, consisting of several layers of wire, and with a large
Fig.  108.   central opening.  It is represented at C, in Fig. 108.  The
ends of the wire are left free,
in order to be  inserted  into
the cups of the battery.  At
I C      -D are shown two semicircular
pieces of soft iron, provided
with handles for pulling.  The
handles are attached to the
semicircles, by ball and socket joints, to prevent
them from being twisted or wrenched by irregular
pulling. When connection is made with ti.e battery, and the semicircles are passed within the coil,
as shown in the cut, they adhere with very considerable force.  The induction of magnetism  in these
15




170            D AVIS S  MANUAL.
semicircles, by means of the current from a thermoelectric battery, has been mentioned in ~ 129.
272. With thicker semicircles, such as are repreFg. 109.          sented in Fig. 109, the
induced magnetism  is
sufficient to  support
a hundred weight or
more, even  with  a
small battery.  The
mutual attraction between the semicircles
when near each other,
but not in actual contact, is comparatively
very feeble.  If the
flow  of the current
in the coil is stopped
while they are applied
to each other, they still
continue firmly attached; but if once separated, will
not adhere again.
273. A heliacal ring, with a smaller central opening, will sustain a large mass of tacks or brads while
the current is flowing through it, as shown in Fig.
110. They adhere together in rings, surrounding
each portion of the circumference, as the filings
enclose the single wire, in Fig. 106. The poles of
a magnetic needle brought near the coil are attracted
and repelled as if its axis were a magnet.  An
iron bar becomes strongly magnetic when passed
half way through it. In ~ 185 is described the
polarity exhibited by a coil when it has freedom of




HELIACAL RING.                 171
motion.  The polarity of the helix bears so close a
resemblance to that of a magnet, that it is usual to
Fibg. 110.
speak of it as magnetic, and to give the names of
north and south poles to the extremities of its axis.
274. In Fig. 111 are represented, at a a, two
double cylinders of iron, enclosing a coil. A sectg. 111.     tional view of one of them is
given separately at A. It contains a cavity in the form of a
hollow cylinder, adapted to rea  ceive one half of the coil seen
at C, the remaining half passing  into the other cylinder
e   when they are fitted together.
A longitudinal opening on one
side of the cylinders allows
the wires of the coil to pass
out.  In this arrangement, the
- C                 cylinders adhere with great
force when in contact; but as
soon as the current ceases,
their magnetism instantly dis



172           DAVIs' S MANUAL.
appears, and the adhesion does not continue as with
the semicircles. When one of the cylinders is passed
over the coil, the part exterior to the coil exhibits no
sensible polarity.
275. RIBBON SPIRAL.- Fig. 112 represents a
strip of sheet copper, coiled into a spiral.  This
s   instrument is described here. 12.  in consequence of its possessing considerable magnetizing
N        power, though its principal
uses will not be mentioned
till the  subject of electrodynamic induction comes under consideration, in book
III, chapter I. The copper ribbon may be an inch
wide and one hundred feet long, the strips being cut
from a sheet, and soldered together. Being then
wound with strips of thin cotton cloth, it is coiled
upon itself, like the mainspring of a watch; instead
of covering it with cotton, it may be coiled with a
strip either of cotton or list intervening.  Two
screw-cups are soldered to the ends of the ribbon;
the internal end, for convenience, is brought from
the centre, underneath the spiral, to its outside, care
being taken to insure insulation where it passes the
edges of the ribbon. The whole may be firmly cemented together, if desired, by a solution of shellac
in alcohol.
276. The spiral being connected with the battery, its two faces exhibit strong polarity: a dipping
needle, placed on any part of its surface, or near it,
will always direct one of its poles towards the centre,




MAGNETIZING  HELIX.               173
as seen in the cut, where a dipping needle, N S, is
represented on the spiral.  On reversing the battery
current, the other pole of the needle will turn towards the centre.  If the spiral is fixed in a vertical
position, a horizontal magnetic needle may be used
with the same result.  When brought near to one
side of the coil, it will be found to direct its north
pole constantly towards the centre; when on the
other side, its south pole.  When either the horizontal or dipping needle is placed near the outside,
with its centre of motion in the same plane as the
spiral, neither pole will be directed towards the
centre, but the magnet will place itself at right
angles to the plane of the spiral.
277. The magnetizing power of the spiral may
be shown by connecting it with the battery, and
placing a rod of iron or steel in the central-opening,
or upon it in the direction of a radius, when the iron
becomes temporarily magnetic, and the steel permanently so. If the bar, when laid upon the coil,
extends across the central opening, both ends will
become similar poles, and the part over the centre,
a pole of the opposite denomination.
278. MAGNETIZING HELIX. —For the purpose of
experimenting on the magnetic power developed by
the electric current in small bars of iron or steel, a
helix of the form represented in Fig. 113, is well
adapted. It consists of a number of layers of wire,
and has a small central opening. An iron bar passed
within it, becomes strongly magnetic. When the
coil is in a vertical position, the iron bar is sustained
15*




174          DAVIS' S MANUAL.
within it in consequence of the force with which it is
drawn towards the middle of the coil. With a large
Fig. 113.   battery, a considerable weight may
be suspended from the bar, as represented in the cut, and the whole
will be sustained without any visible support. The helix represented
in Fig. 107, will lift a smaller rod in
the same manner.
279. The action of the coil is the
same, except in amount, as that of a
single circular turn of the wire. At
any two points of the circle diametrically opposite, the directions of
the current are also opposite. Such
points may be represented by the
sections of the wires, W and W', in
Fig. 105, the action being precisely
the same as explained with reference to them. The resultant of the forces exerted
by all the points, tends to bring the centre of the
magnetized bar within the circle. The action of all
the circles of which the helix is composed, draws
the bar into it until its middle lies within the middle of the helix, in which position only can the forces
neutralize each other. The terms axial force and
axial motion are used to designate this peculiar force
and the motion occasioned by it.
280. AXIAL BELL ENGINE. - Fig. 114 represents
an instrument in which the axial force of a vertical helix raises an iron rod whose centre is below




AXIAL  REVOLVING  BAR.             175
that of the coil. This motion is communicated to
a hammer, which strikes a bell. A wire, fixed to
the iron rod below,
Fig. 114.
rests on a spring near
B.  When the rod is
lifted by the current
in the coil, C, its
upper end raises the
handle of the hammer. A guiding wire,
R, keeps the rod vertical.  The circuit is
I3~g~S~ ~    - Dbroken at B, by the
lifting of the handle,,i:,~,Ilmltm~!i~ and is renewed when
the rod falls.
281. AxIAL RErVOLVING BAR. -In the instrument
represented in Fig. 115, an iron bar is suspended
without visible support by the action of a current
flowing in the helix which surrounds its upper half,
the bearings above and below serving only as guides.
The bar is made to rotate by transmitting the battery current through either half. This mode of
suspending the rotating bar was first suggested by
Dr. Boynton.  The screw-cup A, on the stand, connects with the brass pillar, and thence with one end
of the coil. The other end of the coil dips into mercury, contained in a circular cistern of ivory. This
cistern is supported below the helix by an arm attached to the pillar, and has an opening in its centre
to allow the bar to pass through. A bent wire, pro



176          D AV I S'S MANUAL.
jecting from the middle of the bar, conveys the current from the mercury through its upper half to the
Fig. 115.         cup  B.  Some  nonconductor of electricity, interposed at I, insulates B from the arm
which supports it.  If
A and B are connected with  the battery,
and the cistern contains
mercury, the  current
traverses in succession
the helix and the up-fi  t b  2  per half of the bar.
Since the bar becomes
an electro-magnet by
the influence of the coil, it rotates rapidly on its
axis, in the same manner as the magnet described in
~ 167. The cup C connects with the lower end of
the bar, and, by uniting A and C with the battery,
the current traverses the lower half, also producing
rotation.
282. In Silliman's Journal for March, 1847, will be
found a figure and description of a similar instrument,
contrived by Dr. Page, in which no mercurial connections are used. The bar is sustained without
visible support, as in Fig. 115, and the current is
conveyed by a wire tipped with silver, which plays
upon a silver ferule on the middle of the bar.
283. INCLINED  REVOLVING  BAR.- This instrument, represented in Fig. 116, is designed to show




INCLINED  REVOLVING  BAR.    177
the change produced in the direction of rotation by
altering the course of the current. The helix, which
surrounds the lower end
of the bar, is represented
in section, in order to show
a wide glass tube within,
which can be filled with
mercury.  The iron bar
rests in an agate cup at
the bottom  of the tube.
Its upper end connects
with a brass cup on an
ivory arm proceeding from
the upright brass pillar. This cup may be'united by
a bent wire with a similar one on the pillar, as represented in the cut. When the screw-cups on the
base board are connected with a battery, the current
passes from one of the cups up the pillar and along
the bent wire at the top; thence it passes down
the inclined bar. A little mercury in the glass tube
conveys the current to the brass at the bottom of the
tube. It then passes through the helix to the other
screw-cup. The bar now rotates on its axis; a circular card attached to it renders the motion perceptible at some distance.
284. The course of the current is changed by removing the wire at the top of the pillar, and, having
filled the glass tube with mercury, dipping into it
another bent wire, fixed by a binding-screw  to the
middle of the pillar.  The inclined position of the
bar is adopted to prevent this wire from accidentally




178           DAVIS' S MANUAL.
obstructing its movement.  Only the lower part of the
bar is now traversed by the current, which divides
itself between that and the mercury. A rotation of
the bar in the opposite direction is thus obtained.
285. INCLINED BAR REVOLVING ROUND CONDUCTOR. -The instrument represented in Fig. 117 is
MFg. 117.        similar to the one last
described, except that the
inclined bar of iron revolves round a vertical
copper wire, which conveys the current from the
mercury to the pillar, with
which it is connected by
a brass arm.  The iron
bar rotates on its axis in
the opposite direction to
that of its revolution around the wire.  A revolution
of the mercury may also be observed in the same
direction as that of the iron around the wire. The
glass tube requires to be nearly full of mercury to
produce contact with the vertical conducting wire,
which enters a short distance only within it. An
ivory ring, surrounding the wire, insulates the little
arm by which the bar is sustained in its inclined
position, so that the current does not traverse the
upper part of the bar. When enough of the mercury
is removed to interrupt contact between itself and
the vertical wire, and the bent wire is inserted so as
to convey the current from the middle of the pillar,
the iron rod changes the direction of its motion.




AXIAL  REVOLVING  CIRCLE.    179
286. A short iron rod, placed in the glass tube
instead of the inclined bar, floats in the mercury.
When the current is passed through the vertical
wire, the rod is drawn down into the tube, and revolves around the wire and on its axis.  By covering the rod with sealing-wax, it no longer conveys
any part of the current, but is carried round by the
rotation of the mercury. If the vertical wire is so
arranged as to have freedom of motion, it revolves
in the same direction as the iron bar, the two keeping
opposite, and moving round a common centre.
287. AXIAL REVOLVING CIRCLE. —BY employing
two curved helices, the axial motion of an iron bar
Fig. 118.         may be converted
into a direct rotation. In Fig. 118
is seen a revolving circle, composed
of two semicircular
bars, the shaded one
of iron, the other
of brass. This circle is supported by
four grooved  friction rollers, so as to move freely within the two
coils. Its circumference is cut into teeth, and its motion is communicated to two smaller toothed wheels,
on the axis of one of which is the break-piece.
When the current passes, one of the helices becomes charged, and draws the iron bar within it.
As soon as its centre reaches that of the coil, the




180          D A V I SS  MANUAL.
current is interrupted; and, as it moves on by its
acquired momentum, the action of the break-piece
causes the other helix to be charged, and the iron
to be drawn into it.
288. VIBRATING HELIX. -Instead of a oar moving
within the helix, the latter may be made to move
Fg. 119.         over the bar. In the
instrument represented
in Fig. 119, there is
a curved bar of iron
of the U form, and the
helix is so suspended
as to pass along one
of its poles.  When....r             Ithe coil hangs freely,
one of its ends dips
into a mercury cup on
the brass pillar.  The
action of the current makes the iron an electromagnet, and the helix passes along over the pole.
This motion lifts the wire out of the mercury, which
breaks the circuit, and the helix falls back. The
instrument is similar in principle to the one described
in ~ 187, except that an electro-magnet is employed
instead of a steel magnet.
289. In place of a straight iron bar moving within a helix, a bar having the form of the letter U
may be employed with two coils placed side by side.
In Fig. 120, C C are the two helices, secured together
by a brass clamp, not represented; and M is the bent
iron bar. Suppose the bar to be resting on a table




Dn o iTBE  AXIAXl  BELL   N  INE.  18S1
in the position indicated by the dotted lines in the
cut, with its poles within the double helix, which is
Fig. 120.
raised a little from  the table.  When the battery
connections are made, the bar instantly rises until its
bend reaches the helices.  The movement is more
powerful than that of a straight bar.
290. DOUBLE AXIAL BELL ENGINE.- In the instrument represented in Fig. 121, motion is produced
Fig. 121.         in the manner just
described. The force
with which the bar
is drawn up is increased by its attraction for a straight
armature fixed above
the coils.  The bent
bar, becoming an electro-magnet, moves
towards the armature,
as the latter would
do, if free to move; towards a fixed magnet.  Near
B is a break-piece, so arranged that the current is
interrupted when the bend of the bar reaches the
helices, causing it to fall back. As the electro-magnet rises, it communicates motion to a hammer which
strikes the bell.
16




182           DAVIS'S MIIANUAL.
291. UPRIGHT AXIAL ENGINE. -111 the instrument
represented in Fig. 122, the alternating movement
of two bent bars is conveyed by cranks to a
balance-wheel above the
helices.  As the wheel
revolves, the current is
conveyed to each double
helix in succession, by
means of a break-piece
on its axis. This is sinmilar in construction to
/ i                that described in  187,
I      111  lllllllllllll    except that the  two
Q  wires which  press on
the break-piece are connected respectively with one
end of each double coil, while their other ends both
connect with the axis.
292. HORIZONTAL AXIAL ENGINE. -In the instrument represented in Fig. 123, the two double helices
ae fixed/horiontaly ig. 123.
are fixed horizontallv in the same line.  The iron bars




S INGLE B EA   AXIA L E NGi NE.  183
are connected at their Lends by a transverse bar,
through the ends of which pass guiding rods. These
slide in the tops of the small pillars, and keep the
bent bars in place. On the shaft of the balancewheel is the break-piece.
293. DOUBLE BEAM AXIAL ENGINE. —  A1 instrument in which there are two horizontal beams, each
Fig. 1241.
connected with a bent bar moving in an upright
double helix, is represented in Fig. 124. The motion of the beams is communicated to a single flywheel, by means of toothed wheels moved by cranks.
294. SINGLE  BEAM  AXIAL ENGINE. — Fig. 125
represents an instrument in which a reciprocating motion is communicated to a horizontal beam,
by the alternate movement of two bent bars. The
iron bars are not attached to the ends of the beam,
but simply rest upon them. When the current
passes, one of the bars is lifted, and the weight of




184           D A V I  S  MANUAL.
the other depresses that extremity of the beam on
which it rests. There is a break-piece of peculiar coniFig. 125.
struction at B, by means of which the current is
alternately transmitted to each of the double helices.
To Dr. Page is due the invention of the first instruments in which the source of motion was the axial
force exerted by helices on either straight or bent
bars of iron.
295. ELECTRO-MAGNET. — A bar of iron, wound
with insulated wire, so as to be enclosed in a helix, is termed an Electro-Magnet.
During the passage of an electric
current along the wire, the bar
exhibits a remarkable degree of
magnetic power, far superior to
that of a steel magnet of the
same size.  In all the instruments
in which a current is transmitted
through a coil surrounding an iron
bar, the iron becomes temporarily an electro-magnet,




ELECTRO-M- AGNET. IN  FRAME. r15
For the purpose of showing its liftillg t ower, the
electro-mnagnet is made of the U  form, (Fig. 126.)
The battery withi which it is used should be in proportion to the length and finerness of the wire, in
order to obtain the greatest power.  With a thick
and rather short wire, a battery of one or two pairs
will be preferable.   With a long and  fine wire, a
number of pairs should be employed.  A small electro-magnlet wvill sustain a large mass of iron nails or
filings about its poles, which fall when the flow of
the current is stopped.
296. ELECTRO-MAGNET,  IN  FRAME. — Fig. 127
FKg. 127.
E  11 1111i li 11111{!1111 1111 M E
represents an electro-m agnet, fixed in a frame, for the
16 ~




1(            - A v DAVI S'  MANU AL,
purpose of supporting heavy weights. The armature, A, consists of a semicircular piece of iron.
With the electro-mnagnet, this form is preferable to a
straight bar.  If the iron of the magnet is soft and
pure, its magnetic power is immediately communicated and lost, according as the connection with the
battery is made or broken.  NWheln, however, the
armature is applied to the poles, and the flow of the
current stopped while it is attached, it will continue
to adhere for weeks or months with great force, so as
to be able to sustain one third or one half as much
weight as while the current was circulating. But if
the keeper be once removed, nearly the whole magnetism disappears, and the magnet, if of good iron,
will not even be able to lift an ounce. The purest
iron retains, for a time, a certain amount of magnetic power, after being strongly magnetized, even
when an armature is not applied.  This amount
increases with the size of the bar.
297. The poles of the magnet are, of course,
reversed by changing the direction of the current.
When the change is made rapidly, while the armature is applied, it does not fall off. If a considerable
weight is suspended from it, it may fall a very slight
distance, and be attracted again.  The time required
to destroy the previously existing polarity, and to
renew  it in the opposite direction, is exceedingly
short.
298. ELECTRO-MAGNET, IN CASE. —An electromagnet of considerable power is represented in Fig.
128, fixed horizontally in a case.  The poles project




POWER OF ELECTRO-M i GNETS. 187
from one end, and a semicircular armature may be
applied to them, as in the cut. At the other end of
Fig. 128.
the case are seen two screw-cups, for making connection with the battery. There are two strong
rings, one attached to the bend of the magnet, and
the other to the armature. The magnet, with its
case, may be suspended by the former ring and
weights hung from the latter.
299. Very large electro-magnets have been made
to lift 3000 lbs. The proportion of the power to the
weight of the bar increases as their size is diminished,
up to a certain limit; and a small electro-magnet
may be made to sustain 400 or 500 times its own
weight, excluding that of the coil. Increasing the
battery current does not increase the magnetism indefinitely. Small bars acquire their full power, oi
become saturated with magnetism, with a moderate
battery. A current from the thermo-electric battery
(Fig. 41), when transmitted through the wires of
an electro-magnet, induces a considerable charge of
magnetism.
300. An electro-magnet, like the steel magnet




188          ) AVI S'S MANUAL.
exerts its attractive force through intervening substances; and the phenomena are more striking with
the former, in consequence of its greater power. It
will often be, able to lift its armature, with a plate
of glass interposed; and when a few thicknesses of
paper only intervene, a considerable additional weight
will be supported.
301. ELECTRO-MAGNET, WITH THREE POLES. -
This instrument, represented in Fig. 129, consists
Fig. 129.     of an iron rod, wound with
~st(~ /               insulated  wire, which  is
-  carried  in  one  direction
around half the length of
the rod, and then turns and
is wound in the other direction.  The effect of this
arrangement is, that, when
_#~"*, Ad~k~._   connection is made with the
battery by means of the
screw-cups on the stand, the two extremities of the
bar become similar poles, while the middle acquires
a polarity opposite to that of the ends. The middle,
as well as the ends, will sustain a considerable weight
of iron. By reversing the direction of the current,
all the poles are reversed. The arrangement of the
poles may be shown by passing a magnetic needle
along the bar.
302. COMMUNICATION OF MAGNETISM TO STEEL BY
THE ELECTRO-MAGNET.- The great power possessed
by the electro-magnet renders it peculiarly fitted for
inducing magnetism in steel; hence it is very con



ST E E L MAGNETS C H A RG E D.    189
venient for charging permanent magnets. A short
steel bar, if applied like an armature to the poles
of an electro-magnet of the U form, will become
strongly magnetic, the end which was in coltact
with the north pole acquiring south polarity.  A
longer bar may be charged, by employing the same
process that has been described in ~ 251, for touching
by steel magnets.
303. Bars of the U form are most readily magnetized by drawing them  from  the bend to the
extremities across the poles of the U-electro-magnet,
in such a way that both halves of the bar may pass
at the same time over the poles to which they are
applied.  This should be repeated several times,
recollecting always to draw the bar in the same
direction. Then, if it has a considerable thickness,
turn it in the hand, and repeat the process with its
opposite surface, keeping each half applied to the
same pole, as before. In Fig. 130, the arrow indiFig. 130.
catos the direction in which the motion should take
place.  Of course, the result will be the same, if
the steel bar is kept stationary, and the poles of




190           DAVIS' S MANUAL.
the electro-magnet are passed over it in the reverse direction.
304. In order to remove the magnetism of a steel
magnet of the U form, it is only necessary to reverse
the process; that is, to place one of its poles on each
pole of the electro-magnet, and draw it over them in
the opposite direction to that of the arrow in Fig.
130.  In this way, the magnet may be so completely discharged, as to be unable to lift more than
a few iron filings.
305. A bar magnet may be deprived of its magnetism, in a great degree, by passing the north pole
of an electro-magnet over it, from its south pole to
its middle, and then lifting it off perpendicularly; if,
then, the south pole be passed ill the same manner
over the other extremity of the steel bar, it will be
found to have lost the greater part of its polarity.
If necessary, this process may be repeated several
times. A still more effectual mode is to make use
of two electro-magnets; place the north pole of one
on one end of the bar, and the south pole of the
other on its other extremity, and draw  the poles
along the bar till they meet at its middle; then lift
them off perpendicularly.
306: The electro-magnet described in ~298 is
very convenient both for communicating and removing magnetism. On account of its power, bars
of considerable size may be fully charged by it. If
the steel magnet has not the same, or nearly the
same, width, from pole to pole, as the electro-magnet,
it can be charged by drawing each half separately




MAGNETOMETER.                  191
over the proper pole of the electro-magnet, in the
manner described in ~ 251, for bar magnets.
307. MAGNETOMETER. —A simple application of
the electro-magnet brings us at once to a very useful
instrument, the Magnetometer, represented in Fig.
131. Its use is to measure the magnetizing power
of galvanic currents. The form in which it is now
described is new. It consists of a vertical electromagnet, of the U form, with an armature above it,
attached to the short arm of a balanced lever. The
long arm of the lever is graduated decimally to measure, by means of iveights of from 100 grs. to 10,000
grs., the force required to detach the armature from
Fig. 131.
the electro-magnet when connected with the battery
whose power is to be determined. The lever is supported on an axis with knife-edge bearings. The
armature may also be suspended on knife edges attached to the beam. On the under surface of the
armature is brazed a thin plate of brass, to prevent
its adhesion to the poles. A difference of magnetizing power of 10 grs. can be estimated in a series
extending from 100 grs. to more than 100,000 grs.,
or the limit of saturation of the magnet. Two sets




192          DAVIS' S MANUAL.
of screw-cups will be seen on the board; one of these
is connected with a short coil round the magnet, the
other with a long coil. By making this long coil of
fine wire, the instrument compares currents differing
in their intensity.  Two batteries are first estimated,
as to quantity, by their magnetizing power through
the short coil. Their relative intensity is then shown
compared with their quantity, by their magnetizing
power through the long coil, their intensity being in
mathematical proportion to the conducting power of
the wire for each, and therefore to the amount of
electricity which passes.  In comparing the power
of different batteries,- a matter now of some practical importance, - this instrument gives a rapid and
uniform result.
308. Instead of using a long coil of wire, surrounding the magnet, in estimating intensity, the
current may be passed through a detached coil of
fine wire, and through the short coil of the instrument, which would give a similar result.
309. AXIAL MAGNETOMETER. -Another form  of
Fig. 132.
the magnetometer is represented in Fig. 132. In
this, the axial attraction of a double helix is made




ELECTRO- 1AGNETIC TELEGRAPH. 193
use of, as in the Axial Galvanometer invented by Dr.
Page, and described in Silliman's Journal, vol. xlix.
p. 136.  In other respects, the construction is entirely different from his. A. double coil of fine wire
may be added, as in Fig. 131, to compare intensity
currents.
310. ELECTRO-MAGNETIC TELEGRAPH.  Within a
few years, the electric telegraph, the most important
application of galvanism ever made, has been contrived and brought into practical use.  A  slight
sketch of the history of this invention, drawn partly
from Vail's book, "The American Electro-Magnetic
Telegraph," and partly from the original sources,
will be of interest here as an introduction.  This
is the more needed, as there has been great confulsion as to the originators and inventors of the telegraph. We find that the first electrical telegraphs
were put in operation by Lomond, in 1787; by Reizen, in 1794; and by Salva and Belancourt, in 1798.
In these, the common electrical machine was used,
with which, of course, no important result could be
obtained. In 1809, Sernmering laid the foundation
of the galvanic telegraph, by proposing to employ,
with the Voltaic pile, a wire for every letter in the
alphabet, terminating in gold pins inserted in a glass
cell containing water, the letters being indicated by
the evolution of oxygen and hydrogen from the pins.
A similar plan was proposed by Dr. Coxe, of Philadelphia, in 1816.  In the same year, the use of
machine electricity for the telegraph, was revived
by Ronalds, in England.
17




L94          I) AVI S     MANU Al.
311, On the discovery of electro-inagnetisin, in
1819, Ampere very early proposed the deflection of
the needle in place of the decomposition of water
for indicating the passage of the current.  In the
treatise on new discoveries in electricity, by Ampere
and Babinet, published in Paris, in 1822, the telegraph of Wheatstone is anticipated by fifteen years,
in all essential details, with the exception of the
greater number of wires required in the former.
Here it is necessary to draw a line between the proposers and introducers of an invention. The claim
of originality belongs to the former, but public gratitude is usually accorded to the latter. The deflection
of the needle for telegraphic purposes was subsequently referred to by many other writers; but it
seems first to have been introduced into practice by
Schilling, in Russia, at the end of 1832; by Gauss
and Weber, at Gottingen, in 1833; in Vienna, in
1836; and, finally, on a large scale, by Wheatstone,
in England, and by Steinheil, at Munich, in 1837,
or soon after.  The credit of the construction of the
galvanic telegraph belongs thus to Schilling, Steinheil, and Wheatstone, by the latter of whom, with
some of his English coadjutors, many of the practical
difficulties in the modes of transmitting the current
were overcome.  The invention of Grove's battery
was also an important era in the introduction of the
telegraph.
312. We come now to the electro-magnetic telegraph, which, in its beautiful simplicity and efficiency, surpasses all others yet introduced into practice




ELECTRO-MAGNETIC TELEGRAPH. 195
Barlow, in England, seems to have made some early
suggestion of this kind; but it was not until 1830,
on the construction of the first powerful electromagnets, by Professor Henry, of Princeton, N. J.,
that this form of telegraph became possible; and in
his first paper on the results of these experiments,
in some of which long wires were used, he at once
applies the new facts to the telegraph. The present form of the American telegraph is claimed to
have been suggested in 1832, by Dr. C. T. Jackson,
and by Professor Morse. It was finally matured by
Professor Morse, and introduced by him between
Baltimore and Washington, in 1844, after a delay
before Congress of more than six years.
313. ELECTRO-MAGNETIC TELEGRAPH. — Fig. 133
represents the recording part of the telegraph.  It
Fig. 133.
consists of an electro-magnet, armature, and lever,
arranged in a similar manner to the magnetometer,
(Fig. 131.) At the extremity of the lever is a blunt
point, which marks the strip of paper when the electro-magnet is in action. If one wire from the battery
is placed in one of the screw-cups, whenever the
other wire is touched to the remaining cup, the arma



196          DAVIS S  IMANUAL.
ture is powerfully attracted by the mag. et, and the
point on the lever presses the paper into the corre.
sponding groove of the roller, so that lines or dots art
made according to the time during which the contact
with the battery is maintained, the paper being slowly
drawn under the roller.
314. The following table exhibits the signs employed by Professor Morse: —
MORSE'S TELEGRAPHIC ALPHABET.
ALPHABET.      n - -
a --             0 - -          NUMERALS.
b ---- I p...2c --  -         q              1 ----
d —  r-  2.
e-              s —-.        3...
/ —-_            t             4 —---
g...    uo-.-'    5 —----
he              S- - -        36..k   - _          --- y...  9..... a.            -- -           -  ----
m —--                           0
It will be obsgrved, that the characters are, formed
by different combinations of dots, and of'short and
long lines, with -short and long spaces. Between
each letter of a word, a short space is used; and 1bng
ones between the words themselves;. -Sentences are
separated by still longer spaces.




CLOCK-WORK TELEGRAPH.               197
315. TELEGRAPH, WITH CLOCK-WORK. —In Fig.
134, the telegraph is shown with all the appendages
k. 134.
usually employed. The electro-magnet near A, and
the lever, are the same as in the instrument last
described; -connected with them is an apparatus,
moved by clock-work, for carrying the paper, which
is unwound from a large spool, S, by the rollers
between which it passes, and thus drawn slowly over
the registering point. The first movement of the
lever sets the clock-work in action, and a break and
friction-wheel are sometimes added, by which it is
sto.ped shortly after the signals cease to be transmifted.  A bell, seen at B, is connected with the
leve, so that the first motion communicated by the
battery gives a signal to the attendant.
316. The battery used in operating Morse's tele17*




198          D A V I SS MANUAL.
graph, is a modified form of Grove's, (~ 45.) Instead
of a galvanic series, the magneto-electric machine,
to be described hereafter, has been used as a source
of electrical power for working the telegraph. The
first instance, it is believed, in which this was accomplished with Morse's telegraph, is given below, as an
example of telegraphic writing, taken from the scroll
used on that occasion:W         r       i     t      t    e      n
b         y       t       h      e       p
o        w       e      r          o       f
o      n      e        o       f           D
a         v        i        s       e       s
M  r      a         g          n      e     t
o           E       I       e       c       t
r      i      c          M        a       c
h      i      n      e      s            B
o      s        t      o      -n          D
e      c         r           9             1
8          4           4




SIGNAL  KEY.                199
317. SIGNAL KEY.- Fig. 135 represents the instrument usually employed to make the various conFig. 135.         tacts, differing in succession and length,
by which the system
of lines and dots, representing the various
letters, are produced
|   e 3 4 \at the other end of
the telegraph.  The
fingers  are  shown
resting upon a knob, attached to a metallic spring,
by the depression of which contact is made with a
metallic conductor on the baseboard, connected with
a screw-cup at one extremity of the instrument;
the other screw-cup is connected with the spring.
One battery wire passes into the first screw-cup.
The other screw-cup receives one of the wires of the
telegraph, which proceeds to the registering apparatus
at the other station. By the other telegraph wire,
the remaining extremities of the battery and of the
registering apparatus are directly connected.  The
circuit is therefore completed by depressing the key,
and is immediately broken by the action of the spring
when the fingers are removed.
318. Instead of using the second wire directly
connecting the battery and register, the earth is
sometimes employed as a conductor, by connecting
the pole of the battery and register with a large metallic plate, sunk in the ground at each terminus of
the telegraph. In this case, the number of wires
needed is reduced to one.




200           I) I' V I  S MANUAL.
319. Tile wires are well insulated, and are carried
from station to station along high posts, about 300
feet apart.  On some of the lines they are of copper;
oni others, of iron.  An iron wire is imuch inferior ill
conducting power to a copper one of the same size;
but the greater cheapness admits of the use of
much thicker wires.  These are found to convey
the current well, and are less liable to be broken than
the copper wires.
320. The number of galvanic pairs required to
work the, telegraph varies with the distance and
consequent length of the wires. From twelve to
thirty or fifty have been employed upon different
lines. The largest number would still be inadequate
to give sufficient power to the electro-magnet at very
long distances, were it not for the invention of the
receiving magnet, by Professor Morse. An electromagnet of the most sensitive construction is placed
at the end of the telegraphic line, which moves an
armature by the galvanic power, sufficiently only
to make contact between a platinum point and surface, which are in the circuit of another small battery, immediately on the spot. This at once works
the registering apparatus with great force, its current having to traverse only a few feet of wire in its
passage to the registering electro-magnet.
321. AXIAL RECEIVING  MAGNET. -In Fig. 136
is represented a receiving magnet of a new  construction. The movement is produced by the axial
force, an iron bar of the U form being drawn within
a double helix, as described in ~ 289. The bent bar




AXIAL  RECEIVING  MAGNET.    201
is horizontal, but is so suspended that its own weight
withdraws it from the coils when the current is
interrupted. In the cut, the instrument is shown
in connection with a local battery for igniting an
explosive charge in blasting rocks. er for submarine
or other explosions.  The loss of power by conFig. 136.
duction, renders it difficult, with a moderate sized
battery, to explode the charge by heating a wire at
a considerable distance. In the arrangement here
described, the receiving apparatus is used to complete
the circuit of a small battery in a pwtected position
near the place where the explosion is to occur.




202          DAVIS'S MANUAL.
322. In the figure, the coils are placed horizontally, and the U-shaped iron bar, which moves in
connection with a vertical bar, suspended by an axis
above, enters them  at M. The influence of the
helices is aided by an armature, as in Fig. 121. The
connections of the single galvanic pair, seen in the
foreground, are so made that its circuit is completed
whenever the vertical bar comes in contact with the
platinum point, A, immediately in front of it. This
takes place as soon as the long battery circuit is
completed by the operator at a distance, and the
axial magnet drawn into the coils. This instrument,
even without the armature, is more sensitive than
the receiving magnet of Professor Morse, and may
be used with some advantage as a receiving apparatus for tile telegraph.
323. AxIAL TELEGRAPH.- Fig. 137 represents a
form of telegraph in which the axial force is used,
motion being obtained in
the manner described in
289. This is introduced as a new instrument,
and its originality is here.... claimed.  Two upright
coils, C, are supported a
short distance above the
baseboard.'Entering these from below is a U-shaped
rod of soft iron, fitted to be drawn up into the coils,
under the influence of the galvanic current.  When
not thus drawn up, it rests on a spring, shown in
the cut, by which the instrument is rendered more




CLOCK-WORK  AX I AL TLEGRAPL F  A  I. 203
sensitive.  A grooved roller, for carrying the paper,
is seen above the coils.  To the iron bar is attached
a blunt point, so as to project above its poles, in the
centre, and il opposition to the groove on the roller.
Contact being made with the battery, the soft iron
rises up within the coils, and marks the paper.  On
the whole, this form of telegraph is more sensitive
and efficient than the electro-magnetic, and less liable
to derangement; from the absence of the lever, it is
also more compact. A small armature may be placed
across the coils above, as described in ~ 290, by which
the mark is made with still greater force; but in this
case, the motion is partly electro-magnetic.
324. AXIAL  TELEGRAPH, WITH  CLOCK-WORK. —
In Fig. 138, the axial telegraph is represented, with
Fig. 138.
clock-work annexed, for carrying the paper, in the
manner already described in connection with the
electro-magnetic telegraph. The coils of the axial




204          D AVIS SS MANUAL.
apparatus are partly seen at M. The large scroll of
paper is shown at S, with the strip, P P, proceeding
from it. The arrangement for giving the alarm by
the bell, B, is also annexed.
325. AXIAL TELEGRAPH, WITH ENGINE. — A different arrangement of the telegraph is shown in Fig.
139, where the machinery for carrying the paper is
i.Rg. 139.
moved by an axial engine, driven by a small local
battery, and set in action, if desirable, by the completion of -the circuit of the telegraph. The soft
iron bar of the telegraphic apparatus is seen at A,
and that of the engine at M. There is a peculiar
arrangement added for commencing and suspending
the action of the engine when the telegraph begins
and ceases to work.
326. The deflection of the gold leaf galvanoscope,




RECIPROCATING  ARMATU1RE ENGINE. 205
(Fig. 63), has recently been proposed as a means of
telegraphic indication. The extreme delicacy of this
instrument enables it to give a result with a single
pair of plates through great lengths of wire. There
are no means, however, of recording its action,
which would als, be seriously interfered with by
the influence of atmospheric electricity on the conducting wire. The transmission of the battery current through the telegraphic wire, however long, is
practically instantaneous. Experiments which have
been made on the velocity of electricity, appear to
indicate that it is considerably greater than that
of light, which travels nearly 200,000 miles in a
second of time.
327. RECIPROCATING ARMATURE ENGINE. -In this
instrument, contrived by Dr. Page, two electro-magnets of the U form, represented at M M (Fig. 140),
Fig. 140.
are firmly secured in a vertical position, the four
poles appearing just above a small wooden table.
The two armatures, A A, connected by a brass bar,
18




206          DAVISS'  MANUAL.
move upon a horizontal axis in such a manner that
while one is approaching the poles of the magnet
over which it is placed, the other is receding from
those of the other magnet. The brass bar is connected with one extremity of a horizontal beam, the
other end of which communicates motion by means
of a crank to a fly-wheel. On the axis of the flywheel at B is the break-piece. Each magnet being
charged in succession, the armatures are attracted
alternately, communicating a rapid reciprocating motion to the beam, and consequently a rotatory one
to the fly-wheel.
328. HORIZONTAL RECIPROCATING ENGINE.- Two
electro-magnets of the U form, M M (Fig. 141), are
K~g. 141.
supported in a horizontal position, with a single
armature A fitted to vibrate horizontally between
them. When the battery connections are made with
the screw-cupS on the baseboard, one of the magnets
will be charged, provided the break-piece is in such
a position with regard to the springs as to complete
the circuit. The armature will now be attracted
towards the charged poles. Just before it reaches




REVOLVING ARMATURE.               207
them, the movement of the break-piece interrupts
the current in the magnet, destroying its polarity,
and then causes the current to be transmitted through
the opposite one; this becomes charged in its turn,
and attracts the iron bar, A, which imparts motion
to the balance-wheel, W. By means of clock-work,
the rotation of the wheel causes a hammer to strike
the bell placed over one of the electro-magnets.
329. REVOLVING ARMATURE.-In this instrument,
invented by Dr. Page, a small bar of iron is arranged
to revolve horizontally just above the poles of an
electro-magnet of the U form, fixed in a vertical position, as seen in Fig. 142, where A is the iron bar,
and M the electro-magnet.  To
the axis of motion of the bar is
affixed a break-piece, 1made by
filing away two opposite sides
ffW    of a small solid cylinder of silver.
A            Upon the narrow prominent portions thus left, play two silver
springs, shown at W in the cut,
opposite to each other.  One of
these springs is connected with
a screw-cup on the stand; the
other communicates with one extremity of the wire enveloping
M         the electro-nagnet, the other end
of this wire being fixed to a second cup on the baseboard.  The
break-piece is so arranged as to
release the springs from their bearing just as the




208          D A V IS'S MANUAL.
armature passes over the poles; and to restore them
to it again when it has moved on somewhat more
than a quarter of a circle, so as to be a little inclined from a'position at right angles to the plane
of the magnet.
330. On placing the bar in this position, and coinnecting the cups orn the stand with a battery, the
electro-magnet becomes charged, and consequently
attracts the armature towards its poles: as soon as it
reaches their plane, the springs leave the projecting
parts of the break-piece, and the current is cut off.
The polarity of the magnet is now destroyed, and it
ceases to attract the armature, which moves on by
the momentum it has acquired, until it passes a little.
beyond a position at right angles to the plane of the
magnet.  At this point, the springs again come in
contact with the break-piece, and the flow of the
current is renewed. The attraction now exerted by
the poles gives a new impulse to the armature, and
the circuit being again broken when it reaches their
plane, it continues its motion in the same direction,
revolving with great speed. Care should be taken
that the springs are in such a state of tension as to
open and close the circuit at the proper points, as
indicated in the above description. The motion of
the bar is not reversed by changing the direction
of the current.
331. REVOLVING ARMATURE ENGINE. -In this instrument there. are several armatures fixed on the
circumference of a vertical wheel, parallel to its axis.
In Fig. 143 three are represented, each of them




REVOLVING ARMATURE  EN G INE. 209
marked A. On the poles of the electro-magnet, M,
is secured a brass plate, from which rise' two pillars
to support the axis of the wheel: as the wheel turns,
the iron bars pass in succession over the poles with
heir extremities very near to them.  At B, on the
shaft of the wheel, but not insulated from it, is the
break-piece, consisting of a small metallic disc, from
which project, in a lateral direction, several pins, equal
in number to the iron bars; or
the disc may be furnished with
A;, A. a corresponding number of teeth.
A silver spring connected with
one end of the wire surrounding
A   ~  the electro-magnet, plays upon
these pins or teeth; the other
end- of this wire is soldered to
the iron of the magnet, which
brings it into metallic communication with the shaft by means
of the brass plate and pillars.
_ Or the wire may be terminated
by a second spring pressing upon
a cylindrical part of the axis.
332. The break-piece is arranged in such a manner, that the
electro-magnet is charged when
any one of the iron bars is brought near it by the motion of the wheel. This bar is then attracted towards
the poles; when it arrives at the plane of the magnet, the current is cut off, in consequence of the corresponding pin or tooth releasing the silver spring
18 




210          DAVIS S MANUAL.
from its bearing. The armature being no longer
attracted, the wheel moves on by its momentum
until the next bar comes into the same position,
causing the magnet to be recharged; this is then
attracted in its turn, and passes on like the precedc
ing one.
333. The spring playing on the break-piece must
be so disposed that the circuit shall be broken when
each bar reaches the poles, and not be renewed again
until it has passed to a greater distance from them
than that between the next succeeding bar and the
poles, or it will be attracted back again, preventing
the continuance of the motion.
334. VIBRATING ARMATURE ENGINE.- Fig. 144
represents an instrument in which the armature, A,
K/g. 144.    is made to vibrate backwards
and forwards above the poles
of an electro-magnet, M. The
armature, when on one side, is
attracted by the poles until it
arrives directly over them. As
it reaches this position, the circuit is broken by the breakpiece, and the bar moves on by
the momentum it has acquired,
for some distance. The circuit
is then renewed, and it moves
back towards the poles. The
arrangement of the- break-piece
is such, that the current circulates while the armature is approaching the poles on




[REVOLVING  ELECTRO —MAGNEIT.  211
either side, and is interrupted while it is receding
from them. A continuous vibration is thus occasioned, which imparts motion to a fly-wheel, and to
a hammer, which strikes the bell seen in the cut.
335. Fig. 145 represents another form of the instrument, in which
the axis of motion
of the armature is
below  the  magnet.
The moving bar exerts its greatest force
when  passing  the
poles, and  at-the
moment when  the
crank is in the best
position for driving the fly-wheel.
336. REVOLVING  ELECTRO-MAGNET. -In the instrument represented in Fig. 146, a steel U-magnet
is fixed in a vertical position, and a small straight
bar of soft iron, enclosed in a helix, is so arranged
as to revolve between its poles. The two extremities of the insulated wire surrounding this electromagnet, are connected respectively with the segments
of a pole-changer (~ 191), on the shaft. The silver
springs, which press upon the pole-changer, are attached to two stout brass wires, passing through the
brass arch surmounting the U-magnet, but insulated
from it by the intervention of ivory or horn; each
of these wires supports a brass cup for connection
with the battery. These springs must be so placed
with regard to the segments, that the poles of the




212            D A V IS' S MANUIAL.
revolving bar shall be reversed at the moment when
it is passing the poles of the fixed magnet.
337. On making connection with the battery,
when the bar is at right angles to the plane of the
magnet, it immediately acquires a strong polarity.
Its north pole is then attracted by the south pole of
the steel U-magnet and repelled by the north pole.
The south pole of the bar, on the contrary, is repelled by the similar pole of the upright magnet, and
attracted by its opposite
Fig. 146.        pole. These four forces
conspire in bringing the
electro-magnet between
the poles of the U-magi U UdN    C net.  When it reaches
this position, each segment of the pole-chlanger leaves the spring with
which it was in contact,
and passes to the other.
As the bar is moving
past the poles by the
momentum it has gained, its magnetism  is
destroyed  for a moment, and immediately
restored in the opposite
direction.  Each pole
of the bar is now repelled by that pole of the permanent magnet which
it has just passed, and attracted by the opposite one;




RITCHIE' S REVOLVING  MAGNET. 213
it consequently moves on, the polarity being reversed
twice in each revolution.
338. Instead of using a pole-changer, the poles
of the electro-magnet are reversed in the following
manner in the instrument known as RITCHIE'S REVOLVING MAGNET. The ends of the wire surroundilnl
the revolving bar dip into mercury contained in a
circular cistern of ivory, fixed between the poles of
the U-magnet below the bar. This cistern is divided
into two separate cells by low partitions of ivory, so
arranged that, when the electro-magnet is passing
between the poles of the steel magnet, the ends of
the wire may be moving across the partitions and
just above them. On supplying the cells with a
proper quantity of mercury, its surface will be found
to curve downwards on every side towards the ivory,
so that its general level will be higher than the partitions; thus allowing the extremities of the wire to
be immersed in it, except when passing across them.
A wire connected with a brass cup, for making communication with the battery, projects into the mercury in each compartment of the basin. At the
moment when the wires quit the mercury to pass
across the partitions, a spark is seen. When the
machine is put in motion in a dark room, these sparks
give rise to an optical illusion of the same character
as that mentioned in the description of the Revolving
Spur-Wheel, causing the bar to appear at rest in.
the position it is in when the sparks are emitted.
339. The revolution is more rapid where the
pole-changer is used, than in Ritchie's instrument,




214            1)AVIS I S MANUAL.
and may be made still more so, by employing a Ushaped electro-magnet in place of the stationary steel
magnet.  In this case, the rotation is not reversed by
changing the direction of the current, as it is when a
steel magnet is used, since the poles of both electromagnets are reversed at the same time, and their
relative polarity remains the same.
340. REVOLVING BELL ENGINE. - This instrument, represented in Fig. 147, is similar in principle
Fig. 14 7.    to the one last figured, the
U-magnet, however, being inverted, so that the revolving
electro-magnet  is near the
baseboard; the pole-changer
is on the axis below it. There
is, in addition, an arrangement
for striking a bell, which is
fixed above the magnet.  To
the axis of the revolving bar
is attached an endless screw;
this screw acts on a toothed
wheel, which is provided with
a pin projecting laterally, for
the  purpose of moving the
hamrnmer.  As the wheel turns,
the pin presses upon the handle of the hammer, raising it
from  the bell until it is released by the pin at a
certain point of the revolution; when a spiral spring,
fixed to the handle, impels the hammer against the
bell.




ROTATION BY EARTH S ACTION. 215
341. If the wheel has 100 teeth, as in the cut,
the electro-magnet must revolve 100 times in order
to produce one revolution of the wheel, and consequently one stroke upon the bell. The velocity
of the rotating bar is measured by counting the
number of strokes in a given time; it may make 100
or more revolutions in a second. In order that the
motion of the wheel may raise the hammer, it is
necessary to transmit the battery current so that the
bar shall rotate in the proper directirn.
342. REGISTERING REVOLVING MAGNET. —Fig.
Fig. 148.    148 represents an instrument in
which the number of revolutions
of the electro-magnet in a given
time is registered by means of
clock-work connected with the
shaft. On the dial-plate are three
pointers, which mark the revolutions up to 1000. The time
occupied by any known number
of revolutions at once enables us
to ascertain the speed of the rotating bar. Though very great,
it is necessarily somewhat less
than when the bar carries no
machinery.
343. ELECTRO-MAGNET REVOLVING BY THE EARTH'S
ACTION. -As the earth itself exhibits magnetic polarity, an electro-magnet may be made to revolve
by its influence; though, in consequence of the
feebleness of the action, the instrument must be




216           D A V I S S MANUAL.
constructed with some delicacy. A small electromagnet, (Fig. 149,) is so supported as to have freedom
of motion in a vertical plane, like the dipping needle,
a pole-changer being secured on its axis of motion.
The springs which press upon the pole-changer
should be disposed in such a manner that the polarity
of the bar may be reversed when in the course of its,evolution it reaches the line of the dip.  In high
Fig. 149.        latitudes, it will be sufficient to  arrange the
pole-changer so as  to
reverse the poles when
the bar becomes vertical. The shaft on which
the bar revolves, rests
on friction rollers.
I   344. On placing the
electromagnet horizontally in  the magnetic
meridian, that is to say,
with its extremities directed north and south, and
transmitting the battery current, its north pole (in
this hemisphere) immediately inclines downwards
towards the earth, in the same manner as that of the
dipping  needle.  The  momentum  thus acquired
carries it past the line of the dip into a vertical direction. As soon as it reaches this position, the poles
are reversed, and it continues to move on in the
same direction as long as the battery connections are
maintained, revolving with a moderate velocity.
345. In Fig. 150 is represented an electro-magnet,




MAGNET  REVOLVING  IN  A COIl.. 217
so supported as to revolve by the earth's action in
a horizontal plane, instead of a vertical one. In this
Fig. 150.     case, the instrument must be
placed in such a position that
the polarity shall be reversed
when the revolving magnet assumes the direction of
the compass-needle, pointing
north and south.
346. When a steel magnet is placed in a proper position near the revolving bar,
in either of the instruments
whose motion depends on the earth's influence, it
rotates with much greater speed than by the action
of terrestrial magnetism alone. Its motion may be
reversed, notwithstanding the opposing influence of
the earth, by disposing the permanent magnet in a
suitable manner.
347. ELECTRO-MAGNET REVOLVING WITHIN A COIL.
-Fig. 151 represents an instrument in which a
straight electro-magnet revolves within a circular coil.
The same current traverses the coil and the wire surrounding the rotating bar, but its direction is changed
only in the latter. When the circuit is completed,
the revolving bar moves so as to bring its poles into
a direction corresponding with those of the coil.
This motion depends upon the same cause as that
of a galvanometer needle, (~ 158.) As the poles of
the coil are situated in the line of its axis, the
springs pressing on the segments of the pole-changer
19




21S            ) AVIS S MANUAL.
are so arranged as to reverse the current when the
electro-magnet is at right angles with the coil. The
north pole of the magnet places itself
within the north pole of the coil, and
not, as might at first sight be expected, within its south pole.  This
apparent anonialy is due to the circumstance of the magnet not being presented to the coil from the outside,
but lying within, with its centre coinciding with that of the coil.  If a
magnetic needle is brought up to a'helix, its north pole is attracted towards the south pole of the helix, (see
g 268), and passes within it, until its centre lies in
the centre of the coil.  In this position, its poles lie
in the same direction as those of the electro-magnet
described above.
348. DOUBLE REVOLVING  MAGNET.- In this instrument, represented in Fig. 152, there are two
FKg. 152.         electro-magnets of the
U form and of the same
size, arranged to revolve on vertical axes.
At s, on the shaft of
the lower magnet, is a
silver ferrule, on which
presses a wire connected with  one  of the
screw-cups.   On  the
shaft of the upper magnet is a break-piece.  When




REVOLVING  WHEELS.               219
the current passes, both magnets become charged,
and, their opposite poles attracting each other, they
move in opposite directions until they come into the
same line. At this moment the circuit is interrupted
by the break-piece, and the poles move past each
other by their acquired momentum. Trhe circuit is
then renewed, and the magnets continue to revolve
in opposite directions.  Since the shaft of the lower
magnet sustains the weight of both, the lower one
moves with less speed than the other, in consequence
of the increased friction. Reversal of the poles by
a pole-changer is not resorted to, on account of the
feeble repulsion between electro-magnets.
349. REVOLVING WHEELS, WITH ELECTRO-MAGrET.- In the instrument represented in Fig. 153,
Fig. 153.       motion is obtained on the
same principle as in the
Revolving   Spur-Wheel
(~ 180), an electro-magnet
taking the place of the steel
magnet.  There are two
wheels; of these, the upper and larger one is partly
supported by springs, its
circumference resting on
that of the smaller wheel.
The current traverses in succession the coil of the
electro-magnet and the wheels. It passes from the
axis of the smaller wheel to that part of its circumference which touches the circumference of the other,
and thence to the axis of the larger one. The lower




220           DAVIS, S MANUAL.
wheel is not merely carried round by the other, but
those parts of both which are conveying the ciurrent
tend to move away from between the poles in the
same direction, causing the two wheels to revolve in
opposite directions.
350. REVOLVING  WHEELS. -A  better form  is
shown in Fig. 154, in which the wheels move beFig. 154.       tween the poles of a
steel U-magnet, whose
legs are brought very
near together. In these
instruments no  mercuryisused. Fig. 154
is similar to the Revolving Disc (~ 184),
and would have been
described in connec-'_ tion with that, had not
that part of the volume
gone to press before this instrument was contrived.
351. ELECTRO-MAGNET, REVOLVING ON ITS AXIS.
- The instrument represented in Fig. 155, is similar
to the one described in ~ 167, except that the revolving bar is of iron, enclosed in a helix which,rotates with it. The battery current traverses the
helix and one half of the bar. As the bar becomes
an electro-magnet, it revolves like the one represented in Fig. 57, except that the direction of the
rotation is not changed by reversing the current,
since the poles are at the same time reversed. From
the power of the magnet, the motion is very rapid.




M F C H ANICAL P O  ER.        22 
The revolution of either an electro-magnet or a steel
magnet on its axis is properly classed with that of a
fig. 155.        conductor around a
magnet.  When the
conductor and magnet are fastened together, the latter is
carried round by the
movement  of  the
former, as stated in
~ 171. Instead  of
fastening a conducting  wire  to  the
magnet, the current
may be transmitted
through the magnet
itself, with the same
result.
352. ELECTRO-MAGNETISM AS A MOTIVE POWER.
— The great velocity of motion, and the strong
attractive force exhibited by many of the small electro-magnetic instruments, naturally suggested the
application of this power to the purposes of the arts
as a mechanical agent; and numerous experiments
have been made with this view, but hitherto without
success. Professor Henry was the contriver of the
first instrument whose motion depended upon magnetic attraction and repulsion. In his little machine,
an electro-magnet, whose polarity was alternately
reversed, was made to vibrate above the north poles
of two straight steel magnets. He, however, made
19*




222           DAVI I  S MANUAL.
no attempt to apply this power to practical purposes
There are obstacles of a purely mechanical character
in the way of its employment; these, though important, are not perhaps insurmountable. But the
most serious difficulties are those which arise from
the nature of the power. The motion of the attracting poles of two electro-magnets towards each
other, actually lessens the attractive force in proportion to the velocity with which they approach;
the same thing occurs in the recession of mutually
repelling poles. These phenomena are due to the
influence of secondary electric currents produced by
the motion, which flow against the battery current,
and of course partially neutralize its magnetizing
power. The secondary currents present a very formidable obstacle, as their opposing influence increases
with the size of the machine in a rapid ratio. An
appreciable time is also required for the communication and removal of the polarity of large electromagnets, and the purest iron retains a considerable
degree of magnetic power after being highly charged.
To the action of these and some other causes is
owing the fact, which was early discovered by those
engaged in these investigations, that the smallest
machines possess by far the greatest proportional
power. Some of these difficulties are obviated by
the axial motion, recently proposed as a source of
mechanical power, by Dr. Page, one of the earliest
and most persevering, as well as ingenious experimenters in this department.  In his instruments,
modified forms of some of which have been described




INDUCTION BY THE EARTH.   223
on previous pages, the force by which an iron bar is
drawn into a helix is the source of the power. A
stroke of some length may thus be obtained, and
there are no attracting or repelling poles to produce
interfering secondary currents.
III. BY THE INFLUENCE OF THE EARTH.
353. An- unmagnetic bar of iron, placed in the
direction assumed by the dipping needle (see ~ 210),
acquires temporary magnetism by induction from the
earth. That extremity which is directed towards
the south magnetic pole (~ 218), receives north polarity, and the other end south polarity. A bar of
soft iron held in a horizontal position, especially
if directed east and west, attracts indiscriminately
either pole of a magnetic needle, as the earth exerts
no appreciable inductive action upon it.
354. In Fig. 156, A B represents a bar of iron, presented in a horizontal position to the north pole of
a magnetic needle, N S. The pole is- now attracted
by the bar. Keeping the end B in the same place,
raise the end A so as to bring the bar into the position
C D. As the bar is raised, the north pole recedes
from C, as indicated by the dotted lines in the cut.
The strongest action is exerted when the bar is in
the ine of the dip, or in this latitude nearly vertically
over the needle. By carrying the bar below the
needle, still preserving the same direction, its upper
end will be found to attract N and repel S. These




224          DAVIS' S MANUAL.
phenomena are sufficient to show that C D has become a magnet, with C for its north pole. On reFig. 156.    9
B bA
versing the bar, so as to bring the end D downwards,
C immediately becomes the south pole: thus the
polarity may be changed at pleasure, the induced
magnetism  being only temporary.  If the bar is
brought very near to the pole of the needle, the inductive action of the earth will be overpowered by
that of the needle, causing attraction to be exhibited
in every position.
355. Except in the vicinity of the equator, it is
sufficient to hold the bar vertically, as the line of dip
approaches the perpendicular in high latitudes. In
consequence of this inductive action of the earth, all
large bars of iron standing in an upright position, ate
more or less magnetic, their lower ends, in this hemisphere, being north poles.  When they have remained for a long time in this situation, the polarity
does not disappear on changing their position.




INDUCTION BY THE EAIRTI.   225
356. The induction of magnetism by the earth is
greatly facilitated by causing a movement among the
particles of the bar, as by percussion or twisting.
Let a bar of iron be held in the line of the dip, and
its lower end brought
near the north pole of a
magnetic needle. The
pole will be repelled,
and will come to rest at
some distance from the
bar, as shown in Fig.
157.  Now  strike the
upper end of the bar
with a hammer, and the
induced magnetism will
be so much increased
that the  needle will'25<'   swing  round, and its
south pole be drawn to
the bar, as indicated by
the dotted lines in the
cut. The polarity thus
induced is not reversed
by merely inverting the
rod, but the aid of percussion will be required, in order to remove or reverse
the magnetism. This is especially the case when a
steel bar is employed instead of an iron one.
357. Take a piece of iron wire, and, placing it in
a vertical position, twist it powerfully. It will now
be found to sustain iron filings at its extremities, and




226           D A V I S'S MANUAL.
to turn itself' north and south, when balanced on a
pivot, as shown in Fig. 158. The end which was
Fig.. 158.      downwards becomes the
north pole.
358. The magnetism,
in these cases, is not due
directly to the percussion
or twisting, either of
which merely favors the
action of the earth. A considerable degree of permanent magnetism may be communicated to a steel
bar, by placing it vertically on a large mass of iron,
and striking its upper end repeatedly with a hammer;
it acquires much greater power if struck while resting on iron than on any other substance.
359. Percussion may be used to facilitate the removal of magnetism. Thus the polarity of a steel bar
magnet may be lessened, or even entirely destroyed,
by repeated blows of a hammer, while held horizontally east and west.  This process is very convenient
for removing slight degrees of magnetism from iron
or steel bars. In discharging steel magnets by the
methods previously described, it is sometimes difficult to effect- a complete removal of their magnetism. In such cases, the desired result may be
attained, after having nearly discharged them, by
following the process now given. Merely falling
upon the floor often injures the power of a magnet
considerably, in consequence of the vibration excited among the particles of the steel.




MAGNETISM.
III.
INDUCTION  OF  ELECTRICITY.
1. BY THE INFLUENCE OF A CURRENT OF
ELECTRICITY.
360. THAT branch of the science of electricity
which treats of the phenomena presented by it when
at rest, is termed Electro-statics: the branch which
relates to electricity in motion, is called Electrodynamics. The phenomena which characterize the
latter state are classified by Professor Faraday as
follows: "The effects of electricity in motion, or
electrical currents, may be considered as, 1st, Evolution of heat; 2d, Magnetism; 3d, Chemical decomposition; 4th, Physiological phenomena; 5th,
Spark." Many of the phenomena presented by electricity in motion are closely related to magnetism,
and it is usual to treat of them in connection with
that subject, as in the present volume, rather than
with electricity.
361. Before entering upon the particular subject
of the present chapter, that is, the inductive action




22t           DAVIS S 7MANUAL.
of currents, it will be advisable to occupy a few
pages with a comparison of some of the phenomena
exhibited by electricity in the two states of motion and rest, as induction is exerted in both. The
nature of the action is entirely different in the two
cases.
362. An electrified body attracts light substances
in its neighborhood, having previously induced in
their nearest ends the opposite electricity to its own;
and on their approach communicates to them a part
of its charge, when, if insulated, they are instantly
repelied by it. A. wire conveying a current exerts
no such influence upon light bodies, although placed
in the immediate vicinity.
363. In the case of electricity at rest, two bodies,
charged either positively or negatively, repel each
other; while if one is charged with positive and the
other with negative electricity, they exert a mutual
attraction. Electrical currents, on the contrary, attract each other when flowing parallel in the same
direction, and repel each other when flowing in
opposite directions. The result is the same whether
two different currents or two portions of one current
are experimented on.
364. ATTRACTING AND REPELLING WIRES.- The
instrument represented in Fig. 159 is designed to
exhibit the attractions and repulsions of currents.
Two wooden troughs for containing mercury are
supported opposite to one another, each being divided
into two oblong cells by a partition in the middle.
Each of the four portions of mercury thus insulated,




ATTRACTING AND REPELLING WIRES. I       29.
is'connected by means of a wire projecting into the
cell, with one of the screw-cups fixed at the ends
Fig. 159.        of the troughs.  The
A  la oints of two rec2il_         I      angular wires, A and
3, rest in the opposite
compartments of the
troughs. This mode
of support allows the
wires to be placed
nearer to or farther
Fa               x     ~ aC from  each other, at:)FEq~D pleasure, still remaining parallel.  These
wires are balanced by
6   b  -'<ibj~j  two brass balls, b b,
attached to them be-                         low, which are capable of being raised or depressed by means of a screw
cut on the wire; they may thus be so adjusted that
the wires will be moved from their vertical position
by a very slight force, their upper portions rocking
towards or away from each other, without requiring
any motion of the points of support.
365. Cups C and E, being united by a copper
wire, connect cups D and F with the galvanic battery. The current will now traverse A and B in
succession, flowing in the same direction in both,
and they will be seen to incline towards each other.
The motion is slight, but may be made considerable
by breaking and renewing the circuit in correspond20




230          DA VI S'  MANUAL.
ence with their oscillations.  The same effect is
produced by uniting D with F, and connecting C
and E with the battery. If a powerful current is
employed, the wires still attract each other when
separated to a considerable distance, by moving the
points which rest in the mercury to the: farther ends
of the cells; with a feeble battery, the wires should
be placed near to one another.
366. Let the cup C now be united with D, and E
anid F be connected with the battery. This will
cause the current to flow in opposite directions in
the two wires, and they will recede from each other;
the extent of the motion may be increased, as before,
by alternately opening and closing the circuit. Cups
C and D may be connected with the battery with the
same result, E and F being united by a wire.
367. The current, instead of traversing the wires
in succession, may be divided into two portions by
uniting C with D, and E with F, by two wires, and
then connecting the battery either with C and F or
D and E. In this case, the two portions of the current flow in the same direction in A and B. causing
them to attract each other. By uniting C with E,
and D with F, the currents in A and B flow in
contrary directions, and the wires exhibit a mutual
repulsion. The movements produced by a divided
current are feebler than when it traverses the wires
in succession, unless the battery employed is so
powerful that one of the wires singly is not able to
convey the whole of the electricity supplied by it.
368. These attractions and repulsions are some



ATTRACTING CURRENTS.              231
times called magnetic, the two currents, when flowing
side by side, acting upon each other like two magnets presented end to end. In fact, if two short
pieces of iron wire be suspended end to end, and at
right angles to the conducting wires, the magnetism
induced in them by the currents (see ~ 254) will
cause them to exhibit similar attractions and repulsions to those of the wires themnselves.  It is, however, preferable to regard this peculiar action as a
primary one; it being highly probable that the polarity of even a steel magnet is due to electric currents circulating within its substance. The mutual
actions of two magnets, or of a magnet and a current,
would thus be secondary effects, depending upon the
attractions and repulsions just described.
369. It is not essential that the current should
traverse metallic wires in order to produce these
effects. Two streams of machine electricity flowing
through a vacuum, or even through the air, exhibit
the phenomena very satisfactorily.  The attraction
of currents moving in the same direction may be
shown by the following arrangement: Connect the
inner coatings of two Leyden jars with either the
positive or negative conductor of a common electric
machine, their outer coatings being insulated sufficiently from each other to prevent the passage of a
spark between them when the jars are discharged in
the mode about to be described.  With the exterior
coating of each jar is connected a wire having one
end free. These ends are left free for the purpose
of being placed on a card over which the charge is




232           D A V I S S MANUAL.
to be passed.  The common enamelled cards should
be used, as they receive a dark-colored and permanent mark from the passage of the spark over their
surface, A third wire, attached to the discharging
rod, is also to rest on the card, at such a distance
from the other two wires that the sparks from the
jars may be able to pass.  The ends of the wires
proceeding from the outsides of the jars should be
placed a quarter or a half of an inch apart, and nearer
to one another than to the third wire, which is to be
equally distant from both, so that two straight lines
drawn from them to the third wire, would form the
letter V.  The jars being charged (during which
process the exterior coatings should, of course, be
uninsulated), arrange the points as directed, and
bring up the ball of the discharging rod to the conductor. The inner coatings being connected, and
the outer ones insulated, the current is obliged to
divide into two portions as it proceeds from the point
attached to the discharger to those in connection with
the outsides of the jars. The two sparks will thus
pass simultaneously over the surface of the card, and,
were they not affected by each other, would leave
a mark in the shape of the letter V. It will be found,
on the contrary, that the track left on the card will
be more or less in the form of the letter Y, the two
currents coalescing in their passage over its surface.
The result will be the same, whether the jars are
charged positively or negatively on the inside. If
the wire connected with the discharger is placed
under the card while the others are on the upper




C; ONTRACTING  HELIX.              233
side, the card will le perforated in one or imrle places
by the passage of thie electricity.
370. The experiment may be varied, by connecting with the discharging rod a wire whose ends
may both rest on the card at the same distance flom
each other as that between the two wires attached to
Ihe exterior coatings of the jars.  The two sets of
points being arranged parallel to each other, and their
distances properly adjusted, the two currents will
remain separate during the whole of their passage
over the card; and it will be seen by the marks
which they leave, that, instead of proceeding in
straight and parallel lines, they form curves whose
convexity is turned towards each other. The curvature of the lines is greater in proportion to their
proximity; but if the points are placed too near
together, both currents flow in one track, not separating until they reach one of the wires connected
with the outside of the jars.  The resistance of the
air and other causes often occasion a stream of electricity to follow a very crooked path in passing over
a card.  Hence the lines traced by the two currents
in these experiments may be irregular, though the
tendency to converge is perfectly evident.
371. CONTRACTING  HELIX.  The mutual attraction between different portions of the same current
moving in the same manner, may be rendered evident by the instrument represented in Fig. 160. A
wire, coiled into a loose helix, is supported in a
vertical position by a brass pillar connected with
one of the screw-cups on the base board.  The lower
20 *




234            DAVIS' S M S   ANUAL.
end of the hlelix dil)s into rnelcury ct;iltained in a
glass cunt  \Vwith a nietal bottomn, by nieuis of which
the nmercury is brought
i;lto connection  with
thae  other screw-cup.
W hell the battery is
app:llied, tile portions of
tile current traversing
the different turns of
the helix of course flow
parallel to each other,
and in the same direc-. tion.   Their  mutual
attractions  cause  the
turns of the helix to
approach each other, shortening it sufficiently to lift
the end of the wire out of the mercury.  This
interrupts the current, and the helix is lengthened
again by the elasticity of the wire composing it,
producing continued vibration.  A spark is seen in
the glass cup at each rupture of contact.  It is
necessary to adjust the quantity of mercury so that
the wire may be raised out of it when the coil
shortens itself.
372. A bar magnet passed partly within the helix,
as in Fig. 161, causes a more active movement.
One pole of a U-magnet, or an iron bar, produces the
same effect. If a bar magnet or an iron rod, of the
same length as the helix, is passed wholly within,
the turns of the helix contract, above and below
the centre, while near the centre there is no move



ELECTRO-DYNAMIC REVOLVING  COIL. My
ment, the influence of the magnet maintaining it it]
a state of rest. When an iron bar is introduced, it
Fig. 161.      is magnetized by the
current in the helix,
and exerts the same
influence as the steel
magnet.  The action
on each turn of the
helix is the same as
with  De  la Rive's
Ring, (~ 186.)  The
direction of the current  determines  in
which  direction  the
steel magnet must be
introduced, in  order
that the helix may
contract and break the
circuit.
373. ELECTRO-DYNAMIC REVOLVING COIL.- The
mutually attractive and repulsive action of currents
may be made to produce rotations analogous to some
of those strictly called electro-magnetic; as in the
instrument represented in Fig. 162, which consists
of a circular coil of insulated wire, B, fitted to rotate
on a vertical axis within a larger one, A, mounted
on a brass pillar. The inner coil has a pole-changer
fixed to its axis of motion for the purpose of reversing
the current twice in each revolution. The current
may traverse the two coils in succession, or be
divided between them, but its direction must be
changed only in the interior one.




236           DAVIS' S MANUAL.
374. The inner coil being placed at right angles
rco the other, alid the cups on the stand connected
Fig. 162.    with the galvanic battery, the
faces of each coil immediately
exhibit north and south polarity,
l alB llA like. those of De la Rive's Ring
(~ 185); and B is obliged to
make a quarter of a revolution in
order to  bring  its north pole
within the north pole of A, so as
to make their directions correspond. As soon as it reaches this
position, the current is reversed
by means of the pole-changer,
and its south pole now being within the north pole
of A, it continues to move on in the same direction.
The motion, in this case, depends upon the same
principle as those of the wires in the instrument
represented in Fig. 159; but it is more convenient
to refer them to the polarity exhibited by a current
flowing in a circle, as was done in describing the
Revolving Coil, (Fig. 69.)
375. It is, however, easy to explain the revolution
with direct reference to the mutual action of the currents. As these circulate in the same direction in
every convolution of each coil, they may be regarded
as two single circular currents. Now, suppose the
current in A to be ascending by its left side and
descending by its right side.  If B is placed at right
angles to A, with the current descending in the side
towards the spectator, this side will be attracted by




ROTATION'OF M ERCU  Y.            237
the right side of A, and repelled by the left.  The
farther side of B, on the contrary, will be repelled by
the right side of A, and attracted by its left side.
These forces conspire in bringing the two coils into
the same direction; when, the current being reversed
by the pole-changer, each side of B is repelled by
that side of A which is nearest, and the motion is
continued in the same direction.
376. ELECTRO-DYNAMIC REVOLVING RECTANGLE.
-Rectangular coils may be used in place of cirFig. 163.    cular ones, with the same result, as
in the instrument represented in Fig.
163, which is analogous to the one
described in ~ 193. The inner rectangle revolves in the same manner
as the coil B in the instrument last
described, and the rotation is more
rapid than with the circular coil.
In consequence of the width of the
rectangular coils being less than their
height, the sides of the interior one
are near those of the other during
the whole revolution.  This circumstance is the occasion of its greater speed.
377. INSTRUMENT FOR SHOWING THE REVOLUTION
OF MERCURY. — The instrument represented in Fig.
164, is designed to exhibit the revolution of mercury
within a helix. The mercury is contained in a brass
rup, within which is fixed a small watch crystal.
The brass is covered by this, except at the upper'edge, where a ring is exposed, which is amalgamated.




238          1) A VlS          A L.
This- ring is in metallic connection with one of
the screw-cups on the base board.  Surrounding the
Fig. 164.       mercury cup is a helix,
one of whose ends connects with the remaining
screw-cup.  Its other end
~-.-  is left free, so as to be
inserted into the shallow
cup. An iron cup, with
its rim  amalgamated, and a watch crystal secured
within it, may be substituted for the brass cup. The
movement of the mercury is then more rapid, as
the influence of the iron, which becomes an electromagnet, aids that of the helix.
378. Sufficient mercury being introduced, and the
battery connections made, insert the free end of the
wire into the mercury. The current now flows in a
circular direction in the helix, but in the mercury it
passes from the end of the wire to each portion of
the circumference of the cup in the direction of a
radi-Ls. The reaction between the radiating and the
circular currents, causes the mercury conveying the
former to revolve around the point of the wire, that
being the centre from which the currents radiate.
This rotation is rapid, and produces sufficient centrifugal force to make the mercury hollow in the
centre.  As the end of the wire is thus uncovered,
the current is interrupted and the revolution ceases.
When the mercury returns to its level, the current is
renewed, to be again suspended as before. The
rotation is rendered more visible by putting a drop
or two of muriatic acid on the mercury.




ELECTRO-nDYNAMIC THEORY                239
379. The circular currents in the helix may be
regarded as made up of short rectilinear currents,
each of which flows at right angles to the radius of
the helix. Confining our attention to the action of a
single one of these currents, it will be seen that two
only of those passing through the mercury are parallel with it. Of these, one is moving in the same
direction, and is attracted; the other, on the opposite
side of the inserted wire, moves in the reverse direction, and is repelled. These two forces conspire in
giving a motion of rotation to those portions of,the
mercury which are acting as conductors. The two
radiating currents which are precisely at right angles
to the rectilinear one, and also all the oblique currents, are attracted and repelled by the rectilinear
current, in such a manner that they tend to place
themselves parallel with it. As the influence of
every portion of the current in the helix urges each
of the radiating currents to move in one and the
same direction, a rapid revolution of the mercury
conveying them is the consequence. The rotation
of the mercury mentioned in ~ 285, is produced in a
similar manner with that obtained in this instrument.
380. In connection with the subject of the attractions and repulsions of currents, it will be proper
to give a slight sketch of the Electro-Dynamic Theory of Ampere.  This theory ascribes the actions of
currents upon each other, of magnets'upon each
other, and those exerted between currents and magnets, to one and the same cause, the mnutual attractions and repulsions of electric currents.  In a steel




240           D AVI S S  M  A N UA A 1,
magnet, the currents are regarded as circulating in a
uniform direction, at right angles to the axis of the
magnet, around the elementary magnetic particles
(Q 249), forming closed circuits.  The resultant action of such a system of currents would be the same
as that of a series of currents flowing round the
circumference of the magnet at each portion of its
length and at right angles to the axis. This assimilates the actiof of a steel magnet to that of the
electro-magnet, with the current circulating in its
helix, and to the action of the helix itself.  How
such elementary currents are excited and maintained
within the substance of a good conductor of electricity, like steel; has not been ascertained.  It is
supposed that the currents are flowing at all times
around the particles of every body susceptible of
magnetism, but that they neutralize the influence of
each other, being turned indiscriminately in every
direction. By the process of charging, the currents
are brought into a uniforri direction, and magnetic
power is developed.
381. Ampere regards the magnetism of the earth
itself as due to electric currents circulating within
it, from east to west, in planes parallel, or nearly so,
to the magnetic equator.  These currents are probably in great part thermo-electric, due to the action
of solar heat on successive portions of the surface;
but there are also many circumstances favorable to
the formation of galvanic circuits in the interior.
382. The theory of Ampere affords a simple
explanation of the tangential action of a current,




INDUCTION  OF C  R FENT.            241
removing the apparent anomaly with regard to the
direction in which the force is exerted.  When a
conducting wire is brought near a magnetic needle
and parallel to it, the current in the wire is flowing
at a right angle to those in the magnet.  In this
position, their mutual action is the same as that
described in ~ 379. The currents in the needle tend
to arrange themselves parallel to that in the wire;
and to effect this, the needle, if free to move, turns
itself at right angles to the wire.  The continued
revolution of a conducting wire around a magnet,
and of the magnet around the wire, depends upon
the same principle as the rotation of one current
under the influence of another. By substituting a
helix in place of the magnet, the revolutions would
occur in the same manner, on transmitting the current.
383. When two helices, in which currents are
flowing in the same direction, are placed end to end,
they attract each other like the individual turns of
the coil represented in Fig. 160. If one of them is
turned round so as to present its other extremity to
the first, repulsion is exhibited, in consequence of
the opposite directions of the currents. In two steel
magnets, presented end to end, the currents, according to Ampere's theory, act in the same manner as
those in the helices, and the poles attract or repel
one another on the same principle.
384. We now proceed to consider the inductive
action of currents, taking first in order those phenomena which are referable to the induction of a
current on itself.  When the poles of a small gal21




242           D A V IS S   A    A L.
vanic battery, consisting of a single pair of plates,
such as Smee's, or the sulphate of copper battery,
are connected by a copper wire of a few inches in
length, no spark is perceived when the connection is
either formed or broken, or at most a very faint spark
at the moment of opening,the circuit; but if a wire
forty or fifty feet long is employed, though no spark
is seen when contact is made, a bright one appears
whenever the connection is broken by lifting one
end of the wire out of the cup in which it rests.
By coiling the wire into a helix, the spark becomes
more vivid.
385. The most advantageous length for producing
the spark depends upon the diameter of the wire, and
also upon the quantity and intensity of the battery.
With a wire one sixteenth of an inch in diameter,
and a single pair of Smee's, or the small sulphate of
copper battery, a length of 50 or 100 feet will probably give the best result.  An increased effect is
obtained by employing a longer wire, of greater
diameter.  If a battery of higher intensity is used,
such as a single pair of Grove's, the length of the
wire may be still further increased. The greater is
the quantity of the battery, the larger or shorter
must be the wire, in order to transmit the whole
current, and obtain the brightest spark.  This peculiar action of a long conductor, either extended or
coiled into a helix, in increasing the intensity of the
current from a single galvanic pair, at the moment
when it ceases to flow, was discovered by Professor
Henry, in 1831.




RI B BON  SPIRAL.              243
386. With a wire two or three hundred feet long,
a slight shock may be felt at the moment of opening
the circuit, if its ends, near their connections with
the poles, are grasped with moistened hands; with a
shorter wire, shocks may be obtained through the
tongue; their intensity increases until a length of
five or six hundred feet is attained. A single pair of
plates gives no shock directly; the peculiar and continuous sensation excited in the tongue when the
current from a single pair is made to pass through it
not being called a shock. With a battery of small
size, or consisting of a number of pairs, greater
lengths may be used with advantage for the shock,
as well as for the spark. The maximum effects of a
small battery are, as might be expected, much inferior to those of a large one. If the requisite lengths
of wire are exceeded, the effects are lessened.
387. The brilliancy of the spark is greatly increased by employing a ribbon of sheet coppet& coiled
into a flat spiral, instead of a wire.  A description
and figure of this instrument have been given in
~ 112. The spiral being connected with the battery, a brilliant spark will be seen, accompanied by
a pretty loud snap, whenever contact is broken; and
if two metallic handles are attached by wires to the
cups of the coil, and held in the hands, a slight shock
will be felt.  If tie battery is in feeble action, the
shocks may be perceptible only when passed through
the tongue.  No shock can be obtained by interposing the body in the direct circuit with the coil, so
that the battery current may traverse them in suc



244            D AVI S'S MANUAL.
cession; as the electricity supplied by a single pair
of plates is of too low intensity to be transmitted, to
any considerable extent, by so poor a conductor as
the human body.  Professor Henry was the first to
employ coils of metallic ribbon for obtaining sparks
and shocks from a single pair of plates.
388. CLOCK-WORK ELECTROTOME.- For the purpose of interrupting the circuit rapidly, in either a
long wire or a spiral, the instrument represented in
Fig. 165 is very convenient. In the cut, it is shown
Fig. 165.
in connection with a ribbon spiral and a single pair
in connection with a ribbon spiral and a single pair
of Grove's battery.  It consists essentially of a bSent
copper wire, which, by means of clock-work set in
motion by a spring, is made to vibrate rapidly, dipping its ends alternately into two glass cups, intended
to contain mercury.  The spring is wound up by
turning a milled head. The glass cups are open at
the bottom, to allow the mercury to come in contact
with the brass pillars into which they are cemented
These pillars are both connected with one of the
screw-cups on the base board; the other screw-cup




CLOCK-WORK E L EC ROTO E.  245
communicates with a brass cup for holding mercury
on the top of a third pillar. Into this dips a vertical wire attached to the vibrating wire.  Sufficient
mercury must be put into the brass cup to keep the
end of the vertical wire covered, and enough into the
glass cups to allow one end of the vibrating wire to
leave the mercury in its cup a little before the other
end dips into its portion.
389. The clock-work electrototne mav be advantageously used in connection with many of the
instruments for affording sparks and shocks, which
will be described subsequently. The current must
be transmitted through the two instruments in succession, by connecting one of the screw-cups with
one pole of the battery, and the other screw-cup with
one of those attached to the spiral or other piece of
apparatus, the remaining cup of which is to com.
municate with the other pole of the battery. It is
better to break the circuit mechanically in this way,
when it is desirable to obtain the best possible sparks
and shocks, rather than by means of any interrupting
apparatus worked by the battery itself, as a considerable part of the power of the current is then expended
in giving motion to the interruptor.
390. On making connection with a flat spiral,
in the manner shown in Fig. 165, and turning the
milled head to put the vibrating wire in motion, a
brilliant spark will be seen at each rupture of contact,
accompanied by a loud snap, and causing combustion
of the mercury at the point where the spark occurs.
With a battery consisting of a few pairs of plates of
21 *




246           D A V I SS  MANUAL.
large size, the size of the spark will be greatly increased, and the snap become as loud as the report of
a Leyden jar. The shock will also be pretty strong,
and may be increased by covering the mercury in
the glass cups with a stratum of oil. A shock may
be obtained, especially when oil is used, on closing
the circuit as well as on opening it, though inferior
to that given in the latter case; a faint spark is also
sometimes seen when the wire dips into the mercury.
391. The requisite length and thickness of the
copper ribbon to give a maximum result depend upon
the size of the battery employed. With spirals of
considerable length, even if the copper be pretty
thick, two or three pairs of plates are better than
one, as the metal opposes some resistance to the passage of a current of low intensity. A ribbon spiral
of moderate length interposed in the circuit of a
compound battery, consisting of a considerable number of small pairs, produces scarcely any peculiar
effect; while a coil containing three or four thousand
feet of fine insulated wire will give an intense shock,
though not a very brilliant spark, under the same
circumstances. The higher the intensity of the electricity and the smaller its quantity, the less is the
size requisite in the metallic conductor, and the
greater may be its length.
392. The sparks and shocks given by long wires
and by spirals are due to secondary currents induced
in the metallic conductor at the moments of opening
and closing the circuit.  Their intensity is higher
than that of the current which produces them, and




S E C O N D ARY CU R R E N T S.   247
which, in this connection, is called the primary current. The phenomena belong to the same class as
those presented by the secondaries induced in another
conductor placed in the vicinity of the one which is
conveying the battery current.
393. These secondary currents may be separated
from the primary by placing a second flat spiral of
copper ribbon over the one through which the battery current is transmitted, as represented in Fig.
166.  Two wires being connected with the cups
Fig. 166.
belonging to the upper -spiral, rub their ends together while the circuit through the lower one is
rapidly broken. Sparks will be seen, and slight
shocks may be felt through the fingers, or by placing
the wires in the mouth. When the ends of the
wires are joined, the sparks and snaps given by the
spiral connected with the battery are considerably
diminished, and no shocks can be obtained from it.
394. Connect the cups of the upper coil with a
delicate galvanometer, such as that represented in
Fig. 39. Whenever the battery circuit is completed
-through the lower spiral, the magnetic needle is




248           DAV IS S MANUAL.
deflected to a considerable extent, but immediately
returns to the meridian, indicating the flow of a
momentary current through the wire of the galvanometer. On opening the circuit, a similar transient
deflection occurs in the opposite direction. There is
no deflection while the battery current is flowing
steadily. Care should be taken that the galvanometer is placed at such a distance from the lower
spiral, that its needle may be unaffected by it.  The
spiral is able to deflect a delicate needle at a considerable distance, in the manner described in ~ 276,
and if the galvanometer is found to be at all affected
by making and breaking contact with the battery,
when unconnected with the upper spiral, it must be
farther removed.
395. A sewing needle will be magnetized if placed
within a helix of small internal diameter connected
with the upper spiral. The polarity produced by the
current which attends the completion of the circuit,
is the reverse of that communicated by the one
attending its interruption. If both currents are allowed to act on the needle, it acquires little or no
magnetism, as they flow in opposite directions, and
neutralize each other's influence. The helix should
consist of a single length of wire, wound so as to
form eight or ten layers of coils, to enable it to be
used with currents of considerable intensity.  Its
power will be greater if its internal diameter is very
small.
396. As was stated in ~ 39'2, the momentary waves
of electricity excited by electro-dynamic induction in




SECONDARY  CURR E NTS.            249
a conductor conveying a current, or in a neighboring one, are termed secondary currents. The wave
which accompanies the closing of the circuit is termed
the initial secondary, and flows in the opposite direction to that of the current which induces it.  The
other, which follows the opening of the circuit, is
called the terminal secondary, and flows in the same
direction as the inducing current.  These currents
were discovered by Professor Faraday, in 1831.
They are, probably, occasioned by the disturbance
of the natural electricity of the wire by the influence
of the primary current.
397. In Fig. 167, a coil of fine insulated wire, W,
is represented placed over a flat spiral, A, one of
Fig. 167.
whose screw-cups is connected by a wire with the
cup P, attached to the platinum plate of a single pair
of Grove's battery. A wire from the cup Z, belonging to the zinc plate, is drawn over a steel rasp
resting on the other cup of the flat spiral, for the
purpose of breaking the circuit rapidly.
398. The ends of the wire coil, W, being fixed in
the screw-cups of the metallic handles shown in the




'250           DAVIS'S MANUA L.
cut, powerful shocks will be felt when these are
grasped in the hands, and the wire connected with Z
drawn over the rasp.  In order to obtain the initial
and, terminal shocks separately, the current should
be broken, not by means of the rasp, but by a
cup containing mercury, into which onle of the battery wires call be dipped at pleasure. The mercury
should, of course, be connected by a wire with one
of the cups attached to the flat spiral.
399. When a battery of a single pair of plates is
employed, the initial secondary is much inferior in
intensity to the terminal, and consequently gives a
feebler shock. Professor Henry discovered that the
intensity of the terminal current is very little increased by adding to the number of pairs; the slight
increase which occurs is due to the greater quantity
of electricity transmitted by the ribbon spiral, when
the intensity of the battery current is increased.
With the initial secondary it is different; every additional pair is found to raise its intensity, so that
with about ten pairs it equals, in this respect, the
terminal, and with a larger number excels it.  The
initial shock may also be increased, though not in
any great degree, by employing a shorter flat spiral,
as, for instance, one fifteen or twenty feet in length,
with a single pair of plates. In quantity, as indicated
by the galvanometer, the two secondaries are equal;
those of the wire coil being inferior in this respect to
the currents afforded by a ribbon coil.
400. The coil represented at W contains three
thousand feet of copper wire, about one-fiftieth of an




DOUBLE  HELIX  AND  ELECTROTOME. 251
inch in diameter, wound with thread; the layers are
firmly cemented together by shellac, careful insulation being requisite, in consequence of the length
of the wire and the high intensity of the current
obtained. Where a small battery is used, this length
of wire is unnecessary, as the shock given by it is
scarcely greater than that from a coil of one thousand
feet. With a larger battery, the longer one will be
much superior. A sewing needle may be magnetized
by the currents from a long wire coil, as well as by
those from a flat spiral. If the wire is fine and very
long, this effect will be diminished.
401. Instead of flat coils, long helices of insulated
wire may be employed for obtaining the secondary
currents, though with less effect when not aided by
magneto-electric induction.  Several of the magnetoelectric instruments, which will be described under
the next head, may be used for this purpose, the iron
bar or bundle of wires being withdrawn from the
helices. A description of one (the double helix and
electrotome) may properly be introduced in this conniection.
402. DOUBLE HELIX AND ELECTROTOME. -In this
instrument, represented in Fig. 168, the double helix,
a a, is confined to the base board by three brass bands.
The inner helix is composed of several strands of
large insulated copper wire.  The similar ends of
these strands at one extremity of the helix are connected with the screw-cup c. Their other ends are
soldered to the middle brass band, which is strmollnted b)v a brass cup, e, for holt il II merc1Irv.




262          DAVIS S MANUAL.
Into this cup descends a copper wire attached to the
wire u TV, which, by means of clock-work set in
motion by a concealed spring, is made to dip its ends
alternately into the glass cups, G. G, which are to
contain mercury. The cups being open at the bottom, the mercury is brought il connection with the
outer brass bands, upon which they are fixed. Both
these bands are connected with a screw-cup c', corresponding to c, but not seen in the cut. A second
helix, consisting of about two thousand feet of fine
insulated wire, encloses the one just described, but is
insulated from it; its ends are soldered to the screwcups to which the handles, seen at H, are attached.
403. The cups c and c' being connected with the
galvanic battery, the current will pass through the




DOUBLE HELIX  AND  ELECTROTOME. 253
inner helix whenever either end of the wire w w
dips into the mercury, which should stand at such
a height in the cups that both extremities of the wire
shall not be immersed at the same time. By turning
the milled head, d, the spring is wound up, and the
wire is made to vibrate rapidly. When either end
leaves the mercury, the flow of the current is interrupted, and a bright spark is seen in the cup. If the
handles are grasped with moistened hands, strong
shocks will be felt whenever the circuit is broken.
Introduce into the helix a brass tube, and the spark
becomes small and the shock feeble; if the tube is
sawn open in the direction of its length, it no longer
produces these effects.
404. When an iron bar or a bundle of soft iron
wires is introduced into the helix, the brass tube
being withdrawn, the brilliancy of the sparks and the
intensity of the shocks areI greatly increased, the
instrument being, under these circumstances, one of
the most powerful belonging to the department of
magneto-electricity.
405. We have seen that a battery current of considerable quantity and low intensity can induce either
a quantity or an intensity current.  By substituting
for the ribbon spiral, through which the battery current is transmitted, a coil consisting of one thousand
feet or more of fine insulated wire, and connected
with a battery of a number of pairs, it will be found
that an intensity current is able to induce secondaries
of intensity in a wire coil, and of quantity in a
ril)bon coil.
22




2b4           DAVIS'S  MANUAL.
406. The shocks obtained when the body is introduced into the circuit of a voltaic battery of a considerable number of pairs, without a coil, appear' to
be due to secondary currents induced in the battery
itself.  During the uninterrupted circulation of the
galvanic current through the body, little or no effect
is perceived; but at the moment of either opening or
closing the circuit, a shock is experienced. When
the series is very extensive, a dull pain is felt during
the continuance of contact.  The primary current
has sufficient intensity to traverse the body, though
not to give shocks, and doubtless induces initial
and terminal secondaries when it commences and
ceases to flow.
407. A flat spiral being in connection with the
battery, let a fine wire coil be placed at a little distance above it; shocks may now be obtained from
the wire, but their intensity diminishes in a rapid
ratio as the distance between the coils is increased.
With  the arrangement represented  in Fig. 1.69,
shocks through the tongue are readily obtained when
the wire coil is a foot or two above the other; and
the distance may be still fuirther increased by using
a larger ribbon coil or a more powerful battery.
This furnishes a convenient mode of regulating the
intensity of the shock at pleasure; the same effect
is produced when one coil lies upon the other, by
sliding the wire coil from its central position more or
less beyond the edge of the flat spiral. The shocks
are in any case much increased by wetting the
hands, especially with salt water. If. however, the




SHOCKS  VARIED.                2"5
quantity of the secondary current is very small, the
shock is lessened by improving the conducting power
of the skin. This may happen when the secondary
coil consists of very fine wire and the battery currentt is feeble.
408. The intensity of the shocks diminishes rapidly as the wire coil is raised from a horizontal
position into an inclined one; and when it reaches a
vertical position, its edge resting on the ribbon coil,
they are no longer felt.  Similar phenomena are presented when the flat spiral has a sufficiently large
central opening to allow the wire coil to pass within
it; no shocks being obtained when their axes are at
right angles to each other.  If the diameter of the
wire coil is considerably less than that of the opening, and it is placed in a horizontal position within
the spiral, the shocks are somewhat stronger when
it is near the side than when in the centre.
409. The interposition of any good conductor of
electricity between the fine wire coil and the one
Fg. 169.      connected with the battery nearly neutralizes the
shocks.  The coils being
arranged as represented in
Fig. 169, interpose a slip
W        of wood or a plate of glass
TV     I _          between A and W, and
the shock will be the same
as if air only inte'rvened.
This will be the case with any non-conductor of electricity.  Now, interpose a lplate of metal, for instance,




256           DAVI S'S MANUAL.
lead or zinc, one tenth of atn inch thick, and as broad
as the coils. The shock will be so mnuch reduced as
to be scarcely perceptible.  If the interposed plate
is broader than the coils, the reduction in the shock
is still greater. The magnetizing power of the current is also lessened, in respect to hard steel; thus,
a sewing needle, placed within a helix, will be but
feebly charged. A certain thickness of metal is required to produce these effects, as several sheets of
tin foil may be interposed without diminishing the
shocks in any appreciable degree.
410. The interposition of a metallic plate does not
prevent the occurrence of the secondary currents, but
causes a great reduction in their intensity. That
the' quantity of the current is not affected, may be
shown by connecting the ends of the upper coil,
especially if it be a ribbon coil instead of a wire one,
with a galvanometer.  The deflections will be the
same whether the plate is interposed or not, provided
the distance between the two coils is not altered'
but when the plate is of iron, the deflections are
somewhat diminished.
411. In Fig. 169, M represents a metal plate, from
which a slip is cut out in the direction of a radius,
the cut extending to the centre. When such a plate
is interposed between the coils, the shocks are not
at all lessened.  Instead of a metallic plate, interpose
a flat spiral between the battery coil and the wire one.
No diminution of the shocks will be perceived. Now,
connect the cups of the interposed coil by a wire,
and the intensity of the shocks will be even more




5 EC ON D ARY  CURRENTS.           257
reduced than by a plate of metal. Whenever the
shocks are diminished, the brilliancy of the sparks
given by the battery spiral are also lessened to some
extent.
412. Secondary currents may also be obtained,
without breaking the primary circuit, by altering the
quantity of the battery current or the distance between the coils. A flat spiral being connected with
a single pair of plates, place a second spiral of the
same kind upon it, with its cups in connection with
a galvanometer. While the current is flowing steadily through the lower spiral, no secondary is excited,
and the needle of the galvanometer is unaffected.
Now, lift one of the plates of the battery partly out
of the liquid. The moment the plate begins to be
raised, the needle moves in the same direction as if
the circuit were broken; the deflection, however, is
not momentary, as in that case, but continues during
the movement of the plate. Then, without taking
the plate out of the solution, which would break the
circuit, depress it again. The galvanometer will
now indicate a current in the opposite direction to
the former one.
413. Similar currents are produced by raising the
upper coil from the lower one, through which the
galvanic current is steadily flowing.  As the coil
recedes, a secondary flows through it in the same
direction as that of the battery current in the other
spiral; as it again approaches, a current in the
reverse direction is induced.  Instead of raising the
urpper spiral, it may be moved laterally from  its
22 *




258           DAV IS'S MIANUAL.
central position oni the lower one with the same
result.
414. These currents produce a greater effect upon
the galvanometer than those excited by closing and
opening the circuit, as they are not momentary, but
last as long as the motion continues.  The more
rapid the movement of the battery plate, or of the
spiral, the more powerful are the secondary currents,
as they depend upon the suddenness of the change
in the quantity of the primary current in the one
case, and in the distance between the coils in the
other.  They are, however, of low intensity, and
are unable to afford shocks.  The interposition of
metallic plates or coils produces no effect upon them.
415. The neutralizing action described in   409
and ~ 411, is due to a secondary current excited in the
interposed metallic plate or spiral, which itself induces a tertiary current in the wire coil, flowing in
an opposite direction to the secondary induced in it
by the battery current, and therefore retarding its
development.  A tertiary current is also induced in
the battery coil, which occasions the reduction in the
spark and shock noticed in ~ 393 and ~ 411. When
the interposed plate of metal is divided to its centre,
no secondary is induced in it, and it exerts no neutralizing action; the same is the case with the ribbon
spiral in ~ 411, when its cups are disconnected.
Similar phenomena are produced by the introduction
of a metallic tube into a wire helix, as described
in ~ 403.
416. Thjis tertiary curl nt can be separated from




T E RT IARY  CU RR  NTS.'259
the secondary, and obtained by itself, ill tile following
manner.  A ribbon coil B (Fig. 170) being laid upon
Fig. 170.
the coil A, through which the battery current is
transmitted, connect' its cups with those of a third
spiral, C, of the same kind, removed to a little distance, so as to be beyond the influence of the current in A. The secondary current induced in B will
now flow through C, and if a fine wire coil, W, is laid
on C, strong shocks may be obtained. If W is raised
up, the shocks are still felt when it is at a considerable height above C.
417. By placing a fourth ribbon coil on C instead
of the wire coil, a quantity current will be obtained,
capable of affecting the galvanometer slightly, and
of magnetizing a-sewing needle placed in a helix of
small i ternal diameter.  Let two fine wire coils be
substituted for the flat spirals B and C. A secondary
intensity current will now be obtained, which will
induce a tertiary of intensity in a third wire coil laid
on the second, enabling it to afford strong shocks, or
a tertiary of quantity in a ribbon coil.
418. If the second spiral, B, is alone replaced by a
wire coil, little or no shock can be obtained from W,




*260          D A V I' S MANUAL.
the quantity of the secondary current futrnished iby
the wire coil nlot being sufficient for the productioll
of a powerful tertiary, unless it is passed through a
conductor of many convolutions.  So, on the other
hand, if a fine wire coil is substituted for C only, no
tertiary is induced by it, or at most a feeble one, the
secondary current from B not having sufficient intensity to enable it to overcome the resistance of the
long wire.  The tertiary current, like the secondary,
can be induced at a distance, and has its intensity
greatly reduced by the interposition of metal between
the flat spiral C and the wire coil.
419. The tert.. y currents may be conveniently
obtained by cat. ng the secondary from  a ribbon
spiral to flow tkh. ugh the inner helix of the instrument represented in Fig. 168, or of almost any of the
magneto-electric instruments, which will be described
under the next head. Thus, if the wires attached to
B, in Fig. 170, are fixed in the cups c and c' of the
double helix and electrotome, strong shocks may be
obtained from the tertiary current induced in the
fine wire helix. The circuit through the inner coil
should not be broken by the electrotome, as the only
interruption wanted is that in the battery current.
The shocks are increased by placing a bundle of iron
wires within the helix, as the inductive action of the
current is then assisted by that of the electro-magnet.
420. Tertiary currents, like secondaries, are induced both when the primary circuit is opened and
when it is closed.  The initial and terminal tertiaries
b(t 1h flow  in the opposite directions to the corres;?olding secondaries.  In fact, each secondary must




TERTIARY  CURRENTS.              261
produce two tertiaries, one when it commences, and
another when it ceases to flow; but in consequence
of the exceedingly short duration of the secondary
itself, they cannot be separated as the initial and
terminal secondaries can; and the current which is
obtained is only the difference between the two.
This accounts for the slight effect it produces upon
the galvanometer, while capable of affording strong
shocks. The two parts may differ very much in
intensity, but, being equal in quantity, would not
affect the galvanometer, did they occur precisely at
the same instant.  The needle, however, is first
deflected by the momentary wave induced by the
commencement of the secondary, and, as soon as it
has moved a degree or two, is arrested by the
wave due to its cessation, and carried in the opposite direction.
421. The effects of the interpositions described
in ~ 409 and ~ 411 may now be more fully explained.
The secondary induced in the interposed conductor,
on opening the primary circuit, itself induces a tertiary in the wire coil at the instant of its commencement, which flows against the secondary induced in
it by the battery current. When the secondary in
the interposed body ceases, another tertiary is excited
in the wire coil flowing in the same direction as the
secondary. The total amount of the current is not
altered, since the same quantity is added at its ending
as was subtracted at its beginning; but its intensity
is greatly reduced, probably in consequence of th(
diminished rapidity of its development.




262            DAVIS' S MANUAL.
422. CURRENTS OF HIGHER ORDERS.-It has been
shown that a secondary current, though only momentary in its duration, can induce a tertiary of
considerable energy.  It might therefore be expected
that the tertiary would produce a current of the
fourth order; this another, and so on; and such is
found to be the case.  It is only necessary to remove
the tertiary out of the influence of the secondary, in
the same manner as the secondary is removed from
that of the primary (see ~ 416), in order to obtain a
current of the fourth order.  The currents of the
third, fourth, and fifth orders were first obtained by
Professor Henry, and two other orders have been
since added. These currents progressively diminish
in energy, but the phenomena presented by them are
similar to those of the tertiary. With a larger number of coils and a powerful battery, the series might
doubtless be extended much farther.
423. In the following table, the directions of the
currents produced both at the beginning and ending
of the battery current are given; those which flow in
the same direction as the primary being indicated by
the sign +, and those in the opposite direction by
the sign —.
Adt the beginenivg.    dt the ending.
Primary current,.......... +...... - +
Secondary current,.............. +
Tertiary current,.....    +-.....
Current of the fourth order,.... --...... +
Current of the fifth order,..... +...... -
Current of the sixth order,.... -..... -
Current of the seventh order,...... -




CURRENTS  OF H IGHER  ORDERS. 263
If the induction at the ending of the battery current
is regarded as opposite to that at the beginning, the
second column may commence with minus instead
of plus, and the second series will then alternate
like the first.
424. Induced currents of the different orders may
be obtained from frictional electricity, though, in consequence of its high intensity, the conductors require
better insulation than is necessary when they are
used with the galvanic battery. The flat spirals and
wire coils may, however, be employed, if their layers
are carefully insulated by means of shellac, or if
covered with silk instead of cotton.
425. Let a fine wire coil be placed over a ribbon
spiral, with a plate of glass interposed; a secondary
shock may now be obtained from the wire when the
charge of a Leyden jar is passed through the spiral.
A still better mode is to employ a second wire coil,
instead of the fiat spiral; if the ends of one of the
coils are held in the hands, a strong shock will be
felt at the moment of discharging the jar through the
other. The secondary current flows in the same
direction as the one which induces it; as may be
shown by passing it through a helix consisting of
several layers of coils, when it will magnetize a
sewing needle placed within it.




264          DAVIS S MANUAL.
II. BY THE INFLUENCE OF A MAGNET.
426. The name of Magneto-Electricity is given
to that branch of science which treats of the development of electricity by the influence of magnetism.
Electric currents are excited in a conductor of
electricity by magnetic changes taking place in its
vicinity. Thus the movement of a magnet near a
metallic wire, or near an'iron bar enclosed in a wire
coil, occasions currents in the wire. They are also
produced at the moment of commencing and stispending the flow of a galvanic current through the
coil surrounding an electro-magnet. In this case, an
induced current may be obtained either from the
wire conveying the primary current, or from a second wire also surrounding the iron; but the current
excited by the influence of the magnetized bar is
obtained in connection with that which is the result
of electro-dynamic induction, and cannot be separated
from it, at least with the usual arrangement of the
wires. The discovery of the induction of electricity,
both by steel magnets and by electro-magnets, was
made by Professor Faraday, in 1831, during the same
course of experiments which led. to the discovery of
electro-dynamic induction.
427. Let the heliacal ring (~271) be connected
with a gold leaf galvanoscope, as represented in Fig.
171. By passing the ring over one of the poles of
a steel U-magnet, the gold leaf xwill be sensibly




MAG N E TO-E L E  CTRIC IT Y.      265
deflected during the continuance of the motion. On
withdrawing the ring from the pole, a deflection in
the opposite direcFig. 17tion wvill occur, and
in the same direction as that obtained
Nt;    4by passing it over
the other pole. The
heliacal ring may be
connected with  a
delicate galvarlometer, such as is described  in ~ 162,
with the same re-  sult.
428. If the helix represented in Fig. 107 is connected with the
galvanometer, and a bar magnet or one pole of a Umagnet is introduced into it, the needle of the galvanoimeter is deflected while the magnet is passing
in, but returns to its former position as soon as the
magnet is at rest within the coil.  On drawing the
magnet out, the needle is deflected in the opposite
direction.  By moving the magnet in and out so as
to keep time with the oscillations of the needle, they
are greatly increased.  Reversing the direction of
the magnet, so as to make it enter by the contrary
pole, reverses the indications of the galvanometer.
If the bar magnet is carried through the helix, and
brought out at the opposite end to that by which it
entered, the effect is the same as if it had been dray -:
23




266           D AVIS' S M ANUAL.
out at the saine end.  No current is excited while
the magnet and helix are both at rest.
429. Connect the cups of a flat spiral (Pig. 112)
with the galvanometer, and ipass a U-magnet over
it, towards the centre, with one of its poles above and
the other below. The needle will be deflected iln
opposite directions as it passes on and off.  A less
effect will be produced by moving a bar magnet in
the direction of a radius over the spiral, or by passing
it into the central opening.
430. Place a bar of soft iron within the helix
(Fig. 107), and connect its cups with those of a
galvanometer. Then bring either pole of a steel
magnet in contact with one extremity of the iron.
The bar instantly becomes magnetic, and the galvanometer needle is deflected by the current produced by this means in the helix. It, however, immediately returns to its former position, the settled
magnetic condition of the bar having no power to
excite a current.  On withdrawing the magnetic
pole, the bar loses its magnetism, and the needle is
deflected in the opposite direction.
431. By alternately bringing the magnet in contact with the iron, and withdrawing it in such a
manner as to keep time with the vibrations of the
needle, they may be greatly increased.  If the opposite poles of two bar magnets are brought in contact
with the ends of the iron bar, and especially if their
other poles are made to touch, so that the two magnets
form  the letter V, the inductive influence is Tnuch
greater.  By opening and shnltt;ng the miagnets, as




MAGNETO-ELECTRICITY.              267
if joined by a hinge at the vertex, the magnetism of
the bar within the helix may be communicated and
removed at pleasure.
432. When an armature or any piece of soft iron
is brought in contact with one or both of the poles
of a magnet, it becomes itself magnetic by induction,
and by its reaction adds to the power of the magnet.
On the contrary, when taken away, it diminishes the
power of the magnet.  This alteration in its magnetic state induces a current of electricity in a coil
surrounding it, as may be shown by passing a wire
coil, whose ends are connected with a galvanometer,
over one of the poles of a U-magnet, as represented
in Fig. 171, keeping the magnet and coil stationary.
The needle will now be deflected -in one direction
when an armature is applied to the poles, and in the
opposite direction when it is removed.
433. The most powerful effects are obtained by
causing a bar of soft iron, enclosed in a helix, to
revolve, by mechanical means, near the poles of a
steel magnet. The principle may be illustrated by
connecting the cups, A and B, of the revolving electro-magnet (Fig. 146) with a delicate galvanometer,
as represented in Fig. 172. On causing the iron bar
to rotate rapidly, by drawing the hand over the axis,
the needle is powerfully deflected. As the iron approaches the poles of the steel magnet, it becomes
magnetic; as it recedes from them, its magnetism
disappears. While one end of the bar is leaving the
north pole, or approaching the south pole, the electric
current flows in a constant direction, the loss of south




268           D A V I SS MANUAL.
polarity by the iron, and the production of north
polarity in it, inducing currents in the same direcFig. 172.
tion. During the other half of the revolution, the
same end of the bar is approaching the north pole
or receding from the south, and a reverse current is
produced. The two currents are conveyed to the
galvanometer in one direction by means of the polechanger on the axis of the revolving bar. The polechanger, in place of altering the direction of a current
conveyed to the coil, as it does when the bar is
made to revolve by a galvanic battery, here performs
the duty of bringing two opposite currents from the
coil into one course.  The current traversing the
wire of the galvanometer is, consequently, not reversed, except by changing the direction of the
rotation.




MAGNET — ELECTRIC  MACHINE. 269
434. If the cups A and B are connected with the
cups c and c' of the double helix and electrotome,
slight secondary shocks, which may sometimes be
felt in the hands, will be obtained from the fine wire
helix by rotating the iron bar, as in the cut. The
hollow of the double helix should be filled with iron
wires, and the vibrating wire be put in motion so
as to break the circuit rapidly.
435. With a sufficiently delicate galvanometer,
any of the electro-magnetic instruments in which
motion is produced by the mutual action between a
galvanic current and a steel magnet, may be made to
afford a magneto-electric current by producing the
motion mechanically. In all cases, the current excited flows in the opposite direction to the galvanic
current which would be required to produce the same
motion.
436. When a galvanometer is used in these experiments, it must be placed at such a distance from
the instrument where the movements and magnetic
changes are made, that the needle shall not be deflected by any influence but that which reaches it
through the connecting wires. With the gold leaf
galvanoscope this precaution is unnecessary.
437. MAGNETO-ELECTRIC MACHINE. -In the instrument represented in Fig. 173, an armature, bent
twice at right angles, is made to revolve rapidly, in
front of the poles of a compound steel magnet of the
U form. The U-magnet, whose north pole is seen
at N, is fixed in a horizontal position, with its poles
as near the ends of the armature as will allow the
23 *




270          DAVIS'S MANUAt.
latter to rotate without coming in contact with them.
The armature is mounted on an axis, extending from
Fig. 173.
the pillar P to a small pillar between the poles of the
magnet. Each of its legs is enclosed in a helix of
fine insulated wire. The upper part of the pillar P
slides over the lower part, and can be fastened in any
position by a binding screw. In this way the band
connecting the two wheels may be tightened at
pleasure by increasing the distance between them.
This arrangement also renders the instrument more
portable than would otherwise be the case.
438. By means of the multiplying wheel W, which
is connected by the band with a small wheel on the
axis, the armature is made to revolve rapidly, so that
the magnetism induced in it by the steel magnet
is alternately destroyed and renewed in a reverse
direction to the previous one. When the legs of
the armature are approaching the magnet, the one
opposite the north pole acquires south polarity, and




MAGNETO-ELECTRIC CURRENTS. 271
the other north polarity. (See ~ 228.) The magnetic
power is the greatest while the armature is passing
in front of the poles. It gradually diminishes as the
armature leaves this position, and nearly disappears
when it stands at right angles with the magnet. As
each leg of the armature approaches the other pole
of the U-magnet, by the continuance of the motion,
magnetism is again induced in it, but inl the reverse
direction to the previous one.
439. These changes in the magnetic state of the
armature excite electric currents in the surrounding
helices, powerful in proportion to the rapidity with
which the magnetic changes are produced. The
helices are so connected as to form a continuous coil.
The ends of this coil are soldered to the segments
of a pole-changer (Fig. 70), secured on the axis.
Two silver springs press upon these segments,
and convey the electricity to the two screw-cups.
Although the currents in the helices are reversed
twice in each revolution, they are turned into one
direction by means of the pole-changer. From the
manner in which they are obtained, they necessarily
vary more or less in power in the different parts
of the revolution, according to the position of the
armature.
440. This primary magneto-electric current has
too low an intensity to afford strong shocks.  But
secondary currents may be obtained by interrupting
the primary circuit, as with the galvanic current.
(See ~ 392.) These have a much higher intensity,
and give powerful shocks. One of the springs press




272            DAVIS' S MANUAL.
ing on the pole-changer is fixed by a binding screw
in B, and the other in a similar pillar opposite to B.
They are bent at right angles, and when both the
horizontal and vertical portions are made to touch
the segments of the pole-changer, the circuit is
broken as the armature revolves.  The horizontal
parts simply bring the two primary currents into one
course. For showing the sparks, another wire-spring,
fixed in the pillar, B, is made to play upon steel pins
set in the small wheel on the shaft.
441. Shocks are obtained at every part of the
revolution; but, with a moderate speed, they are
most powerful when the legs of the armature are
near the magnet. If the motion is very rapid, this
difference is less appreciable, and with a powerful
machine, the torrent of shocks which then results
becomes insupportable.  The muscles of the hands
which grasp the handles are involuntarily contracted,
so that it is impossible to loosen the hold.  The
shocks are, however, instantly suspended by bringing the metallic handles into contact. A spark is
seen when the wire fixed in the pillar B leaves each
of the pins on the wheel.
442. In Fig. 173, the metallic handles are represented in connection with the screw-cups, for the purpose of giving shocks. When these are held in the
hands, the arm connected with the negative cup will
be found most affected by the shocks. This is a
physiological phenomenon, the current producing a
greater effect upon the arm in which it flows downwards, in the direction of the ramification of the




M A GNE T O-E L E cTRI C  S H 0 c KS.   273
nerves, than upon the one ill which it ascends.  The
initial secondary is too feeble to afford shocks, so
that only the terminal secondary need be taken into
account. The intensity of the terminal shock is,
however, constantly varying, according to the position of the armature in respect to the magnet, and
the difference in the effect upon the two arms is not
so distinctly marked as with some of the instruments
which will be described hereafter.
443. The shocks may be'regulated to some extent, by varying the speed with which the armature
is made to revolve. They are considerably lessened
by partially neutralizing the power of the steel magnet, by placing an armature across it near the poles;
also, by passing the current through a piece of wet
cotton wicking, a few inches long, one end of which
is connected by a wire with one of the screw-cups
on the base board, from  which the handle is removed. By grasping in one hand the handle connected with the other screw-cup, and in the remaining hand the disconnected handle, and touching a
short wire attached to this to the wet cotton, the
shock is diminished in proportion to the length of
the imperfect conductor which the current is obliged
to traverse.  The handles represented in Fig. 173
are of German silver.  Between the metallic part
of each handle and the screw-cup attached to it, is
interposed a cylinder of wood. No shock is felt when
one or both are held by the wooden portions.
444. Slight shocks may be obtained from  the
primary current, by grasping the metallic handles




274           DAV iS S  iANUAL.
connected xwilh the sleelw-cupis. The wire at B, which
plays upon the pills, must be removed, so that the
circuit shall not be broken.  It is also essential
that neither of the springs pressing on the polechanger should leave the segment which it touches
before it comes in contact with the opposite segment.
If this is neglected, the circuit will be interrupted
at the pole-changer, and strong shocks obtained.
When the screw-cups are connected with those belonging to the inner toil of the double helix and
electrotome (~ 402), and the central opening of that
instrument is filled with iron wires, secondary shocks
of considerable strength will be obtained from the
exterior helix whenever the armature is made to
revolve.  The vibrating wire should, be put in motion to break the primary circuit. Bright sparks are
at the same time seen in the mercury cups. The
magneto-electric sparks are conveniently shown by
passing the primary current of the machine through
the clock-work electrotome (Fig. 165), the vibrating wire of that instrument being set in motion.
445. When the primary magneto-electric current
is made to pass through water in a constant direction,
the water is resolved into its elements, and the gases
hydrogen and oxygen are given off separately, by
the two wires which convey the current. Two
platinum wires being connected with the screw-cups,
and their ends immersed in water, a slender stream
of gas will be seen to escape from each wire when
the armature is made to revolve. The decomposing
cell, represented i'n Fig. 23, is well adapted to this




MIAGNETO-ELECTRIC DECOMPOSITION. 275
3xperiment. A little sulphuric acid may be added
to the water. but care should be taken not to make
the.conducting power too great, or the amount of
gas evolved will be lessened.
446. The experiments described under the head
of Galvano-Decomposition (~ 66 to ~ 70) may be performed with the magneto-electric machine, though
the effects are on a smaller scale. The primary
current is preferable to the secondary for this purpose. Strips of platinum foil, which are generally
superior to wires in decomposing by a compound
galvanic battery, do not answer so well with the
magneto-electric current, especially when the wire
coiled upon the armature is fine.
447. Let the decomposing tube (Fig. 31) be filled
with a weak solution of iodide of potassium, without any coloring liquid. By causing the armature to revolve, iodine will be abundantly liberated
round the positive wire; this, being slightly soluble,
gives a brown color to the liquid, but most of it
remains in suspension, forming a dense cloud. If a
few drops of a weak solution of starch had been
previously added, an intense blue color will be
developed.
448. When a solution of sulphate of copper is
employed, shlphuric acid and oxygen are set free in
the positive cell, and metallic copper is precipitated
upon the negative wire. If the current is powerful,
it is deposited as a slightly adherent black powder;
but if of moderate strength, a thin coating is formed,
possessing the proper color and appearance of the




276          DAVIS S MANUAL.
metal. In this case little or no hydrogen escapes
from the coated wire, though oxygen is given off by
the positive one.  On reversing the current, the
copper is gradually dissolved off from the coated wire,
and a similar deposit occurs on the other. No oxygen escapes from the wire which is now positive,
until its coating has nearly disappeared. When the
experiment is concluded, the deposited copper may
be removed from the platinum wires by a little
diluted nitric acid. If two copper wires are immersed in the solution, as much copper will be
dissolved off of one as is deposited upon the other.
Sulphuric acid does not act upon copper in the cold,
unless aided in this way by an electric current.
449. Let the tube contain a solution of cyanide
of gold (~ 102), the conducting wires being of platinum. The negative wire will soon become covered
with a coating of gold, which increases in thickness
as the current is continued. Other metals, as, for
instance, silver, copper, and brass, may be gilded by
attaching them to the negative wire, and immersing
them in the solution. The coating does not adhere
firmly, unless the metallic surface on which it is to
be deposited has been perfectly cleaned, as directed
in ~ 103.
450. Many other metallic salts may be decomposed
in the same manner, and the metals precipitated; but
in most cases, the deposit is of a black color. Both
wires may be in the same portion of liquid, no partition being required. The deposition of the metals
from their solutions depends upon the same principles




MAGNETO-ELECTRIC  MACHINE.  277
which are concerned in the case of the galvanic current, as explained under the head of Electro-Metallurgy. The magneto-electric machine is sometimes
employed in electro-gilding and silvering, in place of
the galvanic battery. When the mechanical power
requisite to maintain the motion of the armature can
be conveniently obtained, it may be used with advantage; especially if the coils are constructed in
the manner which will shortly be described, so as
to furnish a current of considerable quantity.
451. The galvanometer is strongly affected by the
primary magneto-electric current. Even a large and
heavy needle, surrounded by a single wire, as in the
instrument represented in Fig. 53, may readily be
deflected. A sewing needle, or a piece of steel wire,
placed in a magnetizing helix, will be fully charged.
When the extremities of the wire surrounding a small
electro-magnet, such as is represented in Fig. 126,
are fixed in the screw-cups, it will sustain a weight of
some ounces while the primary current is flowing.
If the electro-magnet is covered with four or five
layers of coils, the wire being in a single length, it
will lift several pounds.
452. IMPROVED MAGNETO-ELECTRIC MACHINE.
An improved form of the machine is represented in
Fig. 174.  Two straight armatures, surrounded by
coils of insulated copper wire, revolve between two
U-magnets, according to Dr. Page's plan; but much
shorter armatures are used than in the machine contrived by him. The steel magnets are fixed, with
the south pole of one above the north pole of the
24




278           D A V I S S M A N U AL
other, at such a distance as just to allow the armatures to pass between them. These are mounted,
Fig. 174.
one on each side of a vertical shaft, in such a manner that both shall be passing between the opposite
poles at the same time. They are made to revolve
rapidly by a multiplying wheel, as in Fig. 173. A
pole-changer on the shaft conveys the alternating
currents in a constant direction to the screw-cups
with which the metallic handles are shown in connection.
453. For giving shocks, the small wheel on the
shaft is set with vertical -pins, upon which plays an
iron or steel wire connected with one of the screwcups.  Brilliant sparks are seen as the wire passes
over the pins. These sparks are sometimes half an
inch in length. The shocks are stronger than with
the machine last described, and the decomposing
power considerably greater.




VIBRATING  ELECTROTOME.    279
454. MAGNETO-ELECTRIC MACHINE, WITH VIBRATING ELECTROTOME. — In this form, the circuit is not
interrupted by the motion of the machine itself, for
giving shocks; but the vibrating electrotome, which
will be described immediately, is attached for that
purpose, as represented in Fig. 175. The shocks
Fig. 175.
Z 
are of more uniform  strength with this interruptor
than with the mechanical one, as they occur only
when the primary current charges the electro-magnet
sufficiently to draw down its armature. For sparks,
decompositions, &c., the primary current can be used
without passing it through the coil of the electrotome. This is effected by turning the screw S, so
as to raise the platinum point from the spring attached
to the armature.
455. VIBRATING  ELECTROTOME.-In Fig. 176,
the vibrating electrotome is shown in a form adapt.
ed for use with either of the machines described in
~ 437 and ~ 452. There is an electro-magnet, of the
U form, enclosed in a helix consisting of several
layers of coils. Above this is a straight armature,




280          DAVIS'S MANUAL.
fixed to a spring, by which it is held up from the
poles. The screw-cups at one extremity of the base
Ig-. 176.     board are to be connected
with the cups of the machine, and those at the other
end with the handles for
shocks. The primary magneto-electric current is passed through the helix, and a
secondary obtained by interrupting the circuit by the movement of the armature. One screw-cup at each extremity of the stand
connects with one end of the helix, which consists
of a rather long single wire, and the other two cups
with the pillar to which the armature is fixed. The
shock is thus obtained from the same wire which
conveys the primary current.
456. When the machine is put in motion, the
electro-magnet is instantly charged, and draws down
its armature. This motion depresses the spring, and
separates a little platinum disc upon it from a point
of the same metal attached to a set screw, S, above
the spring. On the interruption of the circuit, the
armature is raised by the force of the spring. A thin
slip of brass brazed to the face of the armature prevents it from  being retained by the poles.  The
shocks are varied to some extent, by turning the
screw, S, which alters the distance between the
armature and the poles.
457. The primary magneto-electric current resembles a galvanic current excited by a number of




MAG N E T O-E L E CTRIC MA C H INE.  281
small pairs.  Its quantity and intensity are, however,
both greatly influenced by the size and length of the
wire enveloping the armature.  A long and fine wire
affords a current of small quantity and high intensity,
and is most suitable for giving shocks. A short wire
of large diameter gives a current of moderate intensity, but of considerable quantity, and is, therefore, best for producing sparks, decompositions, and
magnetism.
458. MAGNETO-ELECTRIC  MACHINE, FOR  QUANTITY.-The machine represented in Fig. 177 is
similar to the one described in ~ 452, except that the
armatures are wound with short coils of coarse wire.
By this arrangement, a quantity current is obtained,
fig. 177.
capable of producing motion in some of the electromagnetic instruments, like the galvanic current. In
the cut, the revolving bell engine (Fig. 147) is shown
in connection with the machine. The primary current is employed, and those instruments are best
suited to the purpose which do not require a power24 *




282           DAV S' S MANUAL.
ful current to operate them. Both quantity and intensity armatures may be adapted to any of the
machines, so that one may be substituted for the
other at pleasure'.
459. We now pass to a class of instruments in
which electric currents are induced by electro-magnets whose magnetism is alternately acquired and
lost. These instruments consist essentially of double
helices containing bars or wires of soft iron. The
magneto-electric current is thus obtained in conjunction with that excited by electro-dynamic induction, and the current formed by their union is
called a secondary, though only in part such.
460. Platinum wire may be ignited by this secondary current in the manner represented in Fig.
Feg. 178.
178. A short piece of fine platinum wire is secured
by binding screws to the wires of a large electro



PowER OF BATTERIES.             283
magnet. The wires are uncovered at that point,
and being connected by the platinum, a part of the
battery current passes through it. It should be long
enough not to be fully ignited by this current.
When one of the wires is connected with one pole
of a single pair of Grove's battery, and the remaining
wire brought in contact with the other pole, the
electro-magnet becomes charged. On breaking contact, its magnetism instantly disappears, and a current is induced in the surrounding coil which flows
in conjunction with the secondary excited by the
battery current itself. The combined currents have
no circuit open to them except through the platinum
wire, which is for the moment ignited by their
passage.
461. Since the description of Smee's and Grove's
batteries, in the earlier part of the volume, was written, some experiments have been made in relation to
them, which are of sufficient interest to be mentioned
here, though out of their proper place. The first
trials were made to ascertain the relative power of a
single pair of Smee's battery with the plates at different distances. The plates used were flat, and were
placed opposite to each other, in a porcelain trough,
containing sulphuric acid, diluted with 15 times its
measure of water. In the whole series of experiments, the power was measured by the magnetism
developed in a magnetometer (Fig. 131), through the
coil of which the current was passed.
462. In the following table, the first column gives
the distance of the plates in inches and fractions of




284          DAVIS' S  MANUAL.
an inch. The second and third columns give the
attractive force of the electro-magnet, or the magnetizing power of the current, in grains.  The weights
in the second column were obtained with a smooth
platinum plate; those in the third, with one platinized in the usual manner.
TABLE I.
Distance.      Grains.          Grains.
10......25,000......  30,000
5....  30,000... 34,000
2.....32500.                 37,2,500.....   38,000...         40,500
g...40,000...  42,500
It will be seen that, as the plates are approximated,
the power gradually increases, but not in any very
great degree.  The table shows the advantage of
platinizing the negative plate.
463. With a battery of the form represented in
Fig. 178, the increase of power by adding to the
size of the plates was determined. The negative
plate consists of one or more pieces of zinc, whose
lower ends are placed in the mercury at the bottom
of the battery. The weights in Table II. were obtained with the battery arranged as a Smee's, the
porous cell being removed. In the first column is
given the number of zinc plates, the whole being
connected as a single plate. One piece of platinum
was employed in the three first experiments, and in
the fourth, two pieces, connected as one.




POWER  OF BATTERIES.              285
TABLE II.
NJumber of Zinc Plates.            Grains.
1................    47,000
2...............  51,000
3...............  53,000
3...............  56,000
464. The battery was now converted into a
Grove's, by using the porous cell, and its power
with different exciting liquids examined.  Equal
measures of sulphuric acid and water were poured
into the porous cell, and to this liquid was subsequently added pulverized nitre, in portions of one
spoonful each.  The results are given in the following table:TABLE III.
Fluid in Cell.          Grains.
Sulphuric acid and water,.... 50,000
With 1 portion of nitre,..... 65,000
2 portions' "...       86,000
"  3   "   "  ".....         1,000
465. When the porous cell was filled with strong
nitric acid, the magnetizing power was equal to
96,000 grs. The mixture of nitre and sulphuric
acid, though inferior to strong nitric acid, makes a
good exciting solution. Nitre, being a nitrate of
potash, is decomposed when added to sulphuric acid,
which forms sulphate of potash and liberates nitric




286          DAV S' S MANUAL.
acid. Hence the active agent is here also nitric acid;
but in using the mixture, the disagreeable fumes of
nitrous acid produced by strong nitrio acid alone are
in a great degree avoided.
466. Two of Grove's batteries, of the form represented in Fig. 178, were connected with the magnetometer, at first united as a single pair and afterwards consecutively, strong nitric acid being used
in the porous cell. The following table gives the
magnetizing power obtained:TABLE IV.
Battery No. 1,........ 80,000 grs.
"   No. 2,........    82,000  "
Both, as one pair,...... 88,000
"  consecutively.... 97,000 "
Owing to the magnetizing power being so great as
nearly to reach the limit of saturation of the electromagnet, the numbers in this table do not indicate so
great an increase as really occurs; but they show that
no great advantage is gained by connecting two pairs
as a single pair. The large-sized Smee's or Grove's
batteries are not much superior to the small ones.
Ten pairs of Smee's battery, connected as one, are
but little better than one pair.
467. SEPARABLE HELICES. —-In this instrument,
which is represented in Fig. 179, there are two
helices entirely separate from each other. The inner
one, composed of several strands of insulated coarse
copper wire, is fixed in a vertical position on the base




SEPARABLE  HELICES.                287
board. One of its ends is connected with the screwcup A, and the other with a steel rasp, B. The
Fig. 179.
exterior helix is of fine insulated wire, and can be
lifted off from the other, which it surrounds. Its
ends are enclosed in two brass caps, to which the
extremities of the wire are soldered. To these caps
are attached the screw-cups C and D. A bundle of
annealed iron wires, of which the ends are seen in
the cut, can be removed from the inner helix when
desired.
468. In Fig. 180, the different parts of the instrument are shown separately. The exterior helix, a, is
removed from the inner coil, b, which is fixed to the
base board.  At c is seen a brass tube, within which




288          DAVIS S MANUAL.
is the bundle of iron wires, d, intended to be introduced into the interior helix. For giving the strongFig. s180.
d
S
C_
est shocks, the bundle should fill the hollow of the
helix. The other parts are lettered in correspondence with the last figure.
469. The bundle of iron wires being withdrawn,
let a wire connected with one pole of a galvanic
battery be fixed in the cup A, and the other battery
wire be drawn over the steel rasp.  Bright sparks
will be seen, and if metallic handles connected with
C and D are grasped in the hands, as represented in
Fig. 179, slight shocks will be felt on completing the
circuit at the rasp, and stronger ones when it is
broken, as with the instrument described in ~ 402,
which is on the same principle.
470. If a rod of soft iron is introduced into the




SEPARABLE  HELICES.                289
helix, the spark is much increased, brilliant scintillations are produced, and the shock, when the circuit is
broken, becomes powerful. The iron acquires and
loses magnetism whenever the current begins or
ceases to flow, and induces secondary currents in
both of the coils which surround it. In the coarse
wire coil, which conveys the battery current, this
appears in the increased sparks and scintillations.
In the fine wire coil it is felt in the strong shock
which results.
471. When the bundle of iron wires is substituted
for the soft iron rod, the spark and shock are much
greater. If the rod or bundle of wires is introduced
gradually into the helix, the spark and shock increase
as it enters. The intensity of the shock may be
varied at pleasure, by altering the number of iron
wires, the addition of a single wire producing a
manifest effect. If a glass tube is slipped over the
iron wires in the helix, it does not interfere with
their inductive action on the surrounding coils. But
if a brass tube is passed over them, their influence is
entirely suspended, so far as the shock and the spark
are concerned. When the tube is slipped partly over
them, their influence is partially suspended.  This
also is a means of regulating the shock without altering the battery current.
472. The neutralizing action of the tube is thus
explained. The magnet induces in the tube, as well
as in the two coils, a secondary electric current,
which flows around it when the circuit is completed
or broken.  This secondary induces a tertiary our25




29()          D A V I S S MANUAL.
rent in each of the coils, which flows at the first
instant in an opposite direction to the secondary
induced in the coil by the magnet, and therefore
retards it. As the secondary current in the tube is,
however, instantaneous, it induces another tertiary itn
the same direction with itself when it ceases to flow.
The consequence is, that the quantity of the current
in either helix is not altered, but its intensity is
redluced, owing to the slowness of its development.
This is always the effect of any closed circuit in the
neighborhood of an inducing magnet or current, on
other circuits near it.
473. If the cups of the fine wire coil are joined
by a wire, it will form a closed circuit around the
magnet, and will impair the spark when the current
in the coarse wire is interrupted, though not to so
great an extent as the brass tube, since the latter
offers a freer and shorter circuit for the induced current. The spark is but slightly lessened when shocks
are taken from the fine wire coil, because the hunian
body is too poor a conductor to allow of the ready
flow of the secondary through it. A metallic cylinder surrounding the helices will neutralize the sparks
and shocks as completely as the enclosed tube.
474. When a bar of iron is placed within the
helix, a secondary is induced in it in the same manner as in the brass tube, which somewhat retards the
secondary currents in the coils.  Hence the greater
shock obtained from a bundle of wires, where this
secondary current cannot circulate. To this cause is
added aneother - the more sudden change in the mag



SEPARABLE  HELICES.                291
netism of the wires, when the battery current ceases,
from the neutralizing influence of the similar poles
of the wires on each other.
475. If the secondary current can be hindered from
circulating in the brass tube, its retarding influence
will be prevented.  Thus, if the tube is longitudinally divided' on one side, it no longer diminishes the
shock or spark.  With the solid iron bar, the shock
and spark are increased by sawing it open longitudinally to the centre.  A soft iron tube, divided
like the brass tube, gives a stronger shock than the
bar, but is still inferior to the bundle of wires.  The
two brass caps at the ends of the fine wire coil would
exert a considerable neutralizing influence if they
were not divided on one side, as shown in the cut.
The ends of the caps are also cut through for the
same reason.
476. In this instrument there are some peculiarities in the shock occasioned by the motion of the
battery wire over the rasp. If it is moved slowly,
distinct shocks are experienced; if the motion is
quickened, the arms are much convulsed; and if it
is drawn over rapidly, the succession of shocks becomes intolerably painful. This, however, can be
easily regulated. The shock from the secondary
coil increases within certain limits in proportion to
the length and fineness of the wire of which it
is composed. There is no advantage obtained by
employing a very long wire, unless the battery is
powerful.  The shock is also lessened if a very fine
wire is used, unless its length is moderate.




292           D A V I SS  MANUAL.
477. The strength of the shock depends greatly
upon the extent of the surface of contact between
the hands and the metallic conductors: Thus, if
two wires are fixed in the cups C and D, and grasped
in the hands, the shocks will be slight in comparison
with those given by the handles, and still more so
if the wires are held lightly in the fingers. These
effects, as well as the increase of the shock by wetting the hands, are due to the comparatively low
intensity of the secondary current, which causes it
to be transmitted imperfectly by poor conductors.
With frictional electricity it is well known that no
difference in the shock is thus occasioned.
478. When the quantity of the secondary current
is very small, an imperfect conductor, or a surface of
limited extent, may be able to convey the whole of
it, even if its intensity is not very high; in which
case, the sensation and muscular contractions produced by it will not be increased, but even lessened,
by any further increase in the conducting power.
Thus, if the shocks are received by placing the hands
in two vessels of water connected with the cups of
the outer helix, and the current is rather feeble, it
will produce the strongest sensation when the ends
of the fingers only are immersed. With a powerful
current, the shock is intolerable, whether the surface
of contact with the water is large or small; in the
latter case, it extends to a less distance up the arms,
though it may be felt very strongly in the fingers.
479. The shocks have sufficient intensity to pass
without much diminution through a circuit formed




s E P A R A B Lr. NJ  E L I C E S.  293
by several persons with theei hands joiped, especially
if their hands are moistened.  Different individuals
will be found to manifest remarkable differences in
regard to susceptibility to the shocks; some being
but slightly affected, perhaps feeling the shocks only
in the hands or arms; while others will feel them as
far as the shoulders or across the breast, and will
experience strong muscular contractions in the arms.
480. The difference in the strength of the shock
in the two arms, which has been described in the
case of the magneto-electric machine (see ~ 442,) is
exhibited more satisfactorily by the separable helices,
as a rapid succession of shocks may be obtained of
very nearly the same intensity.  Suppose the handle
connected with the positive cup of the exterior helix
to be held in the right hand, and the one connected
with the negative cup in the left hand.  The left
hand and arm will then experience the strongest
sensations, and be most convulsed. In determining
the positive or negative character of the cups, regard
should be had only to the terminal secondary current,
it being found that the initial secondary, whether induced by means of a voltaic battery or a permanent
steel magnet, produces comparatively feeble physiological effects, and consequently need not, in this
case, be taken into account. This singular difference in the intensity~of the shocks is regarded as a
purely physiological phenomenon, the greatest effect,
both as respects sensation and muscular contractions,
being produced by the electric current when it proceeds in the direction of the ramification of the nerves.
25 *




294          DAVIS'S MANUAL.
481. If the ends of the secondary wire are put
into vessels of water, a peculiar shock may be taken
by putting the fingers or hands into the vessels, so
as to make a communication between them through
the body. If both wires are put into a trough, at
some distance apart, and two fingers of the operator
are placed in the water in a line between the two
wires, a shock will be felt. Here the current prefers a passage through the body to that through the
water which intervenes between the fingers. The
conducting power of the water may be made better
than that of the human body by the addition of a
sufficient quantity of common salt; in which case,
little or no shock can be perceived. If the fingers
are placed at right angles to the line between the
wires, no shock will be felt. The trough should
not be of metal, but of some poor conductor of
electricity.
482. If a delicate galvanometer is connected with
the ends of the fine wire coil. the needle will be
deflected in opposite directions, and equally far, when
the battery circuit is closed and opened. The same
effect is produced when the brass tube is slipped over
the iron wires. In this case, though the shock may
have been prevented, the induced current still passes.
The reduction in the intensity of the current; while
its quantity remains unaffected, depends upon the
same cause as with the flat spirals, when a metal
plate is interposed.  (See ~ 421.)
483. When a flat coil of fine wire, such as that
represented at W, in Fig. 167, is passed over the in.




SEPARABLE  HELICES.              295
terior helix (the exterior one being removed), the
shocks will be found strongest when the coil surrounds the middle of the helix, and to decline coiisiderably in strength as it is either raised or depressed
from this position. Now, the magnetism of the enclosed iron wires, which induces the principal part
of the current, manifests itself chiefly at the ends of
the bundle; it might, therefore, have been expected
that the flat coil would give the strongest shock
when surrounding one of these ends.  The shocks
from the exterior helix are also lessened when it
is raised from the stand so as to enclose only the
upper part of the inner helix.
484. Slight shocks may be obtained from the inner
helix itself, by connecting one of the handles with the
cup, A, and the other with the rasp, B. The bundle
of iron wires should be within the helix. The shocks
are somewhat stronger when one handle is in connection with the rasp, and the other with the battery
wire which is drawn over it; in this case, the battery
is included in the circuit of the secondary current.
485. The most important principles of magnetoelectric and electro-dynamic induction are conveniently illustrated by the separable helices, in consequence of the facility with which the powers and
uses of its several parts can be exhibited. The
observations which have been made with regard to
it apply equally well to the following instrument,
which is a modified form.
486. SEPARABLE HELICES AND ELECTROTOME.In the instrument represented in Fig. 181, the inner




296           D A V I SS  MANUAL.
helix is connected with an electrotome, similar to that
described in ~ 388, fixed on the same base board, in
w Fig. 181.
addition to the steel rasp. There are two cups, A
and D, for the battery wires; these are connected,
through the electrotome, with the inner helix. When
the electrotome is set in motion, the curved wire
dips its ends alternately into the glass cups containing mercury, and rapidly breaks the circuit.  One
end of the coarse wire coil is also connected with
the steel rasp, so that this may be used as in the
last-described instrument, when the current is not
made to pass through the electrotome.  At W  is
seen the end of the bundle of wires, and at T the
brass tube, which may be slipped over them at pleasure. This instrument, and others resembling it in
being provided with a mechanical contrivance for




SEPARABLE HELICES & ELECTROTOME. 297
breaking the battery circuit, may be used with a
very small battery, although its effects are of course
more striking with a powerful one.
487. When the circuit is broken at the surface
of the mercury, anl intensely brilliant spark is seen,
and the mercury is deflagrated, passing off in a white
vapor.  If the quantity of mercury is properly adjusted, the sparks occur alternately in the two cups,
and in such rapid succession as to appear simultaneous. A little water or oil upon the surface of the
mercury diminishes the brilliancy of the sparks, but
increases the intensity of the shocks.
488. These sparks are of so short duration that
moving objects appear stationary by their light. The
revolving armature (Fig. 142), although rotating many
hundred times a minute, appears at rest when viewed
in this way; and where the sparks succeed each
other rapidly, it appears to leap from place to place
as their light falls on it. The revolving electromagnet (Fig. 146), and other instruments which exhibit rapid rotation, present the same phenomena.
Many optical illusions of this kind may be observed,
as in moving the fingers rapidly, when their number
seems increased, or rapidly turning over the leaves
of a book, when they seem to leap in the same
manner as the armature.
489. If the ends of the secondary wire are separated from each other at the same moment that the
battery circuit is broken, a spark will be seen from
the passage of the induced current.  A beautiful
light is produced if prepared charcoal points are




298           DAVIS'S MANUAL.
attached to the ends of the secondary wire and
separated in the same way.
490. Water may be decomposed by connecting
the ends of the fine wire helix with an instrument
for that purpose, having very small platinum wires
guarded with glass, as originally used by Wollaston.
These are prepared by inserting the wires into capillary glass tubes, which are heated till the glass melts
and adheres to their ends so as to cover them completely.  The platinum points are then exposed by
grinding away the glass. It is of course only necessary to cover those parts of the wires intended to be
immersed in the fluid.
491. The extremities of the platinum wires, while
the decomposition is going on, appear in a dark room,
one constantly and brightly, and the other intermittingly and feebly luminous. If the apparatus for
decomposition is removed out of the noise of the
electrotome, rapid discharges are heard in the water,
producing sharp, ticking sounds, audible at the distance of eighty or one hundred feet, and occurring
at the moments when the battery circuit is broken.
Decomposition is effected both by the initial and terminal secondary currents; that is to say, by the currents induced both on completing and on interrupting
the battery circuit; but the ticking noise and sparks,
accompanying the rapid discharges in the water, are
produced only by the terminal secondary current.
Both gases, hydrogen and oxygen, are given off in
small quantities at each wire. The secondary cur.
rent of the magneto-electric machine presents the
same phenomena with the guarded points.




SEPARABLE HELICES & ELECTROTOME. 299
492. A Leyden jar, the knob of which is connected with its inside coating by a continuous wire,
may be feebly charged, and slight shocks be rapidly
received from it, by bringing the knob in contact
with one of the cups of the outer helix, and grasping
with the two hands respectively the outer coating of
the jar and a handle connected with the other cup.
A gold leaf electroscope is readily affected by touching its cap with a wire fixed in either cup of the
exterior helix. If the contact, which should be only
momentary, is made at the instant of the rupture of
the primary circuit, the gold leaves exhibit a considerable divergence without the aid of a condenser.
Or the knob of a Leyden jar may be touched for
a moment with the wire, when it will be found to
retain a feeble charge, capable of diverging the gold
leaves and of giving a slight shock. The wire must
be well insulated from the hand in which it is held,
or the electricity will be conveyed off.
493. If the large thermo-electric battery (Fig.
41) is connected with theb cups A and D, and the
vibrating wire put in motion, faint sparks will be
seen in the mercury cups, attended by audible snaps.
Strong shocks may be obtained by grasping the
handles attached to the fine wire coil, especially if
both heat and cold are applied to the battery. A
single thermo-electric pair, of antimony and bismuth,
or of German silver and brass, connected with A and
D, will give a slight shock to the tongue when
heated by a spirit lamp; it will be more perceptible
when the ends of two wires fixed in the cups are




300          DAVIS IS XMANUAL.
made to touch the tongue than with a more extended surface of contact. This is probably due to
the small quantity of the induced current. These
sparks and shocks are, of course, not strictly thermoelectric, but magneto-electric.
494. When a bar of iron is contained within a
horizontal helix, such as is represented in Fig. 168,
where the circuit can be rapidly broken, and a small
key or some tacks are applied to one end of the bar,
they do not cease to be sustained, notwithstanding
its magnetic attraction is intermitted every time the
voltaic circuit is interrupted, since it is almost instantaneously renewed. This experiment succeeds
best when the iron bar is enclosed in a brass tube
within the helix, the closed circuits of the tube
tending to prolong its magnetism.
495. If an iron tube of sufficient diameter to admit
a long helix of fine wire within it is itself passed into
a coil of coarse wire, no shocks can be obtained from
the enclosed helix, even when the tube is divided
longitudinally on one side, to prevent the flow of a
current in its substance which might neutralize that
of the fine wire. This is the converse of the fact
stated in ~ 265, that a galvanic current passed through
a coarse wire helix, enclosed in an iron tube, induces
no magnetism in the tube. Let one or two iron
wires be placed in a glass tube contained within a
tube of iron, the whole being surrounded by the
heliacal ring described in ~ 273.  The wires are
about as long as the tube, and their ends are made
to extend a little beyond one of its extremities.




DOUBLE  HELIX, &C.              301
When the helix is connected with the battery, polarity is induced in the wires in the same direction
as in the iron tube, and they are repelled until about
half of their length has passed out of the tube. If
the circuit is broken at this instant, the momentum
which they have acquired causes them to be projected to some distance. In this case, the repulsion
overpowers the axial force, which tends to draw the
wires within the coil. The action is strongest when
the helix surrounds that end of the tube from which
they are projected.
496. DOUBLE HELIX AND VIBRATING ELECTROTOME. -The instrument represented in Fig. 182 is
one of the most convenient for the medical application of electricity. It is provided with a self-acting
interruptor, by which shocks can be given with extreme rapidity. The double helix is secured to the
base board, in a horizontal positiion, by two brass
Fig. 182.
bands. The helices are insulated from each other,
but are not separable.  A brndle of iron wires is
shown in the cut, within the inner helix. This can
26




302           DAVIS' S MANUAL.
be removed at pleasure.  The vibrating electrotome,
which is fixed on the stand, is of the same construction as that represented in Fig. 176, except that the
coil surrounding the electro-magnet, M, is shorter.
Near the electrotome are two screw-cups for battery
connections. When the battery is applied, as shown
in the cut, the current traverses in succession the
coarse wire helix and the coil of the electrotome.
The electro-magnet is instantly charged, and attracts
its armature, causing the circuit to be broken in the
manner described in ~ 456. At the other extremity
of the base board are the screw-cups belonging to
the fine wire helix. With these the handles for
shocks are connected. With all the magneto-electric
instruments, the battery connections must be made
by stout copper wires. For attaching the handles to
the cups of the outer coil, rather fine wire is more
convenient.
497. With a battery of even moderate power, the
shocks may be made to follow each other with
exceeding rapidity. When their strength is lessened
considerably by removing nearly all the iron wires
from the helices, instead of distinct shocks, a peculiar
sensation of numbness is experienced, extending a
greater or less distance up the arms, and attended by
loss of power over the muscles as far as it reaches.
The shocks are never so powerful with a self-acting
interruptor as when the circuit is broken mechanically, since the battery current is obliged to maintain
the motion of the interruptor as well as to traverse
a circuit of greater length.




SHOCKS REGULATED.               303
498. There is a steel rasp above the helices, by
means of which shocks are given without using the
electrotome.  For this purpose, the battery wire is
removed from one of the screw-cups near M, and
drawn over the rasp. It will be found by trial from
which of the cups it is necessary to remove the wire.
If the hollow of the helix is filled with iron wires,
bright sparks will be seen as the battery wire leaves
each tooth of the rasp, and strong shocks will be felt
when one of the handles is grasped in each hand.
When the iron wires are withdrawn, the spark becomes faint and the shock feeble.
499. The strength of the shock may be regulated
by varying the number of iron wires which are
placed within the helix, or the distance to which
the bundle is allowed to enter. The addition of a
single wire produces a perceptible increase in the
shock, especially when only a few are already within.
By wetting the hands or other parts to which the
handles are applied, especially with salt water, the
shock is still stronger. It may, on the contrary, be
lessened in some degree by diminishing the extent
of contact between the handles and the surface of
the body. If, however, the current is powerful and
the contact too slight, a disagreeable burning sensation will be experienced at the part touched by
the metal.
500. The shocks may be passed through any portion of the body, by placing the handles so as to
include that part in the path of the secondary current; their intensity is greater when the handles are




304           D A V IS' S  MANUAL.
near each other.  The influence does not extend
beyond the direct course of the current, unless the
shocks are severe.  When, however, one of the
handles is placed directly over a large and tolerably
superficial nerve, the shock will be felt not only
in the parts intervening between the handles, but
through those to which the ramifications of the
nerve are distributed. Thus, if one handle is held
in the right hand, and the other pressed upon the
inside of the left arm over the median nerve, the
sensation will be experienced even to the ends of the
fingers, attended by convulsive movements of their
muscles. This is, unquestionably, a physiological
phenomenon, and not a consequence of the flow of
the current below the position of the handle. The
difference in the intensity of the shock in the two
arms, described in ~ 480, may be observed with this
instrument.
501. COMPOUND MAGNET AND ELECTROTOME.-In
Fig. 183 a double helix is seen attached to the base
board by two brass bands. It is placed in a horizontal position, and within it a bundle of soft iron wires
is permanently fixed. There are two screw-cups for
the battery connections at one end of the stand; one
of these is connected with the band which sustains
the glass mercury cup, C.  To the second screwcup is soldered one end of the coarse wire coil, the
other extremity of which is connected with the band
upon which the brass cup, B, also intended to hold
mercury, is fixed. A bent wire, W, moving on a
horizontal axis, dips its ends into the two mercury




COMPOUND  MAGNET, &C.              305
Clups. On the opposite side of the axis is attached a
curved iron rod, R, the lower extremity of which
Fit,. 183.
approaches nearly the end of the enclosed bundle of
iron wires.
502. When the connections are made with the
battery, the current traverses the wire, W, and the
inner helix, causing the iron wires to become magnetic. They now attract the end of the iron rod,
R; the motion of the rod raises the bent wire
out of the mercury in the cup C, and breaks the
circuit. This destroys the magnetism of the iron
wires, and R is no longer attracted. The wire, W,
then falls back by its own weight, and the circuit
is renewed. A thin slip of brass is brazed to the
end of R, to prevent it from being retained by the
electro-magnet after the rupture of the circuit.
503. In this manner a rapid vibration of the wire
is produced, and brilliant sparks and deflagration of
the mercury take place in the cup C. The proper
26*




306           DAVS'I  S S MANUAL.
balance of the vibrating apparatus is insured by
the brass ball, b, which moves on a screw cut in a
bent wire above the axis. The ends of the fine wire
coil are connected with the two screw-cups at the
opposite end of the base board to those for connection with the battery.  From  these cups, the
shocks are taken.
III. BY THE INFLUENCE OF THE EARTH
504. Currents of electricity are induced by terrestrial magnetism; but, in consequence of the feebleness of the action, it is not easy to render it sensible
by the aid of wire coils alone. Deflections may,
however, be obtained by connecting with a very
delicate galvanometer a helix of coarse wire, such as
is represented in Fig. 113, or a flat spiral, Fig. 112,
and, having placed its axis in the line of the dip,
suddenly inverting it..
505. Stronger deflections are produced by causing
a helix to revolve rapidly, as in the instrument represented in Fig. 184.  The coil which is hollow,
moves in a vertical plane, and its shaft -is provided with a pole-changer, to the segments of which
the extremities of the wire are soldered. The springs
pressing on these segments convey the currents to
the screw-cups on the base board. When the cups
are connected with those of a delicate galvanometer,
and the instrument placed in such a direction that
the helix shall move in the magnetic meridian, it is




INDUCTION  BY THE  EARTH.   307
made to revolve rapidly by means of a multiplying
wheel. As each end of the helix approaches and
recedes from the line of the dip, opposite currents
Fig. 184.
are induced in the wire, their direction changing
as the helix passes this point.
506. These alternating currents of electricity are
turned into one course by the pole-changer, the segments of which may be so arranged as to pass from
one spring to the other when the helix is vertical,
this being sufficiently near the line of the dip, except
in low latitudes.  The galvanometer needle is steadily deflected as long as the motion is maintained
uniformly. By reversing the revolution of the helix,
a deflection in the opposite direction takes place.
507. A still more powerful effect is produced by
fixing an iron bar within the hollow helix. Let
the instrument be now placed in the plane of the magnetic meridian, and the cups connected with a galvanometer.  When the coil and bar are made to re



308           DAVIS' S MANUAL.
volve, each end of the iron becomes alternately a
north and a south pole, the magnetism being induced
in it by the earth, as explained in ~ 354. By the
changes in the magnetic state of the iron, electric
currents are induced in the surrounding coil.  rhese
currents change their direction twice during each
revolution, but they are brought into one course by
the pole-changer on the shaft of the rotating coil,
and the needle of the galvanometer is strongly and
steadily deflected. With this instrument, the current
is somewhat augmented'by the feeble one excited in
the wire coil by the direct magneto-electric induction
of the earth.
508. In this, and in all other instances where
electricity is produced by motion, and motion reciprocally by electricity, the motion must be the
reverse of that which would result from a galvanic
current flowing in a certain direction, in order that
the induced current may have the same direction.
An opposite current- is excited by producing mechanically the same motion as that obtained with
the battery.




INDEX.
Sactiea.
AMALGAMATED zinc,....................................... 25
Ampere's electro-dynamic theory,...........................380
Analogy between magnetism  and statical electricity,.......... 247
Animal electricity,........................................134
Arch of flame between charcoal points,.......................48
Armature,................................................   9
Artificial magnets,..........................................2
Attracting and repelling wires,..............................364
Attraction  of armature..................................... 228
currents, shown by frictional electricity,.........369
iron due to induced magnetism................. 222
Attractions and repulsions, magnetic,........................140
of magnets, cause of.............. 383
Attractive force, instrument for measuring,.................... 237
of bar magnets,........................243-246
-       electro-magnet,.....24...............240
long U-magnet,.........................239
short U-magnet,:........................ 238
Astatic needle,........................................                           156
Aurora borealis affects magnetic needle,.....................219
Axial bell engine,.........................................280
-  force,........................................279
magnetometer...................................... 309
-    - motion of bent iron bar,..............................289
Axial receiving magnet,...................................321
-        revolving bar.......................................281. -circle,...............................                             287
-  telegraph,.........................................  323
with clock-work,...........................324..-...    engine,................................325




310                                    I N D E X.
Section.
Bar magnet,............................................ 7...
—, attractive force of......................... 243-246
Batteries for electro-magnetic  experiments,.................... 23
~ —, power of,........................................461
- -,   protected....................................... 35-40
-, sustaining,........................................ 40
Battery, Grove's,.......................................... 41
------   Sme's,.........................................24
- -     sulphate of copper................................. 29
-     -       thermo-electric,.................................. 128
Blue vitriol, used  in battery..................................  30
Calorimotor,.........................................                                     21
Cell for delicate decompositions.............................. 66
Clock-work  electrotome..................................... 388
Coil of fine wire, sparks and  shocks fioni,...............397-400
Cold and heat produced  by galvanism;n,........................ 131
-,   instrument for showing the production of,..............132
Common  salt used  in battery................................ 35
Compass needle.......................................... 207
Compound  bar magnet,........................................ 7
magnet and electrotome..........................501
-  U-magrnet........................................ 8
Conducting power of metals,.............................                             55-58
Conduction of galvanism.................................... 55
Constant batteries.........................................  40
Contracting  helix,.........................................371
Copper deposited by the magneto-electric current,............448
Copperplates electrotyped,............................... 78, 80
Copper solution for the electrotype............................ 88
Cylindrical sulphate of copper battery.........................     29
De ]a Rive's rinog........................................   185
Decomposing cell.........................................64
Decomposition  of water in  Smee's battery..................... 27
Decompositions in protected  batteries.......................... 39
-  -             sulphate of copper battery,.................                    31
Dip of magnetic needle,.................................... 210
Dipping needle,...................................... 10, 208




INDEX.                                          311
Section.
Direction of poles, rule for finding,..........................257
Divergence  of suspended iron  wires,..........................225
Divided  tumbler for decompositions........................... 67
Double axial bell engine,..................................290
beam axial engine,.................................293
cylinders of iron, with  helix,.......................274
forceps for holding  wires to be ignited,..............58
helix and electrotome,..............................402
helix  and vibrating electrotome,....................496
revglving magnet,.348
therino-electric  revolving  arch,...................205
Earth, induction  of electricity  by the,.......................504
--  __ _-,   magnetism  by the........................ 353
--,  magnetic poles of the,..............................  218
-—, magnetism of the,.............................. 215, 217
-, used as a conductor,................................318
Electric light, transient duration of,.....................                      182, 488
Electrical ray...................................                  138
Electricity, animal,...................1................... 134
-  frictional,............................13
--  -, from steam,.............1..................14, 15
-,   galvanic,..................................... 16
induced by the earth,...........................504,     induction  of,.....................................3
-, instruments for showing induction of, by the
earth,.................................505, 507... —-,   magnetic...................................... 426
—,  mechanical,.....................................13,  velocity of,..................................... 326.-..,  voltaic,.........................................16
Electrodes,.............18..........................18
Electro-dynamic revolving  coil,............................. 373
rectangle,........................               376...... —   theory of Ampere,........................                                    380
Electro-dynamics, phenomena of,...........................360
Electro-etching,......................................                                   94
Electro-gilding  and silvering,............................ 98-104
Electro-magnet,...........................................295




312                                 IND E x
Section
Electio-magnet attracts through glass, &c.,............... 300
in case,.................................. 298
in frame,................................. 296
—,   polarity of, reversed......................... 297,          retained by.......................... 296
-        —,    power of,..................................299
revolving by the earth's action,.......... 343, 345
on its axis,....................... 351
- -  -   within a coil,...................... 347
with three poles..........................   301
Electro-magnetic  induction,......................253
--         -  telegraph,..........................310,  313
Electro-magnetism,...................................... 253
-    -     as a motive power,....................352
Electro-metallurgy,........................................ 71
Electro-statics...........................................360
Electrotype,........................................                            71
copies, bronzed, gilt, or silvered................... 93
Elementary magnets,...................................                           249
Fine wire coil............................................397
Fish-worm, galvanic experiments with the,....................53
Flat spiral,...................................        275
-,    magnetism  induced by........................... 277
-- -, polarity of..................................... 276
---, sparks and shocks from.......................... 387
Fracture of magnets......................................247
Frictional electricity,..................................13,induced  currents from,............             424, 425
mutual attraction of currents shown by,..369
Frog used in galvanic experiments,..........................51
Fusible metal moulds,......................................79
Galvanic batteries,..................................... 24-49
circuit,.......................................17
current,.......................................16-18
-, direction of,...............................16
- --...electricity,....................................16
---...series..................................21




INDEX.                                          313
Section7.
Galvanism,................................................16, conduction  of,..........................55,   physiological effects of,...........................50
Galvano-decomposition,....................................62
Galvanometer,...........................................  158
deflected by secondary intensity  current,......482
quantity currents,.......394
horizontal,..................................160
measures quantity,...................... 163
simple,.................................. 158, upright,.................161
with astatic needle,.................. 162
Gas-carbon,.............................................  115
German  silver,...........................................116
Gilding  and silvering by the magneto-electric machine,....... 450
-   by the galvanic  battery,.............................98
magneto-electric  machine,....................449
- solution for,.......................................102
- without battery,.................................104
Glauber's salt used  in battery,...............................                           35
Glycography,..........................................95
Gold leaf galvanoscope,................................... 179
Grove's battery,..........................................41
construction of,............................                     44
- power of,..................... 4........64-466
series of,..................................                      45
Grasshopper's leg used in galvanic experiments,...............50
Gun-cotton used to show  heating of wire,.....................59
Gymnotus,...............................................136
Heat and cold produced by galvanism,.......................131
Heating  of wires shown by gun-cotton,.......................59
Heliacal ring,...........................................271
-, polarity of,..................................                       273
Helix, action of,.......................................... 263
-, iron bar sustained  by,...............................278
-,  magnetizing,.......................................278, on stand,.....................................262
Horizontal axial engine,...................................292
27




314                               INDEX.
Sction
Horizontal galvanometer,................................. 160
reciprocating engine............................328
Horseshoe magnet........................................... 8
Hydrogen and oxygen obtained separately,....................65
Ignition of wire by galvanism,.5..........................56, 57
--         secondary current,......................460
Impressions of seals electrotyped,............................85
Improved magneto-electric machine,........................452
Inclined bar revolving round conductor,.....................285
- revolving bar,....................................283
Induced currents from  frictional electricity,..............424, 425, table of,..................................423
Induction of a current on itself,........................384-392
electricity.....................................3
magnetism,....................................                3
~-   -- not impeded by glass, &c...........236
secondary currents at a distance,...............407
Initial and terminal secondaries,............................396
_- tertiaries,..............................420
-  secondary, intensity of,..............................399
Instrument for measuring attractive force..................... 237
showing revolution of mercury,...............377
Instruments for illustrating terrestrial magnetism........ 215, 217
Insulation of galvanic electricity easy........................22
Intensity and quantity..................................... 20
how  increased,....................................21
- -    of batteries compared..........................42, 45
- -       the thermo-electric current,.................... 125
Iodide of potassium  decomposed by galvanism,................                 66
magneto-electricity,.......447
Iron, attraction of, by magnet,..............................222
-      bar magnetized by the earth...........................354
- sustained within helix,............................. 278
-   filings around conducting wire........................261
-  -      the poles of a magnet,............... 145, 146
increases sparks and shocks......................470, 471
-   wire in glass tube....................................236
wires superior to solid bar for sparks and shocks........474




INDEX.                                315
Section.
Leather partition used in battery,.........................37, 38
Leech, galvanic experiments with,........................52, 53
Leyden jar charged by secondary current...................4. 92
Liebig's theory of muscular action.......................... 135
Line of no variation,......................................211
Loadstone,................................................. 2
--,   polarity of...................................... 234
Magnet,...................................... 2
Magnet and roller,.................................. 230, 231
attractive power of, long known,.............11
gains power when armature is kept on,...............252, iron tacks sustained by,............................233, polarity of, when discovered,.........................11
revolving on its axis,...............................167
-    ~round conductor,...................166
Magnetic attractions and repulsions.......................... 140
curves,..........................................147
metals,............................................1
needle...........................................10
deflected by galvanism..................... 149
half brass....................1............ 157
observations,................................ 220
polarity, probable cause of,...................... 380
-..- poles,.............................4, 5
of the earth,,.............215, 218
Magnetism  and statical electricity, analogy between,........... 247
communicated by electro-magnets,...........302, 303
percussion,....................356
steel magnets,................. 251
twisting,...................... 357
-—,   definition of term,............................     1
-   induced by secondary currents,................... 395
in iron by galvanism,.................... 254, induction of,....................................3
-..,  - —,  by the earth,...................... 354
of iron bar prolonged,...................494
steel magnets, probable cause of,............... 380
the earth, probable cause of,................... 381




316                                INDEX.
Section,
Magnetism  removed by electro-magnets................. 304, 305
-   percussion,.......................... 359
Magnetizing helix........................................ 278.-        power of the magneto-electric current,.......... 451
Magneto-electric current, how  excited....................... 433
~ -, quantity and intensity of,...........457
currents,.................................. 440
decompositions............................. 446
Magneto-electric machine,.................................437
for quantity,......................  458
used with telegraph,...............306
-   with vibrating electrotome,.........454
shocks,.......................441
different in arms,...................442
varied,............................443
sparks............................... 444, 453
from  coarse wire helix...............487
-  -     * fine wire helix................489
Magneto-electricity,...................................... 426
Magnetometer,...........................................307
Magnets, attractive force of,.....................237
charged........................................251
by the electro-magnet,...................302
discharged,.................................304, 305
by percussion,........................359
Mechanical electricity,......................................13
~-       power from  electro-magnetism,............. 352
Medals electrotyped,.................................... 79, 80
Medical use, magneto-electric apparatus for,............. 496
Melloni's thermo-multiplier................................. 130
Mercury, revolution of,....................................377
Metallic chromes,......................................... 105
Metals, conducting power of,................................58
deposited by galvanism,..............................72
the magneto-electric current,........448-450
Muriate of ammonia decomposed,....................69
Natural magnets,...........................................2
Needle, astatic,..................156....................15




INDEX.                                     317
Section.
Needle, dipping,...................................... 10, 208
-,  horizontal magnetic,...............................207,  magnetic,......................................... 10
Nitre, use of, in Grove's battery,.......................464, 465
Nitric acid, used in Grove's battery,.....................42, 465
One metal, galvanic current from,...........................19
—, thermo-electric current from,.....................108
Optical illusions in electric light,.......................182, 488
Oxygen and hydrogen obtained  from  water,..............64, 445
-       separately,.....................       65
Percussion, induction of magnetism  by,.....................356
-,   magnetism  removed by..........................359
Permanent magnets,................................6........
Physiological effects of galvanism,............................                 50
--     phenomena of shock,.....................480, 500
Plaster casts electrotyped,...................................83
Platinum  deposited by galvanic current,......................97
------  employed in Smee's and Grove's batteries,......24, 41
-------      solution of, for platinizing,....................97
-      -     wire ignited by galvanism,........................57
secondary current.................. 460
Polarity of magnet, when discovered,........................ 11
Pole-changer,...........................................  191
Poles, a little within ends of magnet,........................230
-,   magnetic, of the earth,.......................... 215, 218
of galvanic battery,...................................16.magnet,........................................4, 5
Porous cell used in Grove's battery,......................44, 99........              -- protected batteries.................... 35-38
Powder cup,...........................................   59
Protected batteries........3..............................'5-40
Primary magneto-electric current, effects of............. 445-451......... —, quantity and intensity of,....457
Quantity and intensity of galvanic electricity,.................. 20
magneto-electric current,............457
thermo-electric current,......... 124, 125
27*




318                                    INDEX.
Section
Quantity, how increased,...................................21
- measured  by galvanometer.......................... 163
Receiving magnet,.......................................  320
Reciprocating  armature engine,.........327..............327
coil engine.................................. 189
magnet engine,............................190
Registering  revolving magnet,.............................342
Removal of magnetism by the electro-magnet,...........304, 305
percussion........................ 359
Repulsion  of iron wires,.................................... 226
from  iron tube,.....................495
Revolving armature......................................329
engine,..........................                        331
bell engine............3........................340
coil...........................................191
and magnet,................................195
cylinder,....................................... 170
disc,...........................................184
electro-magnet,.................................336..- -    galvanometer needle,.....................194
rectangle,......................................193
spur-wheel,....................................180
wheels,........................................350
with electro-magnet,.....................349
wire frame,.................................168
Ribbon spiral,............................................275,     magnetizing power of,........................ 277: polarity of,..................................                      276, sparks and shocks from,..................387, 390
Ritchie's revolving magnet,................................338
R lling armature.......................................... 229
Rotating battery.......................................... 173
Rule for finding direction of poles,..........................257
polarity induced by helix......... 264
Seals electrotyped,.........................................85
Secondary current,...................... 392, 396
-, gold leaf electroscope deflected by,........492




INDEX.                                  319
Section.
Secondary current, in helices enclosing iron,................. 459
Leyden jar charged by,.................. 492.-., sparks from,............................489,  water decomposed by,....................490.-.., wire ignited by,.........................460
currents, from  frictional electricity,................424, galvanometer deflected by,............... 394
induced by intensity current,............     405, magnetism  induced by,................... 395
-....-                   obtained by moving battery plate,....412
coils,...............     413
separated from  the primary,..............         393
Sensation occasioned by two metals in mouth,.................54
Separable helices,................................467
-   and electrotome,..........................486
Shock depends on surface of contact......................... 477
from  fine wire coil,.................................398
galvanic battery,..............................406
initial secondary,..............................399
interior helix,.................................484
Shocks and sparks from  double helix,.......................403
long wire,......................384-386
ribbon spiral,.......................387
separable helices................... 469
thermo-electric battery,..............493, neutralization of,..................0 9, 471
-  prevented,.........411, 475
increased by a bar of iron,...470
~~    ~bundle of wires,.......471, 474
Shocks, difference of, in arms,.......................480
differences in,................................. 476
from  double helix and vibrating electrotome,........497
feeble current,..................478....-   fine wire coil varied,.................408
flat coil surrounding helix,.........                     483
magneto-electric machine,.......441
primary magneto-electric current,..............444
-   given to several persons at once,.........479..-     -psed near a large nerve,......................500




320                                  INDEX.
Scdion.
Shocks passed  through any part of the body................. 500
regulated,................................... 443, 499
Signal key,..............................................317
Silurus electricus,.........................................139
Silvering by the galvanic battery,.......................98-103
-,    solution for,.................................... 101
without battery,.................................104
Simple galvanometer.................................... 158
Single beam  axial engine,................................. 294
Smee's battery,.........................2............ 24, 89: —-, power of.................................. 461
Sources of magnetism,............................... 250
Sparks, extremely short duration of,.....................182, 488
- from  secondary current,............................489
-magneto-electric machine,.....................453
taken from  mercury,...............................487
Star plate................................................223
Steam, electricity from,..................................                           14
Steel magnet, iron sustained by,............................233
magnets charged,................................251, 302
discharged,.....................304, 359
plates electrotyped,....................................78
used for magnets......................................6
wire ignited.........................................57
Successive induction of magnetism.......................... 232
Sulphate of copper batteries,.............................29-40
--    decomposed by magneto-electricity,.......448
- soda decomposed by galvanism,....................68
used in battery,.............................35
Sulphuric acid used in batteries,.........................25, 46
- Grove's battery,...................35, 42
Sustaining batteries....................................... 40
Tangential action of current,..........................152, 382
-  forces cause attraction and repulsion.............. 258
Telegraph, axial,.....................................323
-, battery for operating,............................316
-,  electro-magnetic,...........................310,  313. —-, magneto-electric machine used with,..............316




INDEX.                                     321
SectioIc
Telegraph, with clock.work,...............................315
Telegraphic alphabet,................................... 314
writing,......................................316
Terminal secondary current compared with initial,............ 399
Terrestrial magnetism, instruments to illustrate,.........215, 217, probable cause of.................... 381
Tertiary currents,.........................................416
-      from  wire coils........................... 419
Thermo-electric arch between poles of U-magnet,............202
-   battery,...................................128
-, sparks and shocks from,.............493
-  --  combinations, table of,......................117
current, cause of,...........................109,   direction of,........................112
from  a single metal,................108
reversed,..........................120
pairs,................................. 117
revolving arch,............................196
on U-magnet,................19
wire frames......................201.- eries,........................127
Thermo-electricity,................................... 106,  quantity and intensity of,................124
Thermo-multiplier,...................................  130
Torpedo.................................................138
Trough batteries,.................................. 46-49
U-magnet.................................................
U-tube, for decompositions,..................................70
Upright axial engine,....................... 291
galvanometer,....................................  161
Variation of magnetic needle,...............................214., how  found,........................... 216
Velocity of electricity,.................................... 326
Vibrating armature engine,...........................334, 335
coil engine,................................... 187...-    -electrotome,............................                             455
--.helix,.........................................288




322                                 INDEX.
Section.
Vibrating wire,..................................... 175, 177
Voltaic electricity,................................16
gas pistol,.........................................60
Voltameter,...............................................65
Water decomposed by galvanism,.............................63
magneto-electricity,..................              445
secondary currents.................... 490
in Smee's battery......................... 27
Wax moulds......................................... 80, 83
Wire conveying current does not attract light bodies.......... 362
-      ignited by current,...................................56
Wires of electro-magnetic telegraph,........................319
- for conveying electric currents,...................  55, 496
Woodcuts electrotyped.....................................80
Y-armature.............................................. 227
Zinc plates amalgamated.......25.....................   25




DANIEL DAVIS, JR.
NO. 428. WASHINGTON STREET, BOSTON, MASS.
MANUFACTURES AND KEEPS FOR SALE
Instruments to illustrate the Principles of
GALVANISM,                 ELECTRO-MAGNETISM,
ELECTRO-DYNAMICS,          MAGNETO-ELECTRICITY,
MAGNETISM,                 THERMO-ELECTRICITY.
FOR SALE ALSC BY
JOSEPH M. WIGHTMAN,
NO. 33 CORNHtILL, BOSTON.
Mi,"ure.*                                            Price.
1. Two Bar Magnets and Armatures, in box,...$3,00 to 5,00
2. Compound Bar Magnet,..................... 2,50 to 4,00
3. Horseshoe or U-Magnet,....................,25 to,75
4. Compound U-Magnet,...................... 2,00 to 5,00
5. Magnetic Needle, on brass stand,............,75 to 1,00
6. Small Dipping Needle....................... 1,00 to 1,50
9. Smee's Battery, (each pair,)................. 1,50 to 2,00
12. Large Sulphate of Copper Battery,...........       7,00
Sulphate of Copper Battery, (medium size,)...       5,00
Small Sulphate of Copper Battery,...........       2,00
13. Protected Sulphate of Copper Battery,.......       1,25
15. Protected Sulphate of Copper Battery,...... 1,50 to 2,00
* The numbers in the first column correspond with those of the cute
in Davis's Manual of Magnetism, 2d ed.




CATALOGUE
Flgure                                                     Price.
17. Grove's Battery, (single pair,)...............$2,00 to 2,50
18. Grove's Battery, in box, (per pair,).....1,50-2,00 &  2,50
20. Large Trough Battery, (100 pairs,)......         85,00
21.  Grasshopper Battery........................,25 to,50
22. Frog, or Leech Battery,.....................  50 to,75
25. Double Forceps, on stand,................... 2,00 to 5,00
26. Powder Cup, of glass,......................,25
Powder Cup, of brass,......................,50
27. Voltaic Gas Pistol,....................            2,50
28. Decomposing Cell, with one gas tube,.......              2,50
Decomposing Cell, with two gas tubes,......              3,50
29. Cell for delicate decompositions,........... 2,00 to 3,00
30. Divided Tumbler, for decompositions,........ 2,00 to 3,00
31. U-Tube, on stand,..........................             3,50
32. Smee's Battery, (improved,)................ 3,00 to 5,00
34. Smee's Battery, for gilding and silvering, (per pair,)  2,00
35. Smee's Battery, for gilding, &c., (improved,)  1,50 to 2,00
38. Thermo-Electric Pair,.......................,25
39. Delicate Galvanometer,..................... 3,00 to 5,00
40. Thermo-Electric Series, (10 pairs,).........            2,00
41. Thermo-Electric Battery, (60 pairs,).........           25,00
42. Cold and Heat Instrument,.................               3,00
52. Astatic Needle, on stand,................... 1,50 to 2,00
53. Simple Galvanometer....................... 2,50 to 3,50
54. Horizontal Galvanometer...................              6,00
55. Upright Galvanometer,.....................               5,00
56. Magnet revolving round Conductor,......                6,00
57. Magnet revolving on its Axis,..............             6,00
58. Revolving Wire Frames,....................               8,00
59. Revolving Cylinders,......................               8,00
60. Rotating Batteries,.........................             8,00
61. Vibrating Wire, (with magnet,)..............             5,00
62. Vibrating Wire, with fixed magnet,..........             6,00
63. Gold Leaf Galvanoscope,................... 5,00 to 7,00
64. Revolving Spur-Wheel,............. 5,00 to 8,00
65. De la Rive's Ring, in small glass cup,......             1,00
66. Vibrating Coil Engine,.....................              8,00
67. Reciprocating Coil Engine,..................            10,00
68. Reciprocating Magnet Engine,.......                 10,00




CATALOGUE.
Fingur                                                          Price
69. Revolving Coil,............................                  $6,00
71. Revolving Rectangle,......................                   6,00
72. Revolving Galvanometer Needle,............ 6,00 to 7,00
73. Revolving Coil and Magnet,................ 8,00 to 10,00
74. Therino-Electric Revolving Arch,............                  2,00
75. Thermo-Electric Arch, on U-magnet,.........                   4,00
76. Thermo-Electric Revolving Frames,..........                   6,0C
77. Thermo-Electric Arch, between poles of U-magnet,    4,00
78. Double Thermo-Electric Revolving Arch,....                     6,00
79. Magnetic  Needle, (on brass stand,)..........  75 to 1,50
80. Large Dipping Needle, on universal joint,.... 4,00 to 6,00
82. Bar Magnets, in frame, (to illustrate Earth's magnetism,) 6,00
84. Globe, with Coil and two Needles,..........                   4,00
87. Star-shaped Iron Plate,.....................,75
89. U-Magnet and Rolling Wires,...............  1,00 to 2,00
90. Y-Armature,...............................,30
92. Rolling Armature, with U-rragnet,.....2..                   2,00
93. Magnet Armature and Roller,...............                    1,50
94. Short Rolling Wire, with U-magnet.......... 1,00 to 1,50
99  Iron Wire, sealed up in glass tube, (with magnet,),75
100. Instrurnent for measuring Attractive Force,...                15,00
101. Bar Magnet, for fracturing, each,............,12
107. Helix, on stand, (with iron bar,)............. 2,00 to 2,50
108. Heliacal Ring and Semicircles, (ball and socket joints,)  5,00
109. Heliacal Ring and Semicircles,..............  2,50 to 4,00
110. Grove's Battery, with covered top,.......... 6,00 to 8,00
11U. Heliacal Ring and Double Cylinders......... 4,00 to 6,00
112. Flat Spiral, per lb.,........................,75 to 1,00
113. Magnetizing Helix, per lb.,................. 1,25 to 1,50
114. Axial Bell Engine,.........................                   8,00
115. Axial Revolving Bar,....................... 6,00 to 8,00
116. Inclined Revolving Bar..5,............... 5M00 to 6,00
117. Inclined Bar revolving round Conductor,....                   6,00
118. Axial Revolving Circle,.....................  10 to 12,00
119. Vibrating Helix,............................                  4,00
120. Double Helix and U-shaped Bar.............                    1,50
121. Double Axial Bell Engine,..................                  8,00
122. Upright Axial Engine,......................                  15,00
123. Horizontal Axial Engine,...................                  18,00




OF APPARArUS.
Fit                                                          Price.
124. Double Beam  Axial Engine,..............$ 15,00 to 18,00
125. Single Beam  Axial Engine,.................               15,00
L26. -Small Electro-Magnet,....................,75 to 1,00
127. Electro-Magnet, in frame,.................. 8,00 to 12,00
128. Electro-Magnet, in case,....................10,00 to 25,00
129. Electro-Magnet, with three poles,..3,00
130. Steel Bar, of the U  form, for charging,......,25 to 50
131. Magnetometer,............................. 12,00
132. Axial Magnetometer,.......................12,00 to 15,00
133. Electro-Magnetic Telegraph,.................                5,00
134. Telegraph, with Clock-work,................25,00 to 35,00
135. Signal Key,................................                 2,50
136. Axial Receiving Magnet,...................                 20,00
137. Axial Telegraph,................. 6,00 to 10,00
138. Axial Telegraph, with Clock-work,.........25,00 to 35,00
139. Axial Telegraph, with Engine,..............                35,00
140. Reciprocating Armature Engine,............10,00 to 12,00
141. Horizontal Reciprocating Engine,..........18,00 to 20,00
142. Revolving Armature,.......................                  3,50
143. Revolving Armature Engine,................                  6,00
144. Vibrating Armature Engine,.................                10,00
145. Vibrating Armature Engine,................                 8,00
146. Revolving Electro-Magnet,..................                 5,00
147. Revolving Bell Engine,.....................10,00 to 12,00
148. Registering Revolving Magnet,............. 6,00 to 8,00
149. Electro-Magnet, revolving by the Earth's action,             8,00
150. Electro-Magnet, revolving by the Earth's action,             7,00
151. Electro-Magnet, revolving within a coil,.....               8,00
152. Double Revolving Magnet,.................. 7,00 to 8,00
153. Revolving Wheels, with electro-magnet,.....                 8,00
154. Revolving Wheels,.........................        8,00 to 10,00
155. Electro-Magnet, revolving on its axis,........              7,00
156. Iron Bar, for showing magnetic induction of Earth,,75
159. Attracting and Repelling Wires,............               3,50
160. Contracting Helix,.........................                3,50
162. Electro-Dynamic Revolving Coil,............                6,00
163. Electro-Dynamic Revolving Rectangle,......                 7,00
164. Instrument for showing the revolution of Mercury,           4,00
Same Instrument, with iron cup,............               5,00




CATALOGUE.
Fiture.                                                 Pricke
165. Clock-work Electrotome,.................. $ 6,00 to 7,00
167.. Fine Wire Coil, per lb.,...............1.....        1,50
168. Double Helix and Electrotome,..............          20,00
172. Galvanometer, Astatic Needle,..............           5,00
173. Magneto-Electric Machine,.................30,00 to 35,00
174. Improved Magneto-Electric Machine,......... 40,00 to 45,00
175. Magneto-Electric Machine, with Vibrating Electrotome, 45,00
176. Vibrating Electrotome,......................  4,00 to 5,00
177. Magneto-Electric Machine, for quantity,               40,00
179. Separable Helices, with handles,............10,00 to 12,00
181. Separable Helices and Electrotome,...........         20,00
Apparatus for showing the Suspension in Air of
an Iron Rod by Repulsion,..-............           3,00
182. Double Helix and Vibrating Electrotome, with
Battery and Handles,....................10,00 to 12,00
183. Compound Magnet and Electrotome,......... 8,00 to 10,00
184. Apparatus for showing Induction of the Earth,         12,00
INSTRUMENTS IN THE MEDICAL APPLICATION OF ELEC
TRICITY.- SECOND EDITION.
Figure.                                                 Price
1. Galvanic Battery, (according to size,)........ $ 2, 5, 7,00
2. Electro-Magnetic Apparatus, with Battery and
Handles,................................ 8,00 to 10,00
3. Electro-Magnetic Apparatus and Revolving
Armature with Battery,..................          15,00
4. Electro-Magnetic Apparatus and Vibrating
Armature,..............................10,00 to 12,00
5. Electro-Magnetic Box Apparatus, (complete,).          12,00
6. Magneto-Electric Machine.................. 30,00 to 35,00
7. Improved Magneto-Electric Machine,........25,00 to 30,00
8. Box Magneto-Electric Machine,............. 30,00 to 35,00
Covered Copper and Iron Wire, per lb.
Fine Platinum Wire for Experiments.
7 Davis's Manual, second edition, 12mo, 328 pages, with 180
original Illustrations, neatly bound, $1,25.
17  Persons addressing the publisher and remitting $1, post paid,
san receive a copy in paper covers by mail