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INDUCTION COILS
AND
COIL-MAKING
BY THE SAME AUTHOR.
Crown SvOy cloth, with 160 Illustrations, price 3s. 6(2.
PRACTICAL ELECTRIC-BELL FITTING.
A Practical Treatise on the Fitting-up of Electric Bells and
Auxiliary Apparatus.
Crown Svo, cloth, with 209 Illustrations, price 5*.
TELEPHONES: THEIR CONSTRUCTION AND FITTING.
A Treatise on the Fitting-up and Maintenance of Telephones
AND the Auxiliary Apparatus.
Crown 8vo, cloth, with 224 Illustrations, price 5s.
PRACTICAL ELECTRIC-LIGHT FITTING.
A Treatise on the Wiring and Fitting-up of Buildings deriving
Current from Central Station Mains, and the Laying-
down OF Private Installations. With Latest
Edition of Ph(enix Fire Office Rules.
Crown 8vo, cloth, with 177 Illustrations, price 3s. 6d.
ELECTRIC-BELL CONSTRUCTION.
A Treatise on the Construction of Electric Bells, Indicators,
AND Similar Apparatus.
( Frontispiece
PRODUCED BY ELECTRIC DISCHARGES ON PHOTOGRAPHIC PLATES.
See PaQc^fo.').
ispiece )
INDUCTION COILS
AND
COIL-MAKING
A TREATISE
ON THE CONSTRUCTION AND WORKING OF SHOCK,
MEDICAL AND SPARK COILS
AUTHOR OF
* PKACTICAL ELECTEIC-BELL FITTING ' ; ' TELEPHONES, THEIR CONSTRUCTION AND FITTING ' J
♦practical ELECTRIC-LIGHT FITTING'; * ELECTRIC-BELL CONSTRUCTION,' ETC.
ilontion:
E. & F. N. SPON, 125 STEAND
SPON & CHAMBERLAIN, 12 CORTLANDT STREET
1894
BY
F. C. ALLSOP
With 118 Illustrations
PEEFACE.
In compilii^g this book it has been my aim to produce a
practical manual that will prove of service not only to
those engaged professionally in the construction and
repairing of coils but also to the medical man a.d amateur
coil-maker. I have given, therefore, in addition to
dimensions and full details for constructing different
kinds of coils, many practical hints and suggestions that I
trust will enable medical men and others having the care
and working of such instruments under their charge to
keep them in efficient working order with the minimum
amount of trouble and expense.
A large portion of the book has already appeared as a
series of articles in the 'English Mechanic,' and I have
also collected together much interesting and useful informa-
tion on coil constructing gleaned from the last twenty-five
years of back volumes of that journal.
r. C. ALLSOP,
Of F. C. Allsop & Co.,
„ MaNUPACTUEING ELECTErCIANS
165 Queen Victoria Street, ""^^ans.
London, E.G.
696206
CONTENTS.
CHAPTER I.
PAGE
Induction .. 1
CHAPTER 11.
Hints on the Construction of Coils Generally .. 12
CHAPTER III.
Shock and Medical Coils .. ..46
CHAPTER IV.
Accessory Appliances for, and the Application of
Medical Coils .. .. .. 81
CHAPTER V.
Spark Coils .. .. 92
CHAPTER VI.
Experiments with Spark Coils ..
130
viii INDUCTION COILS AND COIL-MAKING.
CHAPTER VII.
PAGE
Batteries for Coil- Working .. .. .. ..137
CHAPTER VIII.
Faults in Medical and Spark Coils .. .. 148
CHAPTEE IX.
Figures Produced by Electric Discharges on Photo-
graphic Plates .. .. .• .. 153
Index .. .. .. .. 159
ILLUSTEATIONS.
FIG. PAGE
Figures produced on sensitive plates by electric discharges Frontispiece
1 Iron filings round poles of magnet . . .. .. .. .. 2
2, 3 Magnetic whirls round wires carrying electric currents .. 3
4 Electro-magnet .. .. .. .. .. 4
5 Apparatus for observing the phenomena of induction . . . . 6
6 Circuits of coil with both primary and secondary . . . . 9
7 Primary shock coil .. .. .. .. .. .. .. 10
8-10 Bobbin with iron core .. .. .. .. .. 17
11-13 Various forms of bobbin ends .. . . .. .. .. 18
14 A coil-winder .. .. .. .. .. 23
15 Continental method of winding coils .. .. .. .. 29
16 Vertical contact-breaker .. .. .. .. .. .. 31
17,18 Horizontal contact-breaker .. .. .. .. 32
19-21 Separate contact-breaker .. .. .. .. 33,34
22-24 Variable contact-breaker 36-39
25-28 Terminals 40
29 A discharger .. .. .. .. .. .. 41
30 Building up the condenser . . .. .. .. .. 44
31 Condenser finished .. .. .. .. .. 45
32 Tube method of regulation .. .. .. .. 47
33 Switch method of regulation .. .. .. 48
34 Sledge method of regulation .. .. .. .. .. 48
35 A method of regulating shock from primary coils .. .. 49
36 Primary shock coil .. .. .. .. .. .. 51
37 Method of winding last layer .. .. .. 52
38 Section through primary coil .. .. .. .. 53
39 Small medical coil .. .. .. .. .. 55
40 Small medical coil (section) .. .. .. .. 56
41 Small medical coil (end elevation) . . .. .. 56
X INDUCTION COILS AND COIL-MAKING.
FIG. PAGE
42 Small medical coil, connections of . . .. 57
43 Bath coil 59
44 Bath coil, side elevation .. .. 60
45 Bath coil 61
46 Bath coil, contact stud for .. .. 61
47,48 Bath coil, switch lever for .. .. .. 61
49 Bath coil, connections for .. .. .. ., 62
50 Bath coil, another method of connecting . . . . . . . . 64
51 Bath coil, another form .. ., .. 65
52 Bath coil, connections for ., .. .. .. 65
53-55 Sledge coil 67
56 Sledge coil, plan of .. .. .. .. 68
57 Sledge coil, connections for . . .. ., .. .. 69
58, 59 Portable coil 70, 71
60-62 Portable sledge coil set 72-74
63 Street coil 78
64 Electro-medical cabinet .. .. .. .. .. 83
65 Connecting cord .. .. .. .. .. .. 84
66-73 Electrodes 84, 86
74 Eye electrode 86
75, 76 Brush electrode 87
77 Roller electrode 87
78 Needle electrode .. .. .. .. 87
79 Galvanometer .. .. .. 88
80 Milliamp^re-meter .. .. .. 88
81 Collector 89
82 Current reverser .. .. ,. .. .. 90
83 Metallic resistance .. .. .. .. .. .. .. 90
84 Liquid resistance .. ., .. .. .. 91
85, 86 An inch spark coil 94, 95
87 Connections of spark coil without commutator .. 96
88 Connections of spark coil with commutator .. .. .. 97
89 Ordinary method of winding .. .. .. 100
90 Coil wound in two sections .. .. .. 100
91 Coil wound in four sections .. .. ., 101
92 Coil wound in eight sections .. .. «. .. 101
ILLUSTBATIONS.
FIG. PAGE
93 Ordinary method of winding .. .. .. 102
94 Coil wound in two sections .. .. .. .. 102
95 Coil wound in four sections .. .. .. .. .. 103
96, 97 A 2-inch spark coil 105, 106
98 A 12-inch spark coil 108
99 Magnetic field of coil ., Ill
100 Section-winder for 12-inch coil .. .. .. .. 112
101 Winding the sections .. 113
102 The Spottiswoode coil 123
103,104 Vacuum tubes 131
105 Simple method of holding tubes . . .. .. 131
106-108 Compound vacuum tube .. .. .. 132
109 Vacuum tube rotator .. .. .. .. .. 132
110,111 Dry battery 138
112, 113 Leclanche battery 141
114,115 Bichromate battery .. .. .. .. 141
116 Edison-Lalande battery .. .. .. .. .. .. 143
117 Bunsen battery .. .. .. 146
118 Galvanometer for testing .. » , .. .. 149
INDUCTION COILS AND COIL-MAKINa.
CHAPTEK L
INDUCTION.
Induction coils may roughly be divided into three different
kinds. We have, first, the ordinary " Shocking " Coil, with
its comparatively mild effects ; second, the Medical Coil, which
is more powerful, and has usually an arrangement allowing
a primary, secondary, and in many forms a tertiary current
to be administered ; and, third, the Spark Coil, which is too
powerful for shocks or medical use, and intended chiefly for
experimenting. All these three kinds of coils — which are
very similar, the main difference being chiefly in the propor-
tion of the windings — depend for their action on the inductive
effects an alternating or interrupted current circulating in
one coil produces in another surrounding it, and before
passing on to their construction it will be as well to just
glance at the phenomena of magnetism and induction.
The space all round a magnet in which its influence is
felt is called the magnetic field of the magnet, and the dis-
tribution of this field we are enabled to see by the aid of
some iron filings. If we take a bar-magnet, and, laying it
in a horizontal position, place on the top of it a piece of
sheet glass on which have been sprinkled some fine iron
filings, it will be found that on gently tapping the glass the
B
2 INDUCTION COILS AND COIL-MAKING,
iron filings will arrange themselves somewhat as in Fig. 1.
They will collect chiefly round the poles, as shown, and set
themselves in definite lines or curves, which spread out
and pass from pole to pole. These lines, which vary
according to the shape of the magnet, show the direction of
the field, and are called " lines of magnetic force " ; or, for
short, " lines of force." If we further investigate the field
of a magnet we find that the lines of force apparently
endeavour to pass from pole to pole, and in doing so
encounter a certain resistance from the air. This latter
point is made evident from the fact that if a piece of iron is
placed anywhere in the magnetic field, those lines of force
Fig. 1. — IRON FILINGS BOUND POLES OP A MAGNET.
close to — and whose path does not naturally lie through —
the iron will be immediately diverted from their course, and
pass through the iron in preference, and the lines of force
passing through the space occupied by the iron will be much
denser than were the iron not there, thus indicating that
iron is a better conductor of lines of force, as it were, than
air. No other metal or substance will be found to greatly
afi'ect the field, which remains practically undisturbed,
whether wood, glass, stone, &c., is placed in it, the lines of
force passing through them just the same.
There is one thing, however, that will affect and seriously
disturb the magnetic field of a magnet. This is a wire
carrying an electric current. Electricity and magnetism are
INDUCTION.
3
closely allied, and Oersted discovered in 1820 that a wire, no
matter its composition, carrying an electric current, possessed
certain magnetic properties which ceased the moment the
current was stopped. All round a conductor in which a
current is flowing, there is in fact a magnetic whirl, and by
the aid of our sheet of glass and iron filings, we are enabled
to see these lines of force outside the conductor. To do this
the glass must have a hole bored in the centre, and be
supported in a horizontal position. If now a wire carrying
a fairly large electric current be passed through the centre
of the glass on which iron filings have previously been
sprinkled, the filings will be found to arrange themselves
Figs. 2, 3. — magnetic whiri s round wires carrying electric
currents.
into concentric circles as shown in Fig. 2. Again, if we
take a suspended compass needle, and stretch near it a wire
carrying an electric current, the direction of the wire being
parallel with the needle, it will be found the needle will
swing round and set itself at right angles to the wire, the
direction in which it swings being governed by the direction
in which the current is flowing, and also whether the wire
be above or below the needle. If we take a piece of insu-
lated wire and twist it into a coil, we find, on passing a
strong current of electricity through it and testing it with
the iron filings or compass needle, that the helix presents a
field of force as shown in Fig. 3, which is similar to that of
the permanent magnet in Fig. 1. On further experimenting
B 2
4 INDUCTION COILS AND COIL-MAKING.
with this coil, it will be found that the magnetic effects are
greatly intensified if a soft-iron rod is slipped through the
centre of helix, as shown in Fig. 4. The iron rod, it will be
found, has become magnetised, and exhibits all the phenomena
that a permanent magnet of that form does, with, however,
this difference, the magnetism only lasts so long as the current
is flowing in the helix, and ceases the moment the current
ceases. This form of magnet is called an electro-magnet.
If, instead of the soft-iron rod, we place in the centre of the
helix a hard steel one, we shall find that the ^teel has also
become magnetised, though in a lesser degree, and that on
the cessation of the current the steel rod retains most of its
magnetism, and has become permanently magnetised. On
Fig. 4. — electro-magnet.
trying hard cast iron we find the same effects as with the
soft iron, but that the cast iron retains some of its mag-
netism ; in fact, the harder the iron the more magnetism it
retains. This magnetism that remains in an electro-magnet
after the magnetising influence has ceased is called " residual
magnetism."
Soft iron, as we have said, loses its magnetism the moment
the magnetising influence is withdrawn ; steel or hard iron
do not ; and from this we see that in the construction of
electro-magnets soft iron is the best — the softer the better.
For this reason the iron core of induction coils is made up of
a number of lengths of thoroughly annealed charcoal iron
wire. No. 20 or 22 gauge, bound into a bundle, thus producing
INDUCTION.
5
an iron core of the softest possible nature, and one that is
most rapidly magnetised and demagnetised. If in Fig. 4 we
have some means of testing the amount of magnetism in the
iron rod, we find that as we increase the current so we
increase the magnetism in the rod. We find also that if,
instead of increasing the current, we increase the number of
turns of wire round the iron rod, we likewise increase the
magnetism. In constructing electro-magnets we are accus-
tomed to speak of the " ampere turns " of the magnet — that
is the number of turns of wire multiplied by the current in
amperes. We can get the same amount of magnetism in a
given piece of rod, whether we have a large number of turns
of wire and a small current, or a large current and a small
number of turns of wire. For instance, 10 turns of thick
wire carrying 40 amperes round a piece of iron rod, will
give approximately the same magnetism as 100 turns of
thinner wire carrying only 4 amperes. We find also that
after a certain stage it is of no use to increase the current or
wind on any more turns of wire, as the magnetism does not
increase, or, if it does, only in a very slight degree. This is
because the iron rod has become so saturated with magnetism
that it will not take up any more, and any further expendi-
ture of power is simply being wasted. The iron rod is said
to be " saturated," and the point at which the iron rod
becomes saturated depends on the size and quality of the
iron. The softer the iron and the thicker the rod the more
magnetism it will take up, as the iron will only admit of a
certain number of lines of force per square inch. Cast iron
will not admit of so many as wrought, and hard steel still
less, so that to get the best results in electro-magnets the
iron should be as soft as possible, both because a soft iron
core can be the most powerfully magnetised, and will the
most rapidly take up and part with its magnetism.
There is another effect produced by a wire carrying an
electric current, and that is its inductive action or the
6 INDUCTION COILS AND COIL-MAKING,
induction " it produces on wires or conductors in its imme-
diate neighbourhood. A wire carrying an electric current
that is alternating or pulsating in its character, will be
found capable of inducing, under certain conditions, a similar
current in any closed circuit within range of its field. The
magnetic whirl round a wire, which increases as the current
increases, extends, there is not the slightest doubt, very much
further than we are able to trace it with iron filings. This
is made evident by the fact that the feeble currents in one
telephone wire, the existence of which it would be difficult
to detect externally, will induce currents in neighbouring
wires, some two feet or three feet off, sufficiently powerful to
Fig. 5. — apparatus for observing the phenomena of induction.
be annoying. The inductive action of one wire carrying an
electric current on neighbouring wires can best be observed
in the following manner : — Procure a narrow wooden bobbin
(A, Fig. 5) six inches long and having a ^-inch hole through
the centre, and on this bobbin wind four layers of No. 20
cotton-covered copper wire, fastening the ends of the coil
under two terminals at the top, as shown in the figure.
Next, get another bobbin (B, Pig. 5) the same length as the
former but considerably wider, and having a hole in the
centre of sufficient width to allow of the coil A being easily
and quickly slipped inside of it. This bobbin wind with
about thirty layers of No. 40 double cotton-covered wire» the
INDUCTION.
7
different layers being insulated with a wrapping of paraffined
paper, and the ends of the wire fastened to two terminals as
previously described. Then connect the coil B by two wires
to a sensitive galvanometer D, and join two wires to the
coil A, so that it can be connected to a powerful battery
C, as in the figure. Let us first experiment with the coil
B by itself. If we plunge a powerful bar permanent mag-
net into the centre of this coil we find that the needle of
the galvanometer swings sharply to one side, finally
returning to its position of rest, and also that on quickly
withdrawing the magnet the needle swings to the other
side. This shows that a current of electricity was twice
momentarily generated in the coil, first at inserting and
again on withdrawing the magnet, the two currents being
in different directions. This is the phenomenon of induction,
and to produce these induced currents in the coil we must
have a movement between the coil and the magnet. The
coil must be moved so as to cut lines of force or alter the
number passing through it, and the currents are momentary,
lasting only while the movement continues. The mere
leaving of the magnet in the coil will have no effect, as will
be seen by the needle of the galvanometer going gradually
back to its position of rest. We saw from Fig. 4 that if we
send a current through a helix we get a magnetic field and
effects similar to a bar magnet. By plunging a bar magnet
into the coil B we send lines of force through it somewhat
similarly distributed to those produced by a current flowing
through the coil itself, so that we might almost have
anticipated that if we create the magnetic whirl around the
coils of wire we shall get a momentary current in the
helix. Let us now lay aside the permanent magnet, join up
the coil A to a battery, and insert a soft iron rod in the
centre. We have now an electro-magnet with a field of
force similar to the permanent one with which we have
just been experimenting, and therefore we naturally infer
8 INDUCTION COILS AND COIL-MAKING,
that if we plunge the coil A into the coil B we shall get
the same effects as we did with the permanent magnet.
This, on experimenting, will be found to be the case ; we
get a momentary current on plunging the coil A into the
coil B, and again a momentary one in the reverse direction
on withdrawing it. We will now go a step further: if
plunging the electro-magnet A into the coil B induces
currents, then making and breaking the battery connection
with the coil A, and thus rapidly magnetising and de-
magnetising the coil A inside the coil B should" produce
the same results, as this is tantamount to plunging in the
magnet and withdrawing it. This, on making the experi-
ment, will again be found to be the case, and in the two
coils we have the main parts of an induction coil, the
primary coil A, through which circulates the current from a
battery, and the secondary coil B, in which we get the
induced currents. It needs only, therefore, to insert an
automatic make-and -break in the primary or battery cir-
cuit, and we shall then get a quick succession of currents in
the primary coil which induce others in the secondary coil,
these latter being of considerably higher tension and of a
rapidly alternating character.
Fig. 6 shows how the two circuits of an induction coil are
arranged, P and being the ends of the primary and S and
the ends of the secondary. The primary winding is shown
darker than the secondary.
The same effects would be produced by the coil A even if
it had no iron rod in it, but of a very much feebler character.
If we further experiment with the two coils, and compare the
effects produced by inserting the coil A first slowly and then
quickly into the coil B, and also with a hard steel and then
with a soft iron rod in the centre of the coil A, we shall find
that the more rapid the motion, and also the softer the iron
core, the more powerful the effects. By employing a bundle
of soft iron wires for the centre of the primary coil of an
INDUCTION.
9
induction coil, we have a core that will not only absorb the
most magnetism but also allow itself to be the most rapidly
magnetised and demagnetised.
The current in the secondary coil B makes itself manifest
in other ways than by its effect on the needle of the galvano-
meter, the most prominent being, tirst, a peculiar sensation
(called an electric shock) in the wrists and up the arms when
the ends of the coil are held one in each hand, and, second,
a rapid succession of sparks between the ends of the coil if
brought near together. On seeing these large sparks taking
place between the ends of the secondary coil while those in
the primary are comparatively small, beginners are apt to get
r-~r" ®
m^x^Ui- ' ^ ^
P'L i s'
Fig. 6. — circuits op coil with both primary and secondary.
a wrong idea of the action of a spark coil, concluding that
the coil generates more electricity than is supplied by the
battery. This, however, is entirely wrong, for, as a matter
of fact, the electric energy in the secondary coil is less than
that in the primary, and the large spark arises from a higher
potential of the secondary current, which thus enables it to
leap across gaps in the circuit. Even were the winding of
the primary and secondary coils exactly similar, the induced
current would still have a higher potential than the induc-
ing current ; but as in spark coils the secondary consists of
thousands of turns of very fine wire, the current generated in
it is naturally of a very high potential.
Small shock coils are sometimes made, however, with a
primary winding only, as in Fig. 7, and here the shocks arise
10 INDUCTION COILS AND COIL-MAKING.
from the battery current in the coil inducing, at the moment of
cessation, a momentary current of a considerably higher poten-
tial than the battery current in the coil itself. This is called
Fig. 7.— primary shock coil.
the " extra current," and arises from the "self-induction'' of
the coil, and in constructing a primary coil our aim is to use
a bobbin, the self-induction of which is as high as possible.
If we turn again to our battery and experimental coils, as
shown in Fig. 5, we find that when we join the two wires
from the battery and pull them apart again we get a small
spark at the moment of breaking contact. Connect, now, one
wire from the battery to one terminal of the coil A (the iron
core being in the centre), and momentarily touch the other
wire on the other terminal, and it will be seen that the spark
we now get at the moment of opening the circuit is very
much brighter and longer. This arises from the fact that
the current in the coil has, at the moment of breaking the
circuit, induced another in the coil of much higher potential,
thus enabling it to jump across a much wider gap. It is not
that the coil has augmented the original battery current, as
this must necessarily be less (owing to the resistance of the
coil) than when we short-circuited the battery. Moreover,
when we connect into the circuit any old magnet coils we
may have by us, we find that as we connect more in, so we
increase the spark at break, and this is because we have
INDUCTION,
11
raised the self-inductive capacity of the circuit. If we take
the two wires at the point where we have been breaking the
circuit, and twist each end of the wire round a separate piece
of metal, we shall find that if we hold one of these pieces of
metal in each hand, touch them together, and then pull them
apart again, we feel a smart shock, owing to the induced
current passing through the somewhat lower resistance of
the body in preference to sparking across between the two
pieces of metal through the air.
It is in this manner a shock is obtained from a primary
coil, the two handles being connected (see Fig. 7) one each
side of the make-and-break. It is possible to wind wire into
a helix in such a manner that all self-induction in the helix
is practically eliminated, as is done in the case of resistance
coils for testing purposes, where the wire is coiled on the
bobbin so that the inductive effect of the one wire running
from the one terminal is neutralised by that in the one
running back to the other.
12 INDUCTION COILS AND COIL-MAKING.
CHAPTER 11.
HINTS ON THE CONSTRUCTION OF COILS GENERALLY.
Before we can proceed with the construction of any coil, the
correct sizes and amounts of primary and secondary windings
necessary to prodiice the shock or length of spark desired
must be worked out, in order that the proper size of bobbin
to employ can be determined. The bobbin is the first part
that should be constructed, and the right dimensions for this
having been determined the proper proportions for the other
parts of the coil can at once be decided upon. As soon as the
size of bobbin is ascertained a rough sketch to scale should be
made, showing in plan and elevation the complete coil, the
six principal parts of which are enumerated below in the
order in which they should be constructed.
These are : —
1st. The bobbin.
2nd. Iron core of bobbin.
3rd. Primary winding.
4th. Secondary winding.
5th. Contact-breaker and terminals.
6th. Base or stand.
These comprise the essential parts of every coil, though
to the list must occasionally be added the tertiary winding
with which some medical coils are provided, and the con-
denser used with spark coils.
CONSTRUCTION OF COILS GENERALLY, 13
Determining Size of Primary and Secondary Windings,
The primary winding, as was pointed out in Chapter I.,
is the wire wound directly on the iron core, and is comprised
in the primary circuit, consisting also of the battery and con-
tact-breaker; while the secondary is wound outside the primary,
and has electric currents generated in it by induction from
the primary. The low tension current in the primary is in
the secondary converted into one of high potential, and as we
almost always want, in an induction coil, to produce currents
of very high potential, the primary is wound with a few turns
of thick wire and the secondary with a large number of turns
of very fine wire. We have in the primary circuit a some-
what large current of low electro-motive force, and our object
as regards the primary winding is, therefore, to provide such
a path round the iron core as will make most use of this
current in magnetising it.
It is this consideration almost wholly that governs the
winding of the primary in the spark coils, though with
medical coils giving a primary shock as well as a secondary
the case is somewhat different.
Taking first the case of spark coils, however, our object
is to get a primary winding that will give the most intense
magnetism to the core, occupy a minimum of space, and be
least troubled with self-induction.
Self-induction, as we saw in Chapter I., is the extra current
induced by the primary current in the primary winding itself,
and its chief effect is to momentarily retard the filling and
emptying (if we may so call it) of the primary coil. We saw
also that the self-induction increased with the number of
turns of wire, so it is evident that if we wish to reduce the
self-induction to a minimum there must be as few turns as
possible on the primary consistent with getting the requisite
magnetism. In spark coils it is usual, therefore, never to put
on more than two layers of primary, but to increase the gauge
14 INDUCTION COILS AND COIL-MAKING.
of the wire and the size of the cells as the size and sparking
power of the coil is increased.
Thus, for spark coils giving \io\ inch spark, No. 24 and 22
(B.W.G.) wire is usually employed, No. 20 for J-inch spark,
No. 18 for f to 1-inch spark, No. 16 for sparks from 2 to
3 inches, No. 14 for 6 to 8-inch spark, and No. 12 for 8 to
12 inches.
With medical coils, however, giving primary shocks as well
as secondary, it is usual to put on from four to six layers of
No. 24, 22 or 20, according to the dimensions of the coil.
Since the primary shock arises from the extra current in the
primary on interrupting the circuit, two layers would not be
sufficient, so more are put on, and the effects of self-induction
on the working of the coil are of little moment in this case,
as we do not aim at a very high efficiency from the secondary,
the secondary shock being always quite strong enough for
most people.
The rule, therefore, for the amount of wire to be put on
the primary winding is simple enough — two to three layers
for spark coils, four to six layers of smaller gauge for medical
coils — and once the gauge of wire is determined it is an
easy matter to find the depth of space it will occupy on the
bobbin.
The primary of a spark coil will have, of course, a very
much lower resistance than a medical coil of the same size ;
but then the battery employed with a spark coil is of a much
more powerful kind, and capable of sending a large current
through this low resistance.
The secondary is wound with a large number of turns of
very fine wire. No. 36, 38 or 40 for spark coils, and No. 34 or
36 for medical coils. In spark coils the object is to get as
high electro-motive force or pressure as possible, as upon the
E.M.F. depends the sparking distance or size of spark the
coil will give. Every additional turn on the secondary in-
creases the E.M.F. , and since the finer the wire the more
CONSTRUCTION OF COILS GENERALLY.
15
turns we can get on the coil, a fine wire is always used —
No. 40 or 38 for the coils up to 6-inch spark, and No. 36 for
larger ones. If a thicker wire is used the spark will not bo
so long but it will be brighter and thicker, and thus if a good
thick spark is desired No. 36 should be employed. The inner
layers, of course, contribute more to the effects than the outer
ones, being nearer the inducing current and thus more strongly
acted upon. Each succeeding layer, therefore, although it
must necessarily contain more wire than the preceding ones,
adds less and less to the effects, till, after a certain thickness
has been obtained, it is useless to wind on more layers. In
actual practice, of course, this point is never reached with
induction coils, as if we want to put a large amount of
wire on the secondary we lengthen the bobbin, so that the
last layer is well within the field of induction.
For medical coils a thicker wire is used for the secondary,
firstly, because we do not want anything like such powerful
effects, and, secondly, because the shock produced by a thick
secondary winding is less stinging and more pleasant than a
tbin one. No. 36 is the size usually employed for small
medical coils, and No. 34 for larger ones. Of course, if a
medical coil is wanted that will go in a very small space
No. 38 or 40 can be employed for the secondary, but then a
very small quantity must be used.
In specifying the amount of secondary on a coil it is cus-
tomary to speak of it by weight, so many ounces or pounds
of No. 38, &c. — ounces in the case of medical coils, but it soon
runs into pounds with large spark coils. A rough rule is to
allow about | lb. No. 40 and 1 lb. No. 38 or 36 on the secondary
for every inch of spark required, but so much depends, in a
spark coil, on the skill of the constructor that no really definite
rule can be given. The dimensions and quantities of wire for
several medical and spark coils are given in Chapters III.
and v., and are taken in every instance from coils actually
constructed.
INDUCTION COILS AND COIL-MAKING,
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CONSTRUCTION OF COILS OENEBALLY. 17
On page 16 is given a table of the resistance per lb.,
length of feet to ohm, &c., of the different sizes of copper
wires, which will be found of great use in working out the
proportions for the primary and secondary of coils.
Bohhins.
These should be either of wood or ebonite, and the ends or
cheeks may be round, square, or any special fancy shape
Figs. 8, 9, 10.— bobbin with ikon core.
desired. The form of bobbin generally employed, however,
for medical coils and small spark coils has round ends, as
shown in Figs. 8, 9, and 10, Fig. 8 being a section through
the bobbin showing the iron core, and Fig. 9 a view from the
contact-breaker end. Fig. 10 shows the end of the iron core
fitted with a brass bush, about which we shall treat later on.
The bobbin is intended for a medical coil, and has, therefore,
it will be seen, a space between the iron core and inner sur-
face of the tube of the bobbin for the brass regulating tube.
Next to the round end, the square end, as shown in Fig. 11,
is a favourite one for spark coils, as is also the form of end
shown in Fig. 12, which has its rectangular appearance
G
18 INDUCTION COILS AND COIL-MAKING.
removed by grooving off the corners. Fig. 13 is a form of
end frequently employed in medical coils where a separate
contact-breaker is used, the upright piece at the top being
required for carrying the brass arm of the contact-breaker.
An enlarged view of this and other forms of bobbin ends
will be found in the illustrations of the complete coils in
Chapters III. and V.
Beech and boxwood ebonised, mahogany and walnut, are
the favourite woods out of which bobbin ends are made,
though ebonite is largely used and has the advantage in
being a better insulator. In constructing the bobbin there
are three methods that may be employed: 1st, the bobbin
Figs. 11, 12, 13. — vakious forms of bobbin ends.
may be turned up complete out of a solid block of wood ; 2nd,
the ends may be fixed direct on to the iron core, and, 3rd, a
paper tube may be made and the bobbin ends firmly fixed
on to it by glue. The first method is only admissible for
very small coils, owing to the difficulty of turning down
the tube of the bobbin to the required thinness without
breaking it.
The block of wood out of which the bobbin is going to be
turned should be roughly turned to size, after which the
centre hole is bored, a metal rod (of such size as will only
give just sufficient grip for turning) driven into it, and the
bobbin finished on this.
Sharp tools are lequisite, and great care must be exer-
cised in turning bobbins of this kind, as the body of the
CONSTRUCTION OF COILS OENEBALLY. 19
bobbin must be turned as thin as possible consistent with
safety.
In constructing a bobbin by the second method, first turn
up two bobbin cheeks of the required dimensions and ebonise,
or if made in mahogany or walnut, polish them. Now cut
off the lengths of the iron wire, and work them into a neat
bundle by sliding one of the bobbin cheeks on each. end.
Next procure some thickish cartridge paper, and cut two or
three strips 2 J in. wide, which strips must afterwards be
rolled round the iron core, fastening each layer with some thin
glue till a tube of about ^V^d of an inch in thickness is formed
on it. The bobbin cheeks can now be slipped off, the inside
of the holes glued, the cheeks slipped back on to the core,
and the whole firmly fixed in a clamp till perfectly dry.
The paper tube must, of course, be rolled on the iron core
at the proper place, which can be ascertained by measure-
ment, and the cheeks of the bobbin made to butt against
the ends of the tube.
For spark coils it is advisable to use melted paraffin wax
to fasten the different layers together, and when finished the
inside of the bobbin must be well basted with the wax.
Perhaps the best bobbin can be made by the third method,
to do which proceed thus: — Having turned up or cut out the
two ends of the desired size and shape, bore a hole in the
centre of each, which hole is somewhat enlarged on the inner
sides, as shown in Figs. 8 and 9. A paper tube is next pre-
pared by rolling cartridge-paper smeared with thin glue round
a ruler (the ruler having been previously rubbed with soap
to prevent its sticking), which is afterwards withdrawn, and
this tube, when perfectly dry, has the wooden ends affixed
to it, this being done by smearing the ends of the tube with
glue and inserting them into the enlarged part of the holes,
leaving the bobbin so formed to harden in a clamp if possible.
This form of bobbin is best suited for shock and medical
coils having the tube method of regulation.
c 2
20 INDUCTION COILS AND COIL-MAKING,
For large spark coils it is advisable to employ ebonite ends
and separate primary and secondary by a thin ebonite tube.
The bobbin ends must first be prepared as before described,
except that the hole is of the same size all along in order to
allow the ebonite tube to pass right through and project
beyond the bobbin cheeks. For the tube of the bobbin either
an ebonite tube of the required diameter can be procured or
one can be constructed by warming thin sheets of ebonite and
bending round a ruler, the joins being fastened off with shellac.
Two or three layers are put on until a tube of the required
thickness is formed, care being taken to see that the joins of
the several layers are made at different sides of the tube.
The size of the bobbin is determined chiefly by the amount
of wire there is to be put on, and the quantity being known
the amount of space it will occupy with the layer of paper
and other insulation must roughly be calculated out. It is
advisable to leave a good margin, as so many things may
occur in winding on the wire that it is not possible to cal-
culate exactly, while, moreover, a little space to spare after
all the wire is on is not objectionable, as it can be filled
in with a layer of string or wrapping of velvet or sheet
ebonite to prevent mechanical injury and improve the appear-
ance.
As to the thickness of the bobbin cheeks and tube, the
former, if of ebonite, should be from \ to j, if of wood J to
J inch, while the latter will vary from to according to
the size of the coil and the length of spark it is intended to
give.
Iron Cores.
We saw in Chapter I. what an important part the iron rod
in the centre of the primary coil played, and how necessary
it was that this core should be of the best and softest iron.
The best cores are made of a bundle of No. 22 or 20 best
charcoal iron wire thoroughly well annealed, and as this
CONSTRUCTION OF COILS GENERALLY. 21
wire varies mucli in quality, the reader is advised to use
that of the best makers only. It can be purchased either
in bundles of required lengths, or in one length on a coil,
but if bought in this latter form each length, after being
cut off, requires to be carefully straightened. The length of
the iron core should be such that it projects about a quarter-
inch at each end of the bobbin for small spark coils, but at
one end only if for a medical' coil. The lengths of wire,
after being cut off, are slipped into the centre of the bobbin
(a little patience and care being requisite in getting in the
last one or two), so as to form a good solid core. In medical
coils having a regulating brass tube sliding over the iron,
the core must be made into a bundle so as to leave a small
air-space between the outside of the iron core and the inside
of the bobbin-hole for the tube to work in. This form of
core is shown fixed in its place inside the bobbin in Figs. 8
and 9, the core being fitted with a brass bush r, as in
Fig. 10, which bush is forced into the bobbin, so that the
iron core touches the bobbin nowhere but at this point.
The core is best made into a bundle for this purpose as
follows : — Slip the different lengths, after being cut off, into
a metal tube, the inside diameter of which is the exact size
you wish the core to be. When the tube is full slide out one
end of the bundle of wires ; bind it firmly with some fine
wire, and then dip the end for half an inch into some melted
solder, using spirits of salt as a flux. Let it cool, smooth
off with a file, and then proceed to treat the other end in the
same manner. When quite cool it can be forced into the
brass bush r, and fitted into the bobbin. The brass bush, of
course, goes into the contact-breaker end, and besides its use
in holding the core, also gives a better finish to that end of
the bobbin. With spark coils it is preferable not to make
the core into a bundle, as this process somewhat hardens the
wire, but to force as many lengths of wire into the centre
of the bobbin as possible^
22 INDUCTION COILS AND COIL-MAKING.
Some Continental manufacturers employ an iron core
consisting of thin sheet iron rolled up into a tube of several
layers, as shown in Fig. 15, but this method is not so
efficient as the bundle of iron wires, although for small
medical coils, if the centre of the tube is filled with iron
wire, the difference should be inappreciable.
In medical coils, where the regulation is effected by sliding
in and out the iron core, a solid core is sometimes employed,
on which is marked a scale to show the distance it is drawn
out. A better method, however, where a movable core is
required, is to roll up a stout paper tube that will slide
easily into the centre of the bobbin, fill the centre with
lengths of iron wire, and after fitting a small wood knob
or handle at one end, plug up the other with a short cork,
cutting off any that projects. In such a manner a very neat
core can be made, the surface of which can be covered with
coloured paper and pasted with a printed scale.
The regulating tube of a medical coil is a thin brass one,
of such diameter that it will just slide over the iron core.
It is fitted at the outside ends with a handle or knob for
operating it, this handle being of ebony or ebonised box-
wood, and af&xed to the tube either by some cementing
compound, or what is perhaps better, by making the part of
the knob entering the tube a good fit, and then centre-
punching the tube on either side. The tube should slide in
and out of the bobbin easily and steadily, but without
unnecessary play, the tube bearing on the hole in the centre
of the bobbin, and leaving ample clearance between the core
and the inner surface of the tube. The easy working of the
regulating tube is a most important point in all medical
coils, as it allows the increase and decrease in the strength
of the shock to be made very gradually, and not in a jerky
or sudden manner, as must necessarily be the case where the
tube fits somewhat tightly.
CONSTRUCTION OF COILS GENERALLY. 23
Winding the Primary,
Before passing on to the winding, we will first describe
the construction of a *' coil- winder," a piece of apparatus
which will be found almost indispensable should the operator
not be possessed of a lathe, while even when this is the case,
the employment of a coil-winder will often be found more
suitable and convenient. A convenient form of coil- winder
used by the writer for winding coils is shown in Fig. 14.
The apparatus consists of a stout wood base m, to which are
Fm. 14.— A COIL-WINDER.
fastened the two standards n and r of |--in. by 1-in. iron rod.
At the top of each of these standards a hole is drilled to carry
the iron spindle c, which can be rotated by the handle h.
At the right-hand end of this spindle is the fixed coned
stop /, while at the other end is the adjustable one d,
running on the threaded portion of the spindle. This
adjustable stop consists, it will be seen, of a milled metal
disc secured to the end of a piece of brass tube, which is
drilled and tapped to run on the spindle c. Three pieces of
24 INDUCTION COILS AND COIL-MAKING.
triangular sheet-brass are soldered on to the tube and disc at
equal distances, as shown, thus giving the stops a tapered
form, so that, owing to the one being adjustable to the other,
almost any size and form of bobbin can be readily inserted
and securely clamped in the winder. The spindle can be
removed from the upright standards n and r, by turning
round the catches at the top, which then allow the spindle
and coil to be lifted out bodily. The spindle at the
bottom, carries the reel of wire h, with which the coil is
going to be wound, the reel being removable by turning the
nut on the right-hand end of the bolt g, which causes it to
unscrew out of the upright n. A small ratchet can be
arranged to work on the stop / if desired, so as to prevent
the coil unwinding when the handle is released. To use the
winder the handle h is turned with the right hand, while
the wire is fed on evenly, regularly, and with moderate
pressure with the left, as shown in the figure.
Previous to commencing to wind the bobbin it should first
be carefully looked over, to see that there are no weak points
either in its construction or in the insulation between the
iron core and the space in which we are going to wind the
primary. Having satisfied ourselves that the bobbin is
properly made, some hot paraffin wax should be prepared,
and the inside of the bobbin thoroughly basted with it
and allowed to cool. The paraffin wax, by the way, will
constantly be required during the winding, so it should be
kept close at hand in a suitable contrivance that will keep
it always at the proper temperature. In purchasing paraffin
wax care should be taken to get only the very highly
refined, as some of the qualities of wax that are frequently
offered are very inferior as to insulating properties. The
wax should be white, and clear from any impurities. It is
best heated in a small metal bowl with spirit-lamp under-
neath, great care being taken, however, that it does not get
too hot and burn. It should be carefully kept away from all
CONSTRUCTION OF COILS GENERALLY, 25
metal filings, both when melted or in a solid state, as if only-
one or two filings get in they are liable to be picked by the
brush when applying the wax, and drop into a most impor-
tant place and be the cause of a subsequent breakdown in the
insulation of the coil.
The wire employed for the primary winding should be
the best high-conductivity copper wire well insulated with
silk, or in some cases cotton may be used. For spark coils
the use of silk only is advised, as silk is by far the best
insulator, and considering the tension of the sparks and their
constant tendency to pick out any weak point in the insula-
tion, it is most important no efforts be spared to make the
insulation as perfect as possible. For medical coils, how-
ever, the tension is not very great, and cotton-covered wires
will be quite sufficient, and are, moreover, considerably
cheaper, especially when we come to take into account the
fact that double the length of primary is usually required
for medical coils.
The two ends of the primary winding are usually brought
out at the contact-breaker end of the bobbin. In bringing
out these ends, there are two methods that may be employed ;
first, holes can be drilled right through the bobbin cheeks,
and the wire run down outside the cheek to the base ; or
second (and this has the better appearance), holes are drilled
from the inside of the bobbin in an oblique direction — some-
what as shown by the dotted lines in Figs. 8 and 9 — so that
the ends of the wire come out, not on the outside face of the
bobbin cheek, but on the bottom part of the rim where it
touches the base. Thus the ends of the wire pass directly
from the inside of the bobbin to the underneath side of the
base, and are not visible, while, moreover, the wires being
embedded in the wood cheek of the bobbin are quite pre-
vented from in any way coming into contact with either the
secondary or tertiary windings.
The holes having been drilled in the correct places, the
26 INDUCTION COILS AND COIL-MAKING.
wire (which, should be purchased on a reel) must be arranged
so that it will pay out properly. If the bobbin is going to
be wound in a coil- winder, the reel will merely have to be
slipped on the bottom bar (see Fig. 14) and it will run
out freely, but if it is intended to wind the primary by
hand, as is frequently done, a stout knitting needle can be
driven into the bench and the reel slipped over this. The end
of the wire must be passed through one of the holes in the
bobbin cheek, leaving a tail of about 8 inches, which must
be temporarily twisted up into a spiral on a pencil to avoid
its getting in the way, and the wire is then wound right
along the bobbin and back again, thus making two layers,
the finishing end being passed through the second hole and
finished off into a spiral like the first. The latter end is
known as the outside end and the former as the inside one.
For medical coils, of course, four or six layers will be
required, but still the outside end will finish at the same end
as the inside, only a little higher up the cheek. It is imma-
terial, as regards the action of the coil, in which direction
the primary is wound ; but it will be found more convenient
in winding if the bobbin is so turned that the wire feeds
on to the top part of the coil, as shown in Fig. 14. The
wire must be wound on carefully and evenly, and when the
whole of the layers are put on they must be well served with
paraffin wax. When the winding of the primary is quite
finished, it should be tested by connecting up to a cell, and
noting that the core is well magnetised.
Winding the Secondary,
Before winding the secondary, two or three layers of good
note-paper that has been previously well soaked in paraffin
wax must be carefully laid on over the primary, care being
taken to see that the outside wrapping when finished
presents a firm, level surface for the first layer of the
CONSTRUCTION OF COILS GENERALLY, 27
secondary. If the secondary winding is started with a
good level surface, it is comparatively easy to maintain even
and regular layers throughout ; but commence with a bad
surface, and it will be found that each succeeding layer
becomes more and more rough and irregular than the one
beneath it. For spark coils, as there is a certain amount of
danger of the secondary sparking across at both ends into
the primary winding or iron core, special care should be
taken with this insulation between the primary and
secondary, making it somewhat thicker at the ends than in
the centre. It is customary in this country to use, for the
secondary winding, the very best silk-covered copper wire ;
but in most foreign coils a method of winding is employed,
which we shall presently describe, in which naked copper,
or, in the cheaper coils, even naked iron wire, is used.
The wire for the secondary must be of good quality, and
as free from joins as possible. Of course, in this fine wire
there always will be a certain amount of joins, as it is no
easy thing to draw a mile or so without a break ; but such
joins must be carefully inspected to see that they are
properly made and well insulated. Frequently the wire
drawers merely tie the ends together, and when this is
found to be the case such ties must be carefully joined,
soldered, and insulated by wrappings of paraffined silk, a
twist-joint being made and care being taken to see that
when insulated it is not too bulky. Such joins only become
apparent during the process of winding, being felt as they
pass through the fingers ; but they must be carefully looked
out for, as well as any insufficiently insulated places, which
will be at once detected by the bright gleam of the copper.
The two ends of the secondary are brought out at the
opposite end to the primary ends, small holes being
drilled in the cheeks of the bobbin. It will be found best,
in order to prevent the annoying occurrence of the inside
end breaking off, and consequent rewinding of the coil, to
28 INDUCTION COILS AND COIL-MAKING,
join two more tails of wire to the commencement of the
winding and bring the three ends out. Thus, if one breaks
off there are still two others, or the three wires may be
twisted up into a stranded one. The secondary coil is wound
on in the same direction as the primary, the wire being care-
fully guided on evenly, which is, of course, somewhat more
difficult than with the primary. The hand feeding the wire
on to the coil should be held some 10 inches or 12 inches
away, and the point from which it is fed kept slightly
behind the point where it is winding on to the coil, in order
to cause each succeeding turn to lie close to the preceding
one, the coil being steadily rotated by the right hand.
When one layer is finished it should first be thoroughly
basted with the melted paraffin wax, and then have a layer
of paraffined note-paper, slightly wider than the space
between the two cheeks of the coil, wrapped round, on
which the second layer is wound, and so on, until all the
layers are completed. In winding the secondary coil, care
should be taken not to take the different layers right up to
the cheeks of the bobbin, as the tension between the layers
is highest at the ends, and thus if the layers are continued
right up to the cheeks, the insulation of the secondary is
liable to break down.
The foreign method of winding induction coils is shown
in Fig. 15, which shows one end of the coil partly in
section and partly in elevation. The bobbin cheek, paper
tube, primary and secondary windings are in section, while
the iron core and part of the primary winding are shown in
elevation. Only two layers of the primary are shown, and
the size of the secondary wire much enlarged to make the
illustration clearer. The iron core consists of a piece of
thin sheet iron rolled into a tube as previously described.
The tube forming the body of the bobbin is of paper,
fastened into the wood cheeks in the manner shown, and the
primary (silk-covered copper wire) is wound directly on to
CONSTRUCTION OF COILS GENERALLY, 29
this paper tube. After the primary is wound, a few layers
of paper are put on, and the winding of the secondary com-
menced. For this, naked copper wire of No. 36 gauge is
used, and this is so wound on that each turn is separated
from its neighbours by an air-space equal to the thickness
of the wire. When the one layer is finished, a wrapping or
two of paper is put on, and the winding of the other layer
commenced, layer after layer being so put on until the coil
is finished. The writer has not had much experience in this
method of winding, but he believes it to be performed in a
special automatic machine, though it is possible to do it in a
screw-cutting lathe, the change wheels of which are arranged
for a very fine thread, and a special wire-guide fixed in the
rest. Apparently the secondary is wound by itself, and
afterwards slipped over the primary, as on taking such coils
to pieces it will be found that the secondary can be compara-
tively easily slipped off the primary, while it is not possible
to separate the different layers of the secondary. The wire
is wound on very evenly and under considerable tension, as
it is not easy, even by drawing the point of a knife along
a layer, to cause the adjacent turns to touch one another,
and the secondary winding by itself forms a tube so hard
80
INDUCTION COILS AND COIL-MAKING.
that it is not possible to flatten it by squeezing it with
both hands. The paper used for insulating the different
layers is not saturated with any insulating substance, and
thus the insulation cf the coil, although satisfactory, is not,
of course, equal to the English method, where covered wire
and paraffined layers are employed* The outside of the
coil is covered with a layer of some dark-coloured velvet,
which is affixed to the outside layer by a little thin glue.
In the old form of the hand electric gas-lighters, in which
small spark coils were employed, giving a ^-inch to -j^-inch
spark, naked iron wire was used, wound as described above,
but each layer — and subsequently, on completion, the whole
coil — was thoroughly impregnated with paraffin wax, which
fi.lled up the interstices of the layers.
This method of winding medical coils necessarily makes
them of larger size than the English-made ones of similar
power ; but it is cheap where a quantity are produced at
a time and satisfactory for spark coils if plenty of paraffin
wax is used. The English method of construction is, of
course, more durable, and gives a better insulation.
Contad-hreakers,
The contact-breaker, or interrupter as it is also frequently
called, is an arrangement worked either by the magnetism of
the iron core of the bobbin, by a separate magnet, or some-
times, in very large spark coils, by hand. Its object, as its
name implies, is to interrupt or break the circuit in order
to produce the induction between the two windings.
Too little attention is frequently paid in constructing
induction coils to the contact-breaker, a part of the coil that
is able to influence to a large extent the character of the
shock obtained. Of course, given a well-designed and
constructed coil, almost any form of contact-breaker will
work with it ; but to get the best results from a coil that has
CONSTRUCTION OF COILS GENERALLY. 31
been designed to give powerful, yet not unpleasant, shocks,
the contact-breaker must not be overlooked. Its size, shape,
and even the position of the different parts will be found to
affect the nature of the shocks. In a good contact-breaker
for a medical coil the interruptions should be even and
regular, and vary only as the contact-screw is adjusted. An
intermittent, or a contact-breaker that is spasmodic in its
action, is very objectionable.
Putting aside the hand contact-breaker, which is only
used for very large spark coils, there are three kinds of
contact-breakers in general use : 1st, contact-breakers worked
Fig. 16.— tertical contact-breaker.
by the coil itself ; 2nd, separate contact-breakers ; and 3rd,
variable contact-breakers, in which the rapidity of the
interruptions can be varied from very slow to very fast.
A simple contact-breaker of the first kind, suitable for
very small medical or spark coils, is shown in Fig. 16. It
consists of the upright spring c, carrying at the top the
soft-iron hammer head, and behind it is fixed an ordinary
contact pillar and screw d, such as is used for electric bells.
The bottom of the spring is bent at right angles and fixed
to the base by the screw n.
The most common form of contact for small and medium
size medical and spark coils is shown in Fig. 17.
32 INDUCTION COILS AND COIL-MAKING.
The contact-spring it will be seen is in a horizontal
position, and fixed on a separate pillar, close to which is the
pillar carrying the contact-screw. On the end of the spring
is the soft-iron hammer head, and the other is slotted and
fixed, by means of a screw, to the upright pillar, so that the
position occupied by the hammer head in front of the core
can be adjusted horizontally. The pillars are secured to
the wood base by screws passed up from below.
Fig. 18 shows a form of contact-breaker employed in
portable sets when the coil is in a vertical position, and the
iron core is fixed. The iron core projects up for a half-inch
Figs. 17, 38. — horizontal contact-breakers.
through the wood ^se, and the hammer, fixed on the end of
the thin confcact-spring, works above it. The contact-spring
is carried by a small contact-pillar, seen on the extreme right
of the figure. The contact-screw is carried by a substantial
brass standard, seen on the left of the contact-spring. The
foot of this standard is fixed on to the woodwork of the
coil by a brass screw with ornamental head, while the top
part carries the contact-screw, which is arranged to work
stiffly in the slotted head. The whole forms a very neat
and compact arrangement. All these forms of contact-
breakers are designed, it will be seen, to be worked by the
magnetism in the iron core of the bobbin. Their operation
is as follows : — On the current passing though the primary
coil the iron core is magnetised and consequently attracts
CONSTRUCTION OF COILS GENERALLY. 33
the soft iron hammer of the contact-breaker, causing it to
approach the iron core. The contact- spring bends, owing
to the movement of the hammer, and the platinum contact on
it therefore breaks contact with the platinum point on th^
contact-screw. As the spring and contact-screw are included
in the primary circuit this circuit is immediately inter-
rupted, causing a powerful induced current to flow in the
secondary, while the hammer of the contact-breaker falls
back to its original position, only to repeat its movement so
long as the battery is connected up to the coil. In con-
FlGS. 19, 20. — SEPARATE CONTACT-BREAKERS.
structing all contact-breakers it is important that the iron
hammers are of really soft iron, and the springs either steel
or properly stiffened brass or German silver. The contact-
screws, in order to avoid their continually working back
owing to the vibration, must be provided with efficient set-
screws, though for small coils it is sufficient if the contact-
screws are arranged to work stiffly in the pillar. The
adjustment of the contact-breaker will be found to greatly
affect the working of the coil, and in trying to get the
longest spark from a spark coil, or the most agreeable shock
from a medical coil, the set-screw should be slowly turned
D
34 INDUCTION COILS AND COIL-MAKING.
while the results are carefully watched. Never make more
than a quarter of a turn at a time, as the best position is
easily passed.
Of contact-breakers of the second kind, or separate contact-
breakers, the form shown in Figs. 19 and 20 is chiefly
employed. Although not in any way worked by the iron
core it is fitted quite close to one end of the bobbin, in order
to economise space and make the whole coil more compact.
The contact-breaker consists, it will be seen, of two upright
magnets, above which is the soft iron hammer carried by a
horizontal spring fixed to a brass pillar. Above this spring-
is the contact-screw g, supported by the metal arm screwed
to the top of the wooden standard r, that forms one end of
Fig. 21. — separate contact-breaker.
the bobbin. The contact-breaker magnets are in series with
the winding on the bobbin, and the action of the contact-
breaker is similar, of course, to those worked by the iron
core. Another form of the separate contact-breaker much
employed for portable sets is shown in Fig. 21. It is
largely employed in portable sets, because, being flat, it
requires a space of but little height, and is usually fitted in
the space covered by the lid when it is shut down. It
consists, it will be seen, of two magnet coils, the iron cores
of which are joined together at the left-hand ends by a short
length of iron bar, and fitted with square-shaped pole-pieces
on the right. These magnets are fixed on their sides on the
base of the coil, or, in the case of a portable set, on the
woodwork forming the top of the case when the set is open.
CONSTRUCTION OF COILS GENERALLY. 35
Above the pole-pieces is the soft iron armature, carried by
the thin brass spring, fastened at the left-hand end to the
top of a brass pillar. The contact-screw is carried by a
brass angle piece, as shown, and the point of the screw
works on a platinum contact in the centre of the spring.
Separate contact-breakers are chiefly required for coils where
the regulation of the strength of the primary and secondary
shocks is effected by sliding in and out the iron core, which
is therefore not available for working the ordinary form, and
also for use in portable sets where it is often necessary to
place the coil some way off the contact-breaker, such as at
the bottom of the containing case, while the contact-breaker
is on the top just inside the lid.
Variable Contact-hreaTcers,
Like the separate contact-breaker the variable contact-
breaker is almost exclusively used for medical coils, as with
most medical coils it is desirable to be able to alter the rate
of vibration of the contact-breaker. Very quick vibrations
are required in treating some cases, very slow ones in others.
Fig. 22 shows a very simple form, and a form much used in
America. The figure shows the contact-breaker in perspec-
tive, and part of the coil to which it is fitted. In this form
two armatures and springs are provided, the one being an
ordinary armature for very quick vibrations, and the othe-r
of special form, as shown in the figure. This consists, as
will be seen, of the usual iron armature, having a prolonga-
tion at one end in the form of a rod, on which slides a brass
ball, the distance of this latter along the rod being variable
by unslacking the set-screw to be seen on top of the ball.
When the coil is connected up to a battery, and quick vibra-
tions are required, the ordinary armature is screwed on to
the pillar ; but for slow or variable vibrations this is removed,
and the one with a ball substituted. On adjusting the contact-
D 2
36 INDUCTION COILS AND COIL-MAKING.
screw with this latter form, it will be found the vibrations
are very much slower, owing to the momentum acquired by
the ball increasing the traverse and the period of time
required for the ball to be stopped in one direction and
started in the other ; while, moreover, the rate of vibration
will vary according to the position the ball occupies along
the rod. The vibration will be quicker as the ball is moved
towards the armature, and slower when moved away from it.
Figs. 23 and 24 show the form of variable contact-breaker
employed for all the best class of medical coils, and it is
without doubt the most convenient, reliable, and efficient
form. At first glance it may appear somewhat complicated
Fig. 22. — variable contact-breakee.
in comparison with the ordinary contact-breaker, which is so
simple in its construction ; but on better acquaintance much
of this disappears, while it will be found that the rapidity
of the vibrations are beautifully under control. Fig. 23
shows the contact-breaker from the end of the coil, while
Fig. 24 is a side view. Eeferring to these figures, x x are
the two magnet coils, consisting of an iron core with bone
ends, the wire being wound directly on to the iron core, a
layer of paper only being put on. In this form of contact-
breaker the iron cores usually consist of a special shaped iron
screw with a flat head, the shanks of the screw forming the
cores, and the heads the pole-pieces. Above the two magnet
CONSTRUCTION OF COILS GENERALLY, 37
poles is the soft iron armature a, of the shape shown, which
is fixed at one end of the brass lever this lever being
pivoted in the brass standard 6. This standard, it will be
seen from Fig. 24, has a U-shaped piece at the top, between
the arms of which is lightly pivo+ed on the point of two
Fig. 23. — vakiable contact-breaker.
screws the brass lever v. The two pivot-screws are each
provided with a milled lock-nut to prevent their slacking
out with the vibration. The lever, it will be noticed, is
not pivoted quite at the end, but at a point about J in. from
the left-hand side, and between this end of the lever and a
projection at the bottom of the stand h is stretched the
38 INDUCTION COILS AND COIL-MAKING.
spiral spring t, the tension of which can be adjusted by the
milled screw n. Thus the magnets, when energised, attract
down the armature against the pull of the spiral spring t
Almost centrally above the lever v is the brass arm c, fixed
at one end to the top of wood end of the coil bobbin — this
end being specially shaped to receive it — and carrying at
the other the contact-screw c?, which works on the light
contact-spring /, attached to the lever at a point just
above the pivots. The tension or amount of projection of
this spring can be regulated by the milled-headed screw g.
The portion of the contact-breaker controlling the rate of
vibration consists of the upright rod r carrying the ball m.
The rod r, which is of aluminium, rises up to a height of
6 in. from the centre of the pivot screws, and then bends
over and returns to the other end of the armature. In the
figure it is shown broken away at the top to economise space.
This rod fixes at the one end into a small brass standard
attached to the lever centrally above the pivot screws,
and at the other, which is tapered slightly, into a hole close
by the armature. The rod is thus removable so that the
contact-breaker can be used with and without the upright
arm and ball. When the contact-breaker is not in use the
rod is removed, and placed in the drawer of the case con-
taining the different-shaped electrodes with which such coils
are usually provided. The ball m is of brass, nickel-plated,
and weighs just over i oz. The position of ball upon the
left-hand part of the upright rod can be varied by means of
the set-screw seen on the left side.
The action of the contact-breaker is as follows : The
armature is attracted by the magnets, causing the spring /
to part contact with the point of the screw d, thus inter-
rupting the circuit, and allowing the armature to fall back
to its original position. The more tension on the spring
the quicker the armature will recover itself; while the
more the screw g is slacked out, the less rapidly will the
CONSTRUCTION OF COILS QENEBALLY,
39
spring / break contact with the screw c?, and the further
will be the traverse of the armature. The higher the ball
m is up the rod r the more it will affect the armature, by
increasing its traverse and slowing down its vibrations. It
will thus be seen that, apart from the ball and rod, the
Fig. 24. — variable contact-breaker.
vibrations of the armature are under a certain amount of
control by the springs t and ^, although to get the very
slowest possible vibration (about 60 per minute with most
coils) it is not sufficient to regulate by any one of these
points alone, but all must be adjusted towards the end desired.
40 INDUCTION COILS AND COIL-MAKING.
Terminals.
In order to form a convenient method of coupling up the
coil to the battery, or making connection to the secondary,
apart from the finished appearance it gives to the coil,
suitable terminals mnst be provided for the ends of the
primary and secondary windings. These are usually of
brass, either lacquered, or, if a better finish is desired,
plated.
Figs. 25 and 26 show two forms much used for the
primary connections, the terminals being fixed by passing
the shanks through the base and running up a nut.
Figs. 25, 26.— terminals. Figs. 27, 28— terminals.
Figs. 27 and 28 show two forms suitable for secondary
connections. In these it will be seen the wire is clamped
in a hole in the terminal, this method of clamping being
preferable for secondary terminals. For large spark coils it
is advisable, however, to run the wires from the secondary
winding to a " discharger," a useful form of which is shown
in Fig. 29, as the handling of wires connected to the
secondary of such coils requires great caution, and the use
CONSTRUCTION OF COILS GENERALLY, 41
of a proper discliarger may often prevent the occurrence of
a disagreeable accident. The form of discharger shown in
the figure is fixed on the base of the coil, either at one end
or at the side ; but some forms in use are fixed, half on each
cheek of the bobbin. Keferring to the discharger shown in
Fig. 29, this consists of two upright brass tubes on the
lower ends of which are fixed brass stand-plates, and on
Fig. 29. — a dischakger.
the top of each two brass swivel-pieces, as shown, in which
slide two ^-inch brass rods. The outside ends of these rods
are fitted with ebonite handles, while the points are drilled
centrally to receive fine wire points, which points fit some-
what tightly in the holes. Thus the distance between the
discharger-points can be varied by pulling out the ebonite
knobs, and different shaped points can be readily inserted
as desired. The set-screws just below the swivels are for
the ends of the secondary wires.
Bases for Coils,
These should be of polished walnut or mahogany, and the
shape will vary according to the kind of coil they are in-
tended for. Thus, for a medical coil it need not be very
thick, and should be constructed preferably in two, between
which parts the connections to the different parts are clamped,
and thus prevent their being injured. For spark coils the
base will require to be very much deeper and hollow inside
42 INDUCTION COILS AND COIL-MAKING.
in order to provide sufficient space for the condenser, which
is best placed inside the base. Feet should always be pro-
vided at each corner to raise the base slightly off the
table, as this not only improves the appearance, but also
prevents the coil base getting wet, should it accidentally be
placed in some spilled solution, &c. Illustrations of several
forms of bases for medical and spark coils will be found in
Chapters III. and V.
Putting the Coil together.
In putting coils together, the bobbin should first be fixed
in its correct position on the base, and the ends of the
primary, and, when required, the secondary also, be passed
through the holes that should have been previously marked
and bored for them. Bobbins are in all medical coils and in
most spark coils, fixed by screws passed up through the base
into the bobbin, and great care is requisite in screwing up
these screws to see that they pass centrally into the bobbin
cheek, and not into or near the secondary. Otherwise
sparking may take place to these screws, and thence to the
coil connection beneath the base.
The contact-breaker should next be fixed, taking care
that the correct distances from the end of the iron are
observed for the different parts, and when the contact-
breaker is finished attention should next be given to the
primary and secondary, and any accessory appliances there
may be on the base. The different parts being correctly
fixed, each portion should be thoroughly tested both for
insulation and continuity by means of a galvanometer and
battery, and if all is found correct we can proceed to make
the different connections underneath the base. These
connections will, of course, be made according to the kind
of coil as shown in Chapters III. and V.
When joining all wires the joints should be carefully
soldered, using resin as a flux, and care must be taken when
CONSTRUCTION OF COILS GENERALLY. 43
inserting the ends of tlie wires under the contact-breaker,
nuts and terminals, to see that, when screwed down, the
nuts do not cut or nick the wire, causing it to break.
Condensers.
The condenser of a spark coil consists of a number of
layers of thin sheet-foil, insulated by layers of paraffined
paper, alternate layers of the tinfoil being connected
together, thus forming really two large sheets of foil, one of
which is connected to the pillar holding the contact-spring
of the contact-breaker, and the other to the pillar carrying
the contact-screw. The condenser is thus a bridge across
the make-and-break. To M. Fiseau, of Paris, is due the
invention of the condenser, the important part played by
which in a spark coil can readily be ascertained by experi-
menting, first without, and then with a good condenser.
Its object is to increase the length and strength of the
secondary spark, and this it does by absorbing the extra
current in the primary circuit at " break," and parting with
it at " make." It therefore destroys the detrimental self-
induction of the primary coil, allowing the primary current
to rise and fall at once to and from its full strength. The
self-induction of the primary circuit being reduced to a low
point, the sparking at the contact-breaker is, therefore, at a
minimum.
The following details of the construction of a condenser
refer to the 1-inch spark coil, described in Chapter V., in
which chapter the dimensions for the condenser of other
size coils will also be found.
Get ready 40 sheets of thin tinfoil 6x4 inches (or of
a different shape, but similar area if more convenient), and
60 small strips of the same material. Next cut out 60
sheets, 9x5 inches, of some good note-paper, which must
then be thoroughly soaked in paraffin wax, and allowed to
44 INDUCTION COILS AND COIL-MAKING,
dry. The tinfoil and paper sheets must next be carefully
inspected in a good light, first, to see that there are no sharp
points in the former, and no holes or thin places in the
latter. Now clear a good space on the table or bench, wash
your hands, and carefully inspect the cleared portion of the
table to see that there are no metal filings near, one of
which, if it got between the sheets of the condenser, would
render it useless. First, lay ten sheets of the paraflSned
paper on the table, and on this place one sheet of tinfoil
and on one side one of the small strips or connecting tags.
On the top of the sheet-foil layer place one layer of paraffined
paper, and on this another layer of the tinfoil, leaving the
tag on this one projecting out on the opposite side to the
other. In tliis manner the whole of the sheets are put on,
nntil all are used up. It must be remembered that alternate
layers have the connecting tags joined together, and that
the tinfoil sheets must be got centrally in the paper
separators, leaving a ^-inch space all round. There will
thus be twenty sheets of tinfoil connected together on one
side, and twenty on the other. The condenser must be
finished off with ten layers of the paraffined paper as at the
commencement, and the whole condenser afterwards placed
in a press, and squeezed firmly together, or, if no press is at
hand, placed on a bench with large weights on it. It will
then present the appearance as shown in Fig. 31. After it is
Fig. 30. — building up the condenser.
CONSTRUCTION OF COILS GENERALLY. 45
thoroughly pressed, it must have a sheet of thick paraffined
cardboard or thin wood placed each side, and the whole
Fig. 81.— condenser finished.
wound tightly with cotton to keep it together. The ends
on each side of the condenser must then be pressed together,
and the condenser connected on to the coil as shown in
Chapter V.
46 INDUCTION COILS AND COIL-MAKING.
CHAPTEE nr.
SHOCK AND MEDICAL COILS.
Shock Coils is a name applied to all small coils constructed
to give mild shocks merely for the purposes of amusement,
to distinguish them from the more powerful and scienti-
fically arranged coils designed for the application of induced
currents to the cure or alleviation of disease, and known as
medical coils.
One of the most essential points of all medical or Faradaic
coils is some arrangement whereby the strength of the in-
duced current, or in other words, the intensity of the shock,
can be easily and quickly varied, and before passing on to
the different forms of medical coils, it will be as well just to
glance at the different methods of regulation.
Methods of Begulating Shock.
The three most common methods of regulating the shock
are : (1) by means of a thin metal tube, which is so arranged
as to slide easily over the iron core ; (2) by arranging a
switch on the coil base, so that it cuts out certain layers
of the secondary; and (3) by the "sledge" method, in
which the primary is fixed at one end of the base, and the
secondary slides on two guides, so that its position over the
primary can be varied from right-over to right-off. Of the
three methods, the first is the one most commonly employed,
while the last is the one that is capable of giving the most
SHOCK AND MEDICAL COILS,
47
delicate regulation. Figs. 32, 33 and 34 show diagram-
matically these three different methods of regulation ; the
primary windings in these four figures being indicated by
the thick lines, and the secondary by the thin ones, P and P'
being the battery terminals and S and S' those for the
secondary.
The tube method of regulation is shown in Fig. 32. The
thin brass tube slides over the iron core, and when the
tube is pushed right in, so as to completely cover the core,
the shock from either the primary or secondary is at its
weakest, but when withdiawn altogether the full strength
is obtained. According to the position of the tube, so any
modification in the shock can be obtained, although when
right in the shock does not altogether cease. The principle
of this method of regulation is that we place a closed circuit
in such a position to the primary coil that it absorbs its
inductive effects. For with every momentary current we
have in the primary we get one in a reverse direction in the
tube, and thus the induced current tends to neutralise the
effects of the battery current, and consequently the shock
obtainable from either the secondary or primary winding is
very much diminished. As the tube is withdrawn, so there
is less surface to be acted upon, and the influence of the
regulator is diminished. In the figure P and P' are the
battery terminals, and S and S' the secondary.
Fig. 33 shows the method of regulating the shock by
48 INDUCTION COILS AND COIL-MAKING.
cutting out some of the layers of the secondary winding.
The coil is shown arranged for a secondary shock only, as
this method of regulation only affects the secondary wind-
ing. Connecting wires are taken from different layers of
the secondary coil to the contacts 1, 2 and 3 of switch, the
lever of the switch being connected to one of the secondary
Fig. 33. — switch method of regulation.
terminals. Thus, as the lever is moved from one contact to
another, so the shock is increased or diminished, the varia-
tions not, of course, being very gradual.
The " sledge " method of regulation is shown in Fig. 34.
In coils fitted with this method of regulation the primary
Fig. 34. — sledge method of regulation.
winding, with its iron core and contact-breaking apparatus,
is fixed at one end of the base, while the secondary slides on
two brass guides, the hole in the secondary being of such a
size that it slides over the primary. When the secondary is
right over the primary the shock is, of course, at its maxi-
mum ; and in proportion as it is moved away, so is the
SHOCK AND MEDICAL COILS. 49
strength of the shock reduced. Since it takes a considerable
movement of the secondary coil to much affect the shock,
and this movement can be made as slowly as"* possible, the
strength of the shock from coils of this description can be
varied most gradually and within very wide limits.
Fig. 35 shows a method of regulating the shock from
primary coils devised by the writer, some years ago, in con-
junction with Mr. E. J. Wade. The action of the regulator
is as follows : — The outside end of the coil is connected to a
brass bridge carrying a metal contact-arm which makes
contact, the one end on the bridge, and the other on the
Fig. 35.— a method of begulating shock for primary coils.
outside layer of the coil, this outside layer being of naked
wire. It will thus be seen that when the arm is pushed
along to one end of the bridge (the right-hand end in the
figures) the top layer of the coil is short-circuited on itself,
and forms a closed circuit or metal sheath over the rest of
the layers. The effect of this closed circuit on the layers
underneath is such as to exercise a damping " action on
them, owing to currents having a reverse direction to those
in the coil, being induced in the closed circuit of the regu-
lator, so that the shock to be obtained across the make-and-
break at the moment of opening the circuit is reduced to a
E
50 INDUCTION COILS AND COIL-MAKING.
mininmm. It will readily be understood that as the arm m
is moved back to the other end, so more and more coils are
cut out of the closed circuit, and its length thus reduced
until at the other end the arm makes contact merely with
the brass bridge, and the outside layer has become part of
the coil winding again, and adds to, instead of reducing, its
effects. Of course as the arm m is moved between the two
ends of the bridge, so any effects from zero to full power
are obtained, and the increase or decrease is as gradual as
possible.
Primary Shock Coils.
Primary shock coils are, of course, the simplest form of
induction coil, since there is only the one winding, and that
of thickest wire, although, notwithstanding this, they can
be made capable of giving most powerful shocks. A small
coil of this description, fitted with the method of regulation
just described, is shown in Fig, 36. Eef erring to this figure,
/ and e are the two battery terminals, while c is the contact-
post of the interrupter, smd d the hammer and spring. At
the top of the base are the two handles, while at the bottom
is the slide and scale of the regulator. The connections to
the different parts are indicated by the dotted lines.
The base of the coil, which is 7^ by 4 J by 1 J inches thick,
should be cut of a piece of mahogany or walnut, and after-
wards carefully sand- papered and French polished. The
bobbin (which can be constructed on either of the methods
described in Chapter II.) is 3 inches long by 2 inches across
Nhe cheeks, the thickness of the cheeks being J inch, and the
/lole through the centre of the bobbin | inch. The iron core
consists of about 90 lengths of No. 22 iron wire. To wind
the bobbin, about J lb. No. 24 double cotton-covered wire
will be required, and this must be wound on layer upon
layer, until eleven layers are put on, and this eleventh
layer will finish at the right-hand end of the bobbin. This
SHOCK AND MEDICAL COILS.
51
end of the wire is then secured by drilling two holes in the
bobbin cheek side by side, and passing the end of the wire
through one hole from the inside to the outside, and back
again through the other hole to the inside, leaving a J-inch
tail projecting.
For the regulating layer we shall require about seven
yards of No. 22 naked copper wire ; but before winding on
this layer, three wrappings of thick note-paper must be put
on over the last layer of the other winding, and the paper
well basted with paraffin-wax. This will make a firm and
even surface, on which the regulating layer can be wound —
Fig. 36. — primary shock coil.
a most important point, for the end of the brass spring m, it
will be seen, slides along and makes contact with the bottom
of this layer. If the three layers of paper are not sufficient,
then a piece of thin cardboard should be bent round. In
order to prevent the different turns of wire in this regulating-
layer touching one another, and thus making a permanently
closed circuit, some thin twine, of not larger diameter when
slack than No. 22 B. W.G. wire, is wound on at the same time,
thus leaving a turn of twine between each turn of wire, as
shown in Fig. 37. Previous to winding, however, the begin-
ning of the naked wire must be soldered on to the projecting
tail of the insulated layer beneath. If a certain amount of
£ 2
52 INDUCTION COILS AND COIL-MAKING.
tension is kept on the twine while winding, it will be found
to sink slightly below the level of the naked wire, after the
manner shown in Fig. 37, When this layer is finished the
twine must be fastened off by slipping the end under the last
coil and tying it, while the end of the wire is passed through
a small hole in the bobbin cheek and cut off, leaving a pro-
jecting end of 6 inches. The naked No. 22 wire should not
be continued right up to the left-hand cheek of the bobbin,
but stopped at about \ inch or f inch off, as this will enable
the contact-spring m to slide off all but the last turn of the
Fig. 37.— method of winding last layer.
wire, so that no turns of the regulating layer are short-
circuited when the pointer is at the left-hand end of the
index.
We can now screw the bobbin back on to the base, and
proceed to fit the regulating gear, contact-breaker, and
terminals. Fig. 38 shows an end view of the coil with the
left-hand cheek removed, showing the position of the contact-
spring and slide m and the guide n of the regulator. The
guide is of brass, and is supported off the wooden base, as
shown by two feet, also of brass. On the guide runs the
Blide and spring m. The slide is of brass, with ebonite or
boxwood knob, and the thin spring, also of brass, is soldered
or screwed to the underneath part of the slide, so that it
SHOCK AND MEDICAL COILS.
53
holds it on the guide. The front part of the spring bears
with moderate pressure on the uninsulated layer of wire, as
in Fig. 38, and the outside lapping of velvet, when it is put
on, only extends as far down on each side of the bobbin as
shown in the above-mentioned figure, so that it does not
interfere with the working of the spring.
Eeferring to Fig. 36 it will be seen that the inside end of
the coil is connected to the contact-spring and the outside
end to the guide of the regulator, this guide being also con-
nected to terminal e on the base. The other terminal / is
connected to the contact-pillar, while the two handles are
connected, one to the contact-pillar and the other to the con-
tact-spring. On connecting up a battery to the coil, and
adjusting the contact-screw of the make-and-break, the soft-
iron hammer on the spring should vibrate rapidly, giving
out a humming note, the pitch of which will vary as the
contact-screw is adjusted. The note will be lower as the
contact-screw is slacked back, and higher as it is tightened
up, owing to the more rapid vibration. On taking hold of
the handles while the contact-breaker is vibrating, a smart
shock will be felt if the regulator is to the left-hand side,
which diminishes as it is moved to the right. The action
of the coil is as follows : — The magnetism in the core sud-
denly ceases when the contact-breaker is attracted, and with
the cessation of the current each turn of the coil acts induc-
tively on its neighbours, giving rise to an induced or extra
Fig. 38.— section through primary coil.
54 INDUCTION COILS AND COIL-MAKING,
current of higher E.M.F., which, in its endeavour to complete
its path round the circuit, can only pass by way of the
handles and the person holding them, and hence the shock.
The shock from a primary coil is only felt on breaking the
circuit, although there is also an induced current on comple-
tion ; but this is in oppositiim to the battery current, and its
only effect is to momentarily impede the filling of the coil.
The induced current on breaking the circuit is in the same
direction as the battery current, and thus the nature of
the current passing through the person holding the handles
is partly Galvanic and partly Faradaic.
The strength and character of the shock from the coil is, it
will be found, influenced by the rate of vibration of the
hammer, or, in other words, by the number of times per
second the circuit is interrupted. On slacking back the
contact-screw, so as to get a slower rate of vibration, the
shock will be found increased in intensity, while there
becomes almost a distinct interval between the shocks. On
tightening up the screw the reverse is the case, and this is
because the iron core takes a certain amount of time to take
up its magnetism, or the coil to get fully charged, as it were.
Thus, with very quick breaks, the current from the battery
does not get sufficient time to rise to its full strength, nor the
iron core to thoroughly acquire or part with its magnetism.
Similarly the position of the regulator will be found to
influence the rate of vibration of the contact-breaker, which
can be detected by the alteration in the note as the piece m
is moved along the guide n. This arises from the fact that
when there are no coils of the regulating layer in circuit, the
impedance of the primary winding is greatest ; the shock is
therefore strongest, although the battery current passing is
weakest. With the whole of the regulating layer closed on
itself, as happens when the piece m is to the left-hand side,
the bulk of the inductive effects of the primary winding are
absorbed by this layer, the impedance of the coil is low, and
SHOCK AND MEDICAL COILS.
55
the shock practically nil, notwithstanding a larger battery
current is passing.
Medical Coils,
Most medical coils are so arranged that both a primary and
secondary shock can be administered, and in the larger forms
a| tertiary one also. The reason for this is because the actions
of the primary and secondary currents on the human body
are held by men who have investigated the subject to have a
very different effect from one another.
Fig. 39. — small medical coil.
Figs. 39, 40 and 41 show a very handy little medical coil,
Fig. 39 being a perspective view. Fig. 40 a section, and Fig. 41
an elevation from the contact-breaker end. The coil has two
windings — a primary and secondary — and the three terminals
on the top of the board marked, " Prim.," " Prim. Sec," and
" Sec," respectively, allow either a primary, a secondary,
or a combined primary and secondary to be obtained.
The wood base of the coil is 6 inches by 4J inches, and
should preferably be constructed in two parts, / and x, as
shown in the figures. Eeferring to Fig. 40 the top part /is
f inch thick, while the bottom x \ inch, and the two are
56 INDUCTION COILS AND COIL-MAKING.
fastened together by six screws passed through the bottom
part into the top. Thus the wires connecting the different
parts of the coil, which are let into grooves in the bottom
side of/, are completely hidden, and secured in place when
the part x is screwed to it. These, as well as the five
terminals, " sec." prim, sec," and " prim.," M and N, must
afterwards be properly fixed at the positions shown in
Fig. 39.
The wood bobbin r is 4 inches long by 1| inches across the
bobbin ends, which are f inch thick, and has an iron core s
composed of a piece of sheet iron rolled into a spiral and filled
with lengths of iron wire, one end of the core being fitted
with a solid iron cap at the contact-breaker ends of the coil.
The regulating tube, which, it will be seen, slides in and out
at the opposite end to the contact-breaker, is a thin brass one
Fig. 40. — small medical coil (section.)
Fig. 41. — end elevation.
SHOCK AND MEDICAL COILS, 57
of such diameter that it slips easily over the iron core, and
has a small knob soldered on to the end, by which it is moved
backwards and forwards. The contact-breaker is of the form
shown in Fig 17, Chapter II., g being the contact-screw, c
the spring, and h the hammer head.
The primary windings p consist of two to four layers of
No. 22 cotton-covered wire, over which are wound 15 layers of
No. 36 silk-covered copper for the secondary v. Both wind-
ings must be carefully wound as directed in Chapter II.,
after which the bobbin can be affixed to the base.
Fig. 42. — connections for small medical coil.
Previous to screwing down the bobbin, it should have a
covering of black or dark blue velvet, put over the last
layer of the secondary, which will greatly improve the ap-
pearance of the coil. Eeferring to Fig. 42, which shows the
connections of the coil, the outside end of the primary is,
it will be seen, connected to the terminal w, and the inside to
the post carrying the contact-hammer and spring, the post
carrying the contact-screw being connected to the other
battery terminal m, A wire is also run from the contact-
post carrying the contact-screw to the terminal PS, and
another from the inside end of the primary to the terminal
P. The inside end of the secondary is connected to the
58 INDUCTION COILS AND COIL-MAKING.
terminal P S, and the outside to the terminal S. These con-
nections having been properly made, the part x of the base
can be screwed on to the part /, and the coil is complete.
On connecting up a battery of one or two cells to the
terminals m and and adjusting the contact-breaker, it will
be found that a smart shock can be obtained. First, if the
two handles are connected to P and P S ; second, if con-
nected to PS and S; and third, if connected to S and P.
The first of these shocks is a " primary " shock, the second
a " secondary," and the third a combined " primary and
secondary " shock, the two windings in this latter case being
connected in series and working together. It will be found,
on experimenting, that the shock obtained from the primary
is the weakest, that from the secondary much stronger, while
the combined primary and secondary is the strongest, the
strength of the shock in each case being variable by moving
in and out the regulating tube.
Keferring to Fig. 38, presuming the positive pole of the
battery to be connected to M, then on the breaking of the
circuit by the interrupter, the direction of the shock will be
in the primary from terminal P S to P, and from the secondary
from S to PS, and from the primary and secondary com-
bined from S to P. The induced current in the primary
will be in the same direction as the inducing current on the
opening of the circuit, and the induced current in the
secondary is in the same direction as the induced current in
the primary ; thus, as these two windings are connected in
series (inside end of the one to the outside of the other), the
induced current proceeding from the one coil will augment
that of the other.
Bath Coils.
For the application of electricity to the human body by
means of the electric bath, a specially constructed and some-
what powerful coil is necessary.
SHOCK AND MEDICAL COILS,
59
Figs. 43, 44, and 45, show a form of the bath coil in
side elevation, plan, and perspective respectively. The coil
is provided with a primary, secondary, and tertiary winding,
and on one side of the coil is the switch operated by the
handle h, which allows either a primary, secondary, tertiary,
primary and secondary, secondary and tertiary, or com-
bined primary, secondary and tertiary shock to be obtained.
Fig. 43.— bath coil.
the different position for the handle of the switch being
indicated by the labels P, S, T, P S, ST, and PST. The
two battery terminals are marked + and — , while Ji and
Ji are the terminals to which the handles or bath elec-
trodes are connected. The base of the coil, which is of
polished walnut, is made in two parts, as described for the
previous coil, the dimensions of the top part being 10 inches
X 6|- inches X A i^^ch, and the bottom lOJ inches x 7|
inches X fV i^^c^*
60 INDUCTION COILS AND COIL-MAKING.
The bobbin ends are 3 inches x | inch, and the extreme
length of the bobbin is 7 inches, the ends being made of any-
hard wood ebonised. The iron core, which is composed of
No. 22 highly-annealed iron wire, is 6^ inches in length by
^ inch thick, the ends being soldered together so as to form
one solid mass, and thus allow the regulating tube to slide
easily over the one end, while the other is forced into the
brass bush r, which bush is in turn forced into the right-
hand cheek of the bobbin. The bobbin is built up after the
manner described in Chapter II.
The switch allowing the different kinds of shock to be
administered consists of the plated brass lever x operated by
Fig. 44. — side elevation of bath coil.
the handle Figs. 47 and 48. Beneath the handle, and on
the underneath side of a;, is the ebonite block carrying
two contact-springs, the one of which makes contact between
the contacts (see Fig. 46) on the base and the pillar m, and
the other between the adjoining contact and the circular
metal strip n. The metal part x swings on the screw on the
top of the pillar m, and the shorter of the two springs c
being in connection with it follows that the contact on
the base on which the springs happen to be resting is put
into electrical connection with the pillar m. The longer
spring c is perfectly insulated from the lever a?, and so bent
that the one end presses on the contact adjacent to that on
which the shorter spring is resting, while the other rubs on
SHOCK AND MEDICAL COILS.
61
the brass strip n, and therefore the contact on which this
spring is resting is electrically connected with n. Thus,
when the handle of the switch is over, let ns say, the label p,
the one contact-spring c is on the right-hand contact-stud
and the other on the left-hand one, and the two ends of the
primary are connected to the pillar m and semicircular brass
piece n respectively. Since the terminal h is connected to
the pillar m, and the terminal h! to the brass piece n, it
follows that these terminals, and also the electrodes attached
to them, are in contact with the ends of the primary.
I
Fig. 45.— bath coil. Fig. 48.
switch lever.
For the primary 6 oz. of No. 24 silk-covered copper wire
will be required, and this must be wound on layer upon
layer, as previously described.
A layer of well paraffined paper must be put on over the
primary previous to winding the secondary, and also over
the secondary when finished before winding the tertiary.
The secondary coil is wound with 8 oz. Ko. 36 silk-
covered wire, the ends being brought out at the opposite
cheek to the primary. The tertiary coil is wound with
8 oz. No. 36 silk-covered wire, the ends of the coil being
brought out at the same end of the bobbin as the secondary.
The connections of the coil are clearly shown in Fig. 49.
Starting from the left-hand battery terminal marked +, a
SHOCK AND MEDICAL COILS.
63
wire runs, it will be seen, to the outside end, of the pri-
mary coil, the inside end p being connected to the pillar
carrying the contact-spring and hammer. A wire is also run
from this pillar to contact-studs Nos. 1, 7, and 11 (counting
from the left-hand side) of the switch. The other battery
terminal, marked — , is connected to the contact-pillar
carrying the contact-screw, and this pillar is also connected
to switch contact-studs 2, 3, and 9. The inside end s of the
secondary coil is connected to the contact-stud 3, while the
outside end is connected to the contact-stud 4. The out-
side end of the tertiary coil is connected to the contact-
stud 5, while the inside end t is connected to the contact-
stud 6. The additional connections between the contact-studs
4, 5 and 8 must also be made, and likewise the connections
between the terminal h and pillar m and the terminal ¥ and
the semicircular contact-piece n.
To follow out the course of the induced currents in Fig.
49, there is first the primary induced current starting from
the primary coil to contact-stud No. 1, contact-piece n,
terminal ^, terminal pillar m, contact-stud No. 2, contact-
pillar e, terminal — , battery, terminal -f, back to primary
coil. The secondary induced current starts from the
secondary winding to contact-stud 3, terminals ¥ and h,
contact-stud 4, and back again to secondary. The tertiary
current runs from tertiary winding to contact-stud 5,
through handles to contact-stud 6, and back to tertiary by
the end L The combined primary and secondary currents
run from end of primary p to contact-stud 7, through the
handles to contact-stud 8, then to contact-stud 4, through
the secondary winding to contact-stud 3, contact-pillar e,
terminal — , battery, terminal + , and back to primary. The
combined secondary and tertiary currents run from end s of
secondary to contact-stud 3, then to contact-stud 9, through
handles to contact stud 10, then to tertiary coil, contact-stud
5, contact- stud 4, and back to secondary winding. The
64 INDUCTION COILS AND COIL-MAKING.
combined primary, secondary, and tertiary currents run
from primary coil to contact-stud 11, through handles to
contact-stud 12, from there to tertiary coil, contact-stud 5,
contact-stud 4, secondary coil, contact-stud 3, contact-pillar
e, terminal — , battery, terminal +, and back to primary
coil.
Another way of connecting up the coil shown in the last
article is illustrated in Fig. 50, the two terminals h and h!
and the post m and contact-piece n being omitted, as their
connections are the same as shown in Fig. 49. The main
difference between this method and that previously described
Fig. 50. — another method of connecting up bath coil.
is that whereas in Fig. 47 we had the six ends protruding
from the wound bobbin, we have in Fig. 50 only four, a
state of things that is apt to prove perplexing to many on
first examining a coil of this description. The matter is at
once made clear, however, on glancing at Fig. 53, as it will
be seen that the three different windings, marked p, s, and t
respectively, are all internally connected. Thus ends p and
jp s will give the primary shock, ends p s and t s the secondary,
and ends t and t s the tertiary.
Another form of coil, much used for electric baths, is shown
in Figs. 51 and 52. The coil has a primary winding only,
SHOCK AND MEDICAL COILS. 65
and this winding being of somewhat large wire, a primary
current of comparatively low voltage is obtained, and the
coil is thus specially suitable for the electric bath, and the
treatment of the abdomen by faradisation. The strength of
Fig. 51. — another fokm op bath coil.
shock is regulated in two manners — first, by sliding in and
out the movable iron core ; and second, by a switch
having five contacts, so that as the position of the arm of
the switch is altered, either 2, 4, 6, 8 or 10 layers of the
Fig. 52. — connections for coil.
v/inding are in circuit. This switch is not shown in either
of the above-mentioned figures, but a separate view is given
in Fig. 52, showing the position of the switch, and the
necessary alterations in the connections for its addition to
the coil.
F
66 INDUCTION COILS AND COIL-MAKING.
Sledge Coils,
The Dubois-Eeymond, more commonly called " sledge "
coils, owing to the secondary working on a sledge or slide,
are tindonbtedly the most convenient and efficient for
medical purposes. In these coils the primary winding is
fixed while the secondary is movable, so that its position
over the primary can be adjusted, and thus the induced cur-
rents in the secondary can be made very powerful or very
weak, while the transition to the intermediate strength is
capable of being produced almost imperceptibly. Moreover,
the secondary coil being removable, additional secondary
coils, wound with finer or thicker wire, can be kept by
for use in cases where either a higher or lower potential
secondary current is required. The coils are generally
arranged to give a primary shock as well as secondary — the
variations in the strength of the primary shock being
produced by varying the position of the iron core, which is
made removable for this purpose. The insertion or removal
of the iron core also affects the strength of the secondary
shock, though the alteration of the position of the secondary
itself, as described above, is mainly relied on for regulation.
The iron core being removable, a separate make-and-break is
necessitated.
Figs. 53, 54, 55 and 56 show in side elevation, end eleva-
tion and plan respectively, a form of the Dubois-Eeymond or
sledge coil.
The wood base of the coil, which should be of polished
walnut or mahogany, is 14 inches long by 5 inches wide,
and f inch thick. At 3J inches from the right-hand end is
fixed the upright standard w, carrying the primary bobbin a,
which is secured to the standard by two screws, which
union can be further strengthened with glue if desired.
The iron core in this coil, it will be noticed, slides in and
out at the contact-breaker end, and for this reason the
SHOCK AND MEDICAL COILS.
67
handle actuating it must be made sufficiently long to
prevent the hand interfering with the contact-breaker when
pushing the core right home. The iron core consists of a
tube made by rolling up thin sheet iron into a tube of the
required diameter, and filling up the centre with lengths of
Fig. 53.— sledge coil.
No. 22 iron wire, the wooden handle being driven into one
end of the tube. The primary bobbin is 5 inches long by
across the bobbin ends, with hole in centre, while the
secondary bobbin is 5 inches long by 3 inches across the
bobbin ends, with the hole in the centre \ inch larger than
Figs. 54, 55. — sledge coil.
bobbin ends of the primary, in order to allow the primary
to slide easily through the secondary ; both bobbins should
be constructed of polished walnut or mahogany to match the
base.
The guides for the secondary bobbin consist of two
lengths of wood, x and x\ fastened one each side of the
F 2
68 INDUCTION COILS AND COIL-MAKING.
base, as shown in Fig. 56, these lengths being secured to the
base by screws passed up from underneath. At the top of
each length of wood is a strip of brass -^^ inch thick, and
secured to the wood guides by four round-head brass screws.
The slide consists of a piece of wood (mahogany or walnut,
according to the construction of the base) 4J x 3 inches, the
bottom being cut away, to allow the slide to move steadily,
instead of in a series of jerks. Apart from this, it is neces-
sary that the slide fits the grooves nicely, and this is done
by placing two U-shaped springs on the top of the slide —
one on each side — these springs being fixed by a screw in
Fig. 56. — plan of sledge coil.
the centre, and so arranged that their two ends press against
the metal strips on the top of the side-pieces. The ends of
the wire from the secondary winding are connected one to
each spring, so that the springs serve the double purpose of
keeping the slide pressed firmly to the base, and also com-
pleting the connection of the secondary winding to the
terminals S and S', by way of the metal strips.
The contact-breaker is fixed at the right-hand end of the
coil, and is of the form described on p. 33.
The primary bobbin is wound with six layers of No. 22
silk-covered copper wire, which must be very evenly laid on,
so that the complete bobbin presents a perfectly even
surface, the surface of the last layers being slightly below
SHOCK AND MEDICAL COILS.
69
the cheeks of the bobbin. The secondary is wound with
14 layers of No. 36 silk-covered copper wire, carefully and
Fig. 57. — CONNECTION OF SLEDGE COIL.
evenly wound on, the outside of the bobbin, when completed^
being covered with a wrapping of velvet. Spare secondary
coils can, of coiiri>e, be made, and if care is taken in finishing
70 INDUCTION COILS AND COIL-MAKING.
lip the slide and fitting the secondary on to it, there will be
very little difficulty in making the different bobbins fit in
the guides without touching the primary. Each additional
secondary coil will, of course, require its own slide and side
contact-springs. The number of turns used on coils of this
description is usually primary 500 to 700 turns, secondary
5000 to 10,000 turns.
rig. 57 shows the connections of the coil of this descrip-
tion, the thick lines denoting the primary and the fine ones
the secondary.
Portable Coils,
Portable coils are now to be obtained in a very large
number of different forms, these coils, since the introduction
of dry batteries, having found much favour with the medical
profession ; while, moreover, the majority of persons who
Fig. 58.— portable coil.
desire to have coils for self-treatment usually prefer to have
them in the form of a set that can be quickly set up and put
away when finished.
iig. 58 shows an inexpensive little portable set, consisting
SHOCK AND MEDICAL COILS. 71
of a coil as described on p. 55, fitted into a case as shown,
with a dry cell at the back. The front part of the case
falls over outwards when released, thus allowing the
handles and coil to be readily got at.
Fig. 59. — portable medical coil.
Pig. 59 shows another very convenient form of portable
set. The case is of polished walnut or mahogany, with a
division at the top for the electrodes, and another at the
back for the two dry cells, as shown. The coil is fixed on a
hinged portion of the front, as in the figures, so that it can
readily be got at for adjustment, &c.
Passing on to larger and more elaborate portable coils, we
come to the Dubois-Eeymond set, a portable set which, when
fitted with o-lUts accessories, forms one of the mo^t powerful.
72 INDUCTION COILS AND COIL-MAKING.
useful and convenient. "When worked by dry batteries, and
supplied with the variable make-and-break, as in Figs. 60,
61 and 62, it really leaves little further to be desired.
As usually made up, the set consists of a polished rose-
wood, walnut or mahogany case, the lid, as will be seen
from the figures, being of greater depth than the bottom of
the case. The lid is hinged to the right-hand end of the
case, and at 2f inches from this end is fi.xed the thin upright
Fig. 60.— portable sledge coil set.
partition (also of polished wood) that divides the batteries
from the coil. This partition starts from the very bottom of
the case, rising up to the height shown in Fig. 60. Left of
this partition is the supporting base of the coil and its acces-
sory parts, this base being flush with the top of the bottom
part of the case, as shown in Fig. 60, the space so formed
underneath being fitted with a drawer that pulls out at the
left-hand end of the case. The drawer can be pulled out by
the handle seen to the left of the case ; but when the lid of
SHOCK AND MEDICAL COILS.
73
the case is shut down and locked, the drawer is secured by a
movable pin, seen just below the lock, which prevents it
being pulled or falling out.
The base of the coil, which is usually of polished wood to
match the case, though sometimes of ebonite, is secured in
its place by three brass screws, one on each side, and one
at the left-hand end. The side-pieces or runners for the
secondary bobbin are of polished wood similar to the case and
base, fitted with brass strips at the top, as has been pre-
FlG. 61. — PORTABLE SLEDGE COIL SET (PLAN).
viously explained, these runners butting up against the
upright standard (also of polished wood) carrying the
primary bobbin.
Between the upright standard and the battery partition is
the make-and-break, which is similar in form to that described
on p. 37.
Eeferring to Fig. 61, just below the coil will be seen two
switches, that on the right being to open and close the
battery circuit, while that on the left enables either the
primary or secondary induced current to be directed to the
electrodes. With the battery switch the contacts are simply
74 INDUCTION COILS AND COIL-MAKING,
" on " and " off," but with, the left-hand switch the contacts
are primary " and " secondary," that on the right being
the primary and that on the left the secondary. On each
side of the switches are two terminals, H and H', these being
the connections for the flexible wires of the electrodes.
Fig. 62. — portable sledge coil set.
The connections to the different parts of the coil are
shown by the dotted lines and the spirals of wire in the case
of the battery connections. The battery circuit runs from the
carbon terminal of the top cell (Fig. 61) to coils of contact-
breaker bobbins, thence to pillar of contact-breaker, on the
SHOCK AND MEDICAL COILS, 75
top of whicli is pivoted the armature, then to contact-screw
carried by arm on top of the wood upright, inside end of
primary, outside end, switch arm, and then back to zinc
terminal of lower cell. As shown in the figure, the battery
switch is off and the circuit open. The primary induced
current flows from the outside end of the primary winding
to terminal H, through the electrodes to terminal H',
thence to lever of left-hand side switch, left-hand contact,
the switch being in the position shown in the figure, then to
arm on top of wood standard and back to inside end of
primary. The secondary induced current runs from outside
end of secondary winding to right-hand contact of left-hand
switch, switch lever, the lever being in the opposite position
to that shown in the figure; thence to terminal H', elec-
trodes, terminal H, and then to inside end of secondary
winding by way of the metal strip on the top runner of the
secondary bobbin, the outside end being in a like manner in
connection wdth the bottom runner. The various shaped
electrodes are carried in the wood drawer before described,
and into this drawer is also placed the aluminium upright
arm and brass ball of the contact-breaker when not required.
Street Coils.
" Street coil " is a name that has been applied to all
shock coils designed for use at fairs, in the streets, and other
public places, where, on the usual " high days and holidays,"
the diversion-seeking holiday-makers are " electrified " at
a charge of a penny per head. Every one, of course, is
familiar with the general appearance of such coils — the
massive dial, formidable-looking bobbin, large terminals, and
showy brass mountings, the whole being supported on a
small hand-truck, while by the side stands the loquacious
proprietor extolling loudly, though not always too truthfully,
76 INDUCTION COILS AND COIL-MAKING.
the virtues of " 'lectricity," which, in his estimation, appears
to be a sort of panacea for all ills to which the flesh is heir.
Very numerous are the different forms in which street
coils are to be found — indeed, it is not too much to say that
although there are, of course, certain points alike in all
these coils, one rarely sees two coils of the same design, this
being due, no doubt, to a great extent to the fact that each
proprietor partly, if not wholly, designs and ccmstructs his
own coil, his main endeavour being apparently to outvie, in
showy and mysterious appearance anything he has pre-
viously seen. A showy and attractive appearance is without
doubt the most important point to be looked to in designing
a coil of this description, and the one which is in a, great
measure responsible for some of the very curious forms to
be seen. Plenty of brass mountings are always arranged for,
and all terminals and contacts made unusally heavy.
A bold dial is another important item. This should prefer-
ably be marked up to a high number of gradations, the index-
hand being actuated by drawing out the brass regulating
tube of the bobbin, the tube being connected by a thin cord
or wire, though the apparatus must be so arranged that
there is apparently no connection between them, the im-
pression possessed by the majority of onlookers being
that the pointer is actuated by the electric current in some
mysterious manner. Were it evident that the pointer was
actuated by means of a cord, much of the interest of the on-
lookers would disappear. Another point to which attention
should be paid is to have a large and loud-sounding contact-
breaker, the mysterious humming of the interrupter having
a wonderful effect, both in attracting the passers-by, as well
as being a visible indication to them of the presence of an
electric current.
In many coils there are usually the addition of one or two
electric bells, of more or less curious design, and a small
motor, and frequently an arrangement to ring a bell or
SHOOK AND MEDIOAL COILS.
77
make a figure move when the index-hand indicates a certain
strength on the dial. Suitable switching arrangements are,
of course, made both for switching on and off the battery-
current, as well as directing either the primary, secondary,
or combined primary and secondary current to the handles.
Fig. 63 shows a form of street coil which, while divested
of much of the unnecessary paraphernalia of many street
coils, proves, when made up, a most serviceable instrument,
combining efficiency with a showy and attractive appearance.
Eeferring to this figure, the base of the coil is of polished
walnut or mahogany, having a fancy moulding run round
as shown. At each corner of this base, and also at the centre
of each side, are fixed massive brass pillars of the shape
shown, joined together by brass rods, which rods fix into
holes in the balls of the pillars. From the top end of the
base rise two polished brass pillars, having flanges at the
bottom and a squared portion at the top, finishing off with
an ornamental point, as illustrated. These pillars, which
are, of course, hollow, are secured to the base by screws
passed through the fianges, and the squared portions at the
top carry the dial. The containing case of the dial may be
either of polished mahogany like the base, or of polished
brass to match the pillars. Just below the dial, and slightly
in front of it, is an electric bell that can be set in action by
a small switch (not visible) and also by the regulating tube
when drawn right out, and the needle indicates its fullest
strength, viz. 170. This bell can, of course, be of any
desired form, the form shown being that having two gongs,
and the hammer working from one gong to the other. The
gongs are mounted on a polished mahogany board, which is
supported off the base by four brass pillars, the movement
of the bell being attached to the underneath side of the
board.
The coil itself is supported on a separate base, which base
is in turn fixed to the main base just in front of the bell.
SHOCK AND MEDICAL COILS.
79
The bobbin of the coil is similar in construction to the bath
coil described on p. 60, except that it is wound only with
a primary and secondary winding. More space must also
be left for the tube to work in, as a fine piece of gut is
attached to the end for working the needle of the indicating
dial. This gut passes through the left-hand cheek of the
bobbin, through the base, up the left-hand brass pillar of
dial, round drum of needle three times, and then down the
right-hand pillar, inside which is a small lead weight that
is attached to the end of the gut. At the corners, where
the gut makes a turn at right angles, the edges of the metal
or wood must be rounded off, and it may be necessary in
some cases to fix small pulleys before the gut is got to run
freely. When properly fitted, as the regulating tube (the
knob of which is seen to the right of the bobbin) is drawn
out more and more, so the needle on the dial will indicate a
greater strength, while when pushed back the needle returns,
impelled by the weight on the end of the thin gut cord.
The contact-breaker is fixed on the same base as the bobbin,
and consists of the supporting brass pillar seen on the left,
which carries the thin brass spring that has affixed to the
end of it the soft-iron armature. Below the armature are
the two magnet bobbins in series with the primary winding
of the coil. Half-way between the supporting pillar and
these magnet coils is the brass standard carrying the contact-
breaker screw, this screw being fixed in the top of the ring,
through which passes the contact-spring carrying the soft-
iron armature. Both the supporting pillar and the contact-
pillar are secured by stout screws passed through the base
from beneath. The two magnet cores are fastened to a strip
of soft iron, which is in turn secured by two screws to the
base.
Just in front of the contact-breaker, and at equal distances
from the two centre pillars of the brass rail, are two
switches, having three contacts each. The switch on the
80 INDUCTION COILS AND COIL-MAKING.
right controls the current to the right pair of handles, and
that on the left to the left pair. Of the three contacts, the
first is for the primary, the second for the secondary current,
and the third is an off-block. The contacts and levers are
fixed directly on to the base, and should be of brass, the
different parts being somewhat heavily proportioned.
In front of the two switches are four heavy brass ter-
minals forming the connection for the handles. These ter-
minals are supported off the base by a strip of polished
mahogany, and are bolted through this and the baso. It is
very important that these terminals are of solid design and
firmly fixed, as great strain is liable to be thrown on them
by persons pulling at the handles. The connections to the
handles are likewise made by brass rods a little over ^ inch
thick, for, if fiexible cords were used, they would continually
be being broken by persons pulling or jerking the handles.
These connections are in three pieces, fastened together by
making loops at the end of each piece. The handles them-
selves consist of pieces of brass tube (plated if desired) with
polished mahogany end-pieces, through holes in the centre
of which pass the connecting rods.
The connections to the different parts are not shown in
the drawing; but from what has already been said and
illustrated in regard to previous coils, the reader, if he has
studied these, will have no difficulty in tracing them out for
himself. It must be remembered there are primary and
secondary windings, that the contact-breaker is in series
with the primary, and that the two pairs of handles are
connected in parallel.
As regards the dimensions of the different parts of the coil,
these, of course, can be anything the constructor desires.
A very good size is to make the dial 12 inches in diameter,
and the rest of the coil proportionate thereto. The battery
employed for working such a coil is a four-cell bichromate,
which would be placed in the body of the truck supporting
the coil.
81
CHAPTEK IV.
ACCESSORY APPLIANCES FOR, AND THE APPLICATION OF
MEDICAL COILS.
In the application of electricity to the cure, alleviation, or
diagnosis of disease, there are three kinds of currents em-
ployed. First, currents from batteries applied direct, or
Galvanisation ; second, induced electric currents, or Faradisa-
tion ; and third, static electricity, or Franklinisation. When
the electric current is employed for the destruction of the
tissues of the body by decomposition the application is known
as electrolysis ; while electro-cautery is the process of remov-
ing or destroying diseased portions of the body by metallic
points, rendered white hot by the passage of electric currents.
For Galvanisation, from 20 to 40 cells connected in series
are required, and in addition, a collector for connecting on
more or less cells, a current-re verser for changing the direc-
tion of the current, and a milliampere meter for indicating the
strength of the current passing through the patient. Eheo-
stats are sometimes employed in Galvanisation where there
is no collector, in order to vary the strength of the electric
current by inserting more or less resistance in the circuit.
For Faradisation, a medical coil is required giving primary
and secondary induced currents, the coil being fitted with
suitable means both for regulating the strength of the
current and the rapidity of the interruptions. A suitable
battery, capable of working the coil for an hour or so without
running down, must also be provided.
&
82 INDUCTION COILS AND COIL-MAKING.
For Franklinisation, some form of electro-static machine
such as a Wimshurst, Holtz, &c., will be necessary.
In electrolysis a battery of one or two cells is required, and
special electrodes according to the nature of the disease.
Usually these electrodes consist of needles, insulated except
at their points, which are pushed home to the centre of the
affected place. Double needles are employed or one needle
(negative) only, the patient holding the positive electrode in
his or her hand.
The whole of the current being confined at one electrode
to the point of a needle, the chemical action of the electric
current is concentrated at that point, and the surrounding
tissues, such as the root of a tumour, decomposed.
For electro-cautery a battery that will give a powerful
current and not run down is absolutely necessary. A strong
current is required to render white hot the metallic burners,
and it is, of course, important that the burners are main-
tained at the same heat during the whole of the operation.
A large number of different burners are made for cutting in
this and that direction, and to suit the different forms of
diseased growths.
For the greater convenience of medical men all the dif-
ferent apparatus necessary for the application of the
Galvanic and Faradaic currents are frequently combined
together in one cabinet, a form of which is shown in Fig. 64.
In the cupboard at the bottom are contained the 40 cells for
Galvanisation, and also the two cells for working the medical
coil. This coil is to be seen on the left-hand side of the
desk-shaped part at the top, while in a similar place on the
right-hand side is the milliampere meter. On an inclined
base in the centre of the cabinet are fixed the collectors,
current - reversers and various switches controlling the
working of the induction coil and the primary and secon-
dary currents. The top part of the cabinet is fitted with a
glass front, which lets down and locks to prevent its being
ACCESSORY APPLIANCES, ETC.
83
tampered with. The two terminals for the connecting cord«
to the electrodes are in front of the inclined base.
Fig. 61. — ELECI'KO-MEDICAL CABINET.
G 2
84 INDUCTION COILS AND COIL-MAKING.
Conducting Cords and Electrodes.
The conducting cords, or rheophores as they are also
frequently called, are the wires connecting the terminals of
the instrument with the electrodes. They are, of course,
two in number, one being required for each electrode. The
ordinary conducting cords, as used for Faradisation, consist of
tinsel cord served with one to two layers of cotton, and then
braided usually in fancy colours, the ends of the eords being
provided with special metal tips for convenience in inserting
in the instrument terminals and electrodes. The cords vary
in length from one to three yards. For Galvanisation the
Fig. 66.
Fig. 65. Fig. 67.
electrodes.
conducting cords used are generally more expensive, being
insulated with gutta-percha and more heavily braided. It
will be found more convenient to have the two conducting
cords of different colours, so that the positive and negative
electrodes can easily be distinguished. Fig. 65 shows the
form of cord usually employed.
Of electrodes there are, of course, a great variety of
different shapes and forms, there being special electrodes
designed for use on nearly every part of the body. It may
be as well to mention, perhaps, for the benefit of those not
conversant with electrical terms, that an electrode is the
plate, ball, sponge, or other enlargement at the end of the
conducting cord by means of which the current is applied to
ACCESSORY APPLIANCES, ETC,
85
the bocly. In applying the current, two electrodes are
necessary, the one being attached to the positive conducting
cord, and the other to the negative, the two electrodes being
known as the positive and negative electrodes respectively.
The most usual electrode employed is, of course, the
handle, as shown in Fig. 66. This is usually of brass,
plated, and in the better forms of coils arranged to screw on
to the wooden handle, Fig. 67. This allows the physician
to hold the handles in the patient's hands without himself
receiving a shock, and thus reducing the strength of the
current that is being applied.
Fig. 69.
ELECTRODES.
Fig. 68 shows a wooden handle fitted with a switch for
interrupting the circuit. The metal handle, as in Fig. 66,
or any other electrode, screws on to the metal extension of
the handle, and the physician, holding the wooden part of
the handle in his right hand, can break the circuit at will by
pressing with his finger the button seen underneath.
Fig. 69 shows a double pole electrode, for use w^hen it is
required to confine the current that is being applied to a
small area. The two arms carrying the electrodes are
fixed on swivels, so that the distance between them can be
varied at will.
Fig. 70 shows an improved form of electrode for holding a
sponge. The sponge is pressed into the end of the metal
86 INDUCTION COILS AND C OIL-MAKING.
tube, and, expanding tliroiigli the three holes in the side, t%
thus retained in its proper position.
Figs. 71, 72 and 73 show three of the more common shapes
of electrodes used, and all are adapted to fit on the different
forms of handles just described. These electrodes consist of
tin, brass or carbon plates, covered either with flannel or
Figs. 71, 72, 73. — electrodes.
chamois-leather. They must be wetted for application, and
the flannel or leather after much use must be renewed, as
they rapidly pick up the dirt. The metal plates also
quickly oxidise, and must, therefore, before new flannel or
leather is put on, be cleaned with emery cloth or by scraping
with a knife. Electrodes with flexible gelatine plates that
adhere to the skin are now being much used. Electrodes of
Fig. 74. — eye electrode.
common metal, since they oxidise, must not be placed on the
mucous membrane unless connected to the negative pole.
Fig. 74 shows an electrode for the muscles of the eye,
the handles being provided with a switch, so that the
current can be at once switched on by a slight pressure of the
finger.
ACCESSORY APPLIANCES, ETC. 87
Brushes consisting of bristles of very fine wire are used
for exciting the nerves of the skin. They are connected to
the conducting cords and passed, lightly over the surface of
the skin. Fig. 75 shows a single-pole brush, and Fig. 76 a
Figs. 75, 76.— brush electrodes.
double-pole one, for local electrisation. All the brushes are
provided with wooden handles, so that the physician can
apply the electrode to the patient without himself receiving
a portion of the current.
Fig. 77 shows a roller or wheel electrode, by employing
Fig. 77.— roller electrode. Fig. 78. — needle electrode.
which the place of application can more conveniently and
rapidly be changed. They are also used for combining
massage with the application of electricity.
Fig. 78 shows an electrode composed of nine needles for
88 INDUCTION COILS AND COIL-MAKING.
electrolysis. These are employed for destroying the roots of
hairs, tumours, naevi, &c., by the decomposing action of the
electric current. The needles are composed of gold or plati-
num, insulated throughout their length, except at the very
point, by a special enamel or a coating of vulcanised rubber.
The object of having so many needles on the one terminal
is because should one needle fail another can rapidly be
inserted in its place.
Galvanometers and Milliampere-meters.
Fig. 79 shows a form of galvanometer much used for
electro-medical purposes. It consists of a very delicately
Fig. 79. — galvanometer. Fig. 80.— milliampere-meteb.
suspended needle, round which is wound a coil of fine wire,
the needle being deflected by the passage of an electric
current, and indicatiug the amount of deflection on the dial.
For medical purposes, of course, these galvanometers require
to be very sensitive owing to the feeble currents employed.
To obtain sensitiveness in a galvanometer it is necessary to
have the needle very delicately poised, and to wind the
bobbin with a large number of turns of very fine wire. The
galvanometer is intended more for indicating the pressure of
ACCESSORY APPLIANCES, ETC.
89
a current, while the milliampere-nieter reads off on the dial
the exact value of the current passing in milliamperes, or
thousandth parts of an ampere. The internal construction of
both is much the same, the difference being that the scale of
the latter has been graduated while currents of known
strengths have been passing through it. A form of the
milliampere-meter is shown in Fig. 80.
Collectors,
Collectors are either single or double, according to
whether they have one or two collecting arms. A form of
Fig. 81. — simple form of collector.
the single collector with its connections to the cells is shown
in Fig. 81. It consists of a number of contacts, one for each
cell, arranged in a circle on a wood or ebonite base, in the
centre of which is a brass arm. arranged to work on these
contacts. The connections are run as shown by the dotted
lines, and thus, as the arm is moved round in the direction
of the arrow, more and more cells are connected on to the
terminals + and — . Double collectors have two contact-
arms and are more convenient, as they allow acy portion of
the battery to be included, as only the cells between the two
arms are connected on to the terminals. Thus, if one arm is
on contact 5, say, and the other on contact 15, only the cells
between 5 and 15 inclusive would be in circuit. A small
90 INDUCTION COILS AND COTL-MAKING.
number of cells can therefore be selected from any portion of
the battery. The connections of the double collector are some-
what different.
Current Beversers.
The form of current reverser chiefly employed with
medical coils is shown with its connections in Fig. 82. It
consists, it will be seen, of two switch-arms bolted together
by a piece of ebonite, below which are three contact-studs
Fig. 82. — cuerent-eeversee. Fig. 83. — metallic eesistance.
connected as shown. When the switch is in the position
shown in the figure, the left-hand terminal is -f and the
right — , but if moved to the opposite side the left becomes
— and the right -f .
Bheostats,
Kheostats or resistances, employed for electro-medical pur-
poses, are either metal, liquid or carbon. A form of the
metal resistance is shown in Fig. 83, having 27 contacts and
giving various resistances from 1 to 1000 ohms. The resist-
ances are connected to the contacts so that as the arm is
ACCESSORY APPLIANCES, ETC.
91
moved to each contact resistance to the amount marked is
thrown into circuit.
Perhaps the most simple form of resistance is the liquid
resistance as shown in Fig. 84. This consists of a glass
tube fixed on to a wood base by means of a
brass cap, on which is fastened the one terminal
Another brass cap is fastened to the top of
the tube, through which passes an adjustable
brass rod. A second terminal is attached to
this top cap and the tube three-fourths filled
with water. If this liquid resistance or water
regulator is connected in circuit by means
of the two terminals, it follows that the
strength of the current passing will be
diminished as the brass rod is withdrawn, -ri ~
' rIG. O-t. — LIQUID
owing to the length of the column of eesistance.
water being increased. On the other hand
less resistance is in circuit as the brass rod is pushed down,
and the current therefore increases.
III
92 INDUCTION COILS AND COIL-MAKING.
CHAPTER V.
SPARK COILS.
Spark coils is a name applied to all induction coils designed
and constructed to produce large sparks at the secondary
terminals, as distinguished from those coils intended merely
for administering shocks for amusement or medical purposes,
and known as shock or medical coils.
Although the difference between spark and medical coils
may be said to be mainly in the size, strength and higher
insulation of the different parts, yet very much more care
must be taken with the construction of the former, certain
methods of fixing and insulating different portions that are
suitable for medical being quite inadmissible in a spark coiL
The chief point to which increased attention must be directed
is, of course, the insulation of primary and secondary wind-
ings, and more particularly the latter.
The very high tension of the secondary current enabling
it to spark great distances, necessitates the utmost precaution
being taken both in the winding and insulation of the
secondary coil. Should the spark find an easier path, or path
of less resistance, than between the two ends of the secondary,
whether it be from layer to layer of secondary or secondary
to primary, it will assuredly take it, and, once taken, it will
always follow it in preference if the ends of the secondary are
separated far apart.
A break-down in the insulation of the coil, if not rendering
it utterly useless, always very considerably reduces its spark-
SPABK COILS,
93
ing distance, and as such break-downs may occur on the
first trial of the coil, and generally necessitate unwinding
and rewinding, it will be found advisable not to grudge a
few extra hours during the construction, in order to take all
possible precautions.
Unquestionably the secondary winding is the part of the
coil requiring the most attention, and in very large coils so
great is the tendency of the discharge to break down the
intervening insulation between the layers, that some special
method of arranging the secondary, such as sectional winding,
has to be adopted.
With small spark coils, however — i. e. those giving under
IJ-inch sparks — it is very doubtful, taking all things into
consideration, if anything is to be gained by sectional
winding, as with reasonable care there is little difficulty in
insulating the different portions so as to prevent a break-
down in the insulation.
Our object as regards the secondary is to get on as many
turns as possible, consistent with efficient insulation and not
getting too far from the primary. Every turn of wire adds
to the E.M.F. of the induced current, and as the E.M.F. is
augmented, so, of course, the length of spark is increased.
Nos. 38 and 40 B.W.G. best silk-covered copper wire are the
most suitable sizes for small spark coils; Nos. 36 and 34
for large ones. The finer the wire the more turns can
be got on and the longer the spark, while if a short thick
spark is desired a thick wire, such as No. 34, must be
employed.
In Chapter II. we pointed out the extra precautions to be
taken with regard to the different parts of a spark coil, so we
will now proceed to describe in detail one or two different
sized coils wound both in the ordinary manner and
sectionally.
94
INDUCTION COILS AND COIL-MAKING.
An Inch Spark Coil.
Fig. 85, half full size, and Fig. 86, one-third full size,
show in side elevation and end elevation an inch spark
8PABK COILS.
95
coil complete with commutator. Eeferring to these figures,
the bobbin ends are of ebonite, 3 inches in diameter and
f inch thick, the extreme length of the bobbin being 6 J inches.
The iron core is 1\ inches long and slightly under f inch in
diameter. The body of the bobbin is made up of several
layers of paraffin paper, as previously described, into the
centre of which is forced the iron core of No. 22 charcoal iron
wire. The bobbin having been carefully prepared, the
EiG. 86. — AN INCH SPARK COIL (scaie one-third).
primary is next wound on, this winding consisting of two
layers of No. 16 silk-covered copper wire.
The primary being finished, it must be carefully tested
with a battery of two or three cells to see that it is correctly
wound, and if the magnetism in the core seems sufficiently
powerful, the surface of the primary can be prepared for the
secondary.
For the secondary we shall require lb. of No. 38 best
silk-covered copper wire. This should give us slightly over
1-inch spark with three quart Bunsen cells, and if much
96 INDUCTION COILS AND COIL-MAKING.
over we may consider that we have been fairly snceessful in
our winding. This secondary must be wound on, layer upon
layer, with a wrapping of thick, well-paraffined note-paper
between each layer, and the different layers must not be
run up nearer than J inch from the bobbin ends. The
inside end of the secondary must be brought up inside the
right-hand bobbin-cheek, as shown by the dotted lines in
Fig. 85. This is done by drilling a hole in an oblique
direction, and then another down from the top to meet it.
The inside end can then, previous to winding, be worked
through this hole, and this hole afterwards run in with paraffin
Fig. 87. — connections of spakk coil "without commutator.
wax. After the last layer of secondary has been wound, six
wrappings of well-paraffined, thick note-paper must be put
on, and the whole well basted with wax, finishing with a
layer of black-ribbed paper.
The contact-breaker consists of the brass pillar Ti, carrying
the hammer a, and contact-spring, while behind this is the
pillar c, carrying the contact-screw. The height of these
pillars is If inch, and the contact-spring is 2| inches long by
■J- inch wide. Two substantial terminals are fitted on the
left-hand side of the base for the primary connections, while
the secondary terminals are fixed on top of the bobbin-cheeks,
as shown. Two thin brass rods, fitted at one end with ebonite
SPARK COILS.
97
knobs, slide in the holes of these terminals to form the
discharger.
The commutator, or current-reverser d, is fixed at the
right-hand end of the base, and consists of an ebonite cylinder,
fitted on each side with two brass contact-plates, as shown in
Fig. 86, these two plates being in contact with the uprights
supporting the cylinder, which uprights are in connection
with the two poles of the battery. At each side of the
cylinder is a contact-spring, one spring being connected to
the outside end, and the other to the inside of the primary
coil through the contact-breaker. It will thus be seen that if
the cylinder is turned half-way round, the direction of the
) z ^
Fig. 88. — connections of spark coil with commutator.
current in the primary circuit will be reversed, while if turned
only a quarter of a revolution, the circuit is interrupted.
The connections of the coil, with and without a commutator,
are shown in Figs. 88 and 87, which also show the method
of connecting up the condenser. Fig. 88 is the one with the
commutator. In both the figures P P are the primary ter-
minals, s s the secondary, and m and n the two sheets of the
condenser.
The base of the coil, which is of polished mahogany, is
14J inches long by 6 inches wide, and l£ inch deep, not
including the feet. It is made of J-inch wood, the bottom
H
98 INDUCTION COILS AND COIL'MAKINQ,
being removable for easy access to the condenser. The con-
denser, which occupies very nearly the whole of the inside of
the base, consists of 40 sheets of tinfoil, 6 inches by 4 inches,
put together as described in Chapter II. It must then be
slipped inside the wood base (the connections to the different
parts of the coil having, of course, first been made), and con-
nected up as shown in Figs. 87 and 88, taking care the ends
of the condenser are well apart, and to well soak the inside
of the top part of the base with parafSn-wax.
The following are the dimensions for the different parts of
a J-inch and |-inch spark coil, of a similar construction to
the onejuht described; —
inches.
inches.
1.
3.
2iXi
5i
4
Length and diameter of core
6x|
9x5x2
7i X 3f -f li
5i X 3i
4x2
Number of tinfoil sheets
40
36
6J X 4i
5x3
No. 18
No. 18
No. 40, 1 lb.
No. 40, f lb.
Sectionally-
Wound Coils,
Sectional winding, as applied to spark coils, consists in
winding the secondary, not in a succession of continuous
layers from bobbin-cheek to bobbin-cheek, but in dividing it
up into a number of sections or divisions, which sections are
afterwards joined in series. The object of so dividing up the
secondary is because that with the wire thus distributed, the
ends and those portions of the winding between which there
exists a very high difference of potential, are either placed
very far apart, or else separated by a thin disc of some
material having great insulating properties. In large spark
coils, with the enormous difference of potential that exists at
SPARK COILS.
99
the ends of the secondary winding, there is a tendency to
discharge in all directions — one portion of the secondary to
another, secondary to primary, and, in fact, any part where
the insulation is weak — so that to get the best results, or,
indeed, to get any results worth speaking of, some method of
sectional winding must be adopted.
Sectionally-winding induction coils appear to have been
first introduced by Messrs. Siemens and Halske, of Berlin,
who exhibited a large coil with the secondary wound in
sections at the Great Exhibition of 1851, which gave what
were then considered remarkably good results. The tube
and divisions for the secondary were turned up out of a solid
block of ebonite, from the centre of which the primary could
easily be withdrawn and replaced. This method of con-
struction, apart from its expense, is not one that will recom-
mend itself to those desirous of constructing large spark
coils ; but in the English Mechanic of August 5, 1870, a
method of constructing a sectionally-wound coil giving
8-inch sparks was described by " Inductorium," in which
the divisions (there being 90 sections) consisted of separate
ebonite discs slipped on one by one over the ebonite tube,
with distance pieces or rings between. This method, or
rather a modification of it, as subsequently suggested by
" Inductorium," in which discs of several layers of paraffined
paper are substituted for the ebonite ones, the writer has
found, after experimenting with nearly every other method
of winding, to give the best results, and one that enables
any one possessed with an average amount of patience to
construct a really powerful and efficient coil at a compara-
tively small outlay.
Figs. 89, 90, 91, 92, 93, 94 and 95 are diagrams illustrat-
ing the manner in which section ally-constructed coils are
wound. In Figs. 89, 90, 91 and 92 the whole coil is shown in
section, the black portions being the ebonite ends and divi-
sions slipped on the central iron core, round which is shown,
H 2
100 INDUCTION COILS AND COIL-MAKINQ.
by the dotted lines, the secondary winding. Figs. 93, 94
and 95 are sections of the top half of the coils, only showing
both the primary winding P, and the secondary S. Here
the successive layers are shown, and to make the explanation
clearer, the bottom halves of the windings are omitted.
Fig. 89. — ordinary method of winding.
Eeferring to Fig. 89, which represents the ordinary
method of winding without any sections at all, the secondary
being in a succession of continuous layers from bobbin-
FlG. 90. — COIL WOUND IN TWO SECTIONS.
cheek to bobbin-cheek, it will be seen that one of the
greatest disadvantages of this method is that the inside end
must pass in succession each layer of wire as it rises to the
terminal on the top of the right-hand bobbin-cheek. This
will, perhaps, be better seen from Fig. 93, which shows
SPARK COILS.
101
the same method of winding but figured in a different
manner. Keferring to this figure, the thick dotted lines are
the two layers of the primary P, above which are the thin
ones S, representing the different layers of secondary. The
H ^
i
\
M
H ^
H ^
■ /\ A \
\
MAN.
( 1
\\ 'Jul
Fig. 91.— coil wound in four sections.
passing of the successive layers by the inside end is clearly
shown here, and as it approaches the outside layers the
greater does the difference of potential between this end of
the winding and the layers become. There is, therefore, a
Fig. 92. — coil wound in eight sections.
great tendency for the insulation to break down at this part
of the coil, with the result that the moment such am occur-
rence takes place the coil is ruined. Apart from this, there
being also a great difference of potential between the ends
102 INDUCTION COILS AND COIL-MAKING.
of these long layers, there is a tendency to discharge from
layer to layer, and though there are ways in which the
insulation can be greatly increased at these points, and
the probability of a break-down be reduced to a minimum,
yet the best of coils so wound cannot in any way compare
Fig. 93. — ordinary method of winding.
with one in which the secondary has been sectionally dis-
tributed.
The simplest method of sectional winding is shown in
Fig. 90, another view of the same method being shown in
Fig. 94. In this method the space for the secondary is
Fig. 94. — coil wound in two sections.
divided into two halves by an ebonite disc ^^^^
\ inch thick, this disc fitting tight on the ebonite tube or
other material forming the insulating medium between the
primary and secondary. The secondary is then wound on
in two portions, one portion being wound in each division.
SPAEK COILS.
103
The inside ends of these two portions are joined together, so
that the two free ends of the winding are both outside ends,
and finish off quite close to the terminals to which they are
to be connected. We have now no inside ends passing layer
after layer of wire, and the greatest tendency to spark from
one part of the winding to the other is between the ends of
the layers butting on to the ebonite division disc. As this
is a substantial disc of high insulating material, the insula-
tion is perfectly safe. The tendency to spark through this
disc is greatest, of course, at the two top layers, and though
the spark cannot pass through the disc yet it might jump over
the top ; so to obviate this the ebonite disc is carried up
higher than the bobbin-cheeks, so as to increase the distanc e
the spark would have to travel. In winding the secondary
of a coil constructed on this method, the end of the wire is
passed through a small hole in the bottom of the dividing
disc, and the one half of the coil wound with half of the
quantity of wire that is going to be put on. The coil is
then turned round, and a joint made with the end of the re-
maining half of the wire on to the inside end projecting
through the disc. This portion of the wire is then wound
on, the outside end finishing off at the other terminal, as in
Fig. 90. It is important to remember that before winding
on the second half of the secondary the bobbin must be
104 INDUCTION COILS AND COIL-MAKING.
turned end for end : otherwise the two windings will oppose
one another.
Fig. 91 shows a coil wound in four sections, and Eig. 95
is another view of the same method. The division pieces
are ebonite discs fitted on the ebonite tube containing the
primary. The secondary is divided into four portions, and
we have thus less chance of a break-down in any one division ;
while, moreover, should any one division break down, the
effects on the sparking of the coil are not so very disastrous.
The connections between the divisions are, it will be seen,
made alternately at the top and bottom of the dividing discs.
It is in reality like two coils wound, as shown in Fig. 90,
placed end to end, and connected in series. In winding, the
division at one end is first wound, the inside end being
slipped through a hole on the bottom of the dividing disc,
to which end the beginning of the second division is con-
nected, and, the coil being first turned round, this second
division is then filled. The same process is carried out with
the other two divisions, the two ends of the centre divisions
(which are outside ends) being afterwards joined together
either across the top of the disc or through a hole near the
edge.
Fig. 92 shows a sectionally-wound coil having eight divi-
sions or sections. The process of winding is, of course,
similar to that just described, the difference being that
there are double the number of divisions, and consequently
just half the amount of wire in each division for a coil of
the same size. There must always be an odd number of
division pieces, so as to make an even number of sections,
and thus allow the ends of the two outside sections to finish
on the outside close to the terminals. As the number of
divisions or sections is increased, so the thickness of the
division disc can be somewhat reduced.
SPARK COILS.
105
A 2-inch Sparh Coil,
Figs. 96 and 97 show a 2-incli spark coil, the secondary
of which is wound in two sections, as shown diagramatically
in Figs. 90 and 94. Fig. 96 is a side elevation, while
Fig. 97 is an elevation from the contact-breaker end. The
base, which is of polished walnut or mahogany, is 12 inches
long by 7 J inches wide, the height from the top of the base to
Fig. 96. — a 2-inch spark coil (scale one-third).
the bottom of the feet being 3^ inches. The base is hollow,
and contains the condenser, after the manner described for
the 1-inch spark coil, previously described. The two end
pieces, or bobbin-cheeks, are of ebonite and square in form,
being 4 inches high^by 2| inches wide by | inch thick. The
iron core passes through the centre of these cheeks vertically,
but slightly above the centre horizontally. The distance
between the inside faces of the bobbin-cheek is 6^ inches.
106 INDUCTION COILS AND COIL-MAKING.
After the primary (which consists of two layers of
No. 14 B.W.G. silk-covered wire) has been wound, the centre
ebonite division disc, which is 4J inches in diameter by
\ inch thick, is placed centrally on the primary winding, and
the ebonite end-pieces or bobbin-cheeks slipped on. The
secondary, which consists of 2^ lbs. No. 36 silk -covered wire,
is then wound on as described with reference to Fig. 90,
there being 1 lb. in each division. The outside ends of the
secondary are then connected to the terminals of the ebonite
discharging pillars, the height of which is 5 inches. A
Fig. 97. — a 2-inch spark coil.
wrapping of thin sheet ebonite is then wrapped round the
outside of each division, both to improve the appearance of
the coil and protect the secondary against mechanical injury.
The contact-breaker, which is of a very substantial character
(to scale one-third full size in illustration), is fixed at the
right-hand end of the base. The condenser consists of 60
sheets of tinfoil, 6 inches by 6 inches, put together and
connected to the contact-breaker as described in Chapter II.
The two primary terminals are fixed at the right-hand end
of the base on one side of the contact-breaker.
SPARK COILS.
107
A 12-inch Spark Coil,
We will now pass on to the construction of a somewhat
larger spark coil, viz. one giving 1 2-inch sparks. There are
probably few persons of an electrical turn of mind who have
not some time or other wished to be the possessor of a large
coil, with which so many interesting and instructive ex-
periments can be performed. To purchase a coil of this size
would cost something like 50Z., thus putting it beyond the
reach of many ; but since those with a little spare time at
their disposal and a reasonable amount of patience can con-
struct one equally as efficient for about one-fourth that price,
the objection on the score of expense need no longer exist.
In the back volumes of the English Mechanic will be found
a description of a large number of spark coils, giving 4-inch
to 10-inch sparks, constructed on a similar plan with great
success by several readers of that journal, notably those of
Mr. J. Brown, of Belfast (English Mechanic, Feb. 27, 1880) ;
Mr. E. Baugh (Jan. 4, 1884); Mr. Higgs (Feb. 11, 1887);
and Mr. T. H. Muras (Oct. 7, 1892.).
The main requirements necessary for success are patience
and a determination to construct each part thoroughly.
Never leave one portion until quite satisfied that every
precaution has been taken to render it as perfect as it is
in your power to make it. Remember that even one little
point passed over in a slovenly manner may be the cause of
a complete failure, and that it is an easy matter to alter
certain parts during construction, while, when finished, it is
often impossible to do so without pulling the whole coil to
pieces. If these points, together with the fact that the
secondary discharge is always endeavouring to find out a
weak point in the insulation, are carefully borne in mind,
there is very little reason to doubt a successful issue and the
ultimate possession of a really serviceable coil.
108 INDUCTION COILS AND COIL-MAKING.
Fig. 98 shows the coil in section, one-third full size.
The primary terminals, commutator, condenser, discharger
and secondary terminals are omitted in order to make the
illustration clearer. Referring to this figure, m is the iron
core and n an extension of one pole for the purpose of
working the contact-breaker. The primary winding is
marked jp, over which is the ebonite insulating tube
separating the primary from the secondary, a;, are the
two ebonite cheeks of the bobbin, which is supported up
from the base of the coil by the polished wood supports o, o.
The secondary winding, s, is wound on in 96 sections of an
outside thickness of each section of \ of an inch, w is the
base, which can be of polished walnut, teak, or mahogany, as
desired, and has at the right-hand end the contact-breaker,
which latter is of Apps' improved form.
In constructing the coil the first portion to be made is : —
The Core.
This consists of a number of lengths of highly annealed
charcoal iron wire. No. 22 gauge, which, when made into a
compact bundle, forms a tightly packed core 19 inches long
by inch in diameter. Through the centre of the core
passes a ^-inch soft iron rod, carrying at one end the iron
extension piece, w, and having at the other end a nut which,
when screwed up, clamps the extension piece firmly to the
iron core. The iron wires are best formed into a bundle by
procuring a piece of metal tube, the inside diameter of which
is equal to the required outside diameter of the iron core,
and then forcing the wires into this until packed quite
tight. The tube is then gradually slipped off, at the same
time binding round the core, at intervals of every 3 inches,
some thin binding wire, until the tube is completely with-
drawn and the bundle left complete. The core should then
be immersed in melted paraffin wax and kept at an even
[To Jace page 108.
[To J ace page 108.
Fig. 98.— a 12-inch Spaek Coil. (Scale ird.)
SPARK COILS.
109
temperature until no more bubbles rise to the surface of the
wax. The bundle should then be allowed to cool slightly,
after which it is wrapped from end to end with a layer of
paraffined tape, removing the binding wires as the taping
proceeds. The weight of the core complete will be about
8 lbs.
Primary Winding,
This consists of three layers of double silk-covered copper
wire '110 inch in diameter. The wire is square in section,
whereby less space is wasted, though if this square wire is
not procurable No. 12 circular can be employed. The use
of square wire is, however, preferable, as a lower resistance
of the primary is thus obtained. The winding of the
primary is best done by two persons, the one of whom holds
the iron core firmly in both hands while the other feeds it
on evenly and neatly. A little care is requisite in winding
the primary to see that each layer runs on evenly, so that it
will, when finished, slide nicely into the ebonite tube. To
prevent the end of the wire slipping at the commencement
of the winding it should be bound on to the end of the iron
core with some stout string, leaving a projecting end of
12 inches for the purpose of afterwards making the con-
nection to the contact-breaker. When finished the coil and
primary winding must be immersed in melted paraffin wax
and afterwards allowed to drain and cool.
The Insulating Tube.
This is one of the most important parts of the coil ; its
object being to separate the primary and secondary windings,
between which there exists a great tendency to spark. In
order, also, to prevent any discharge passing over the end of
the tube to the primary it is made longer than the iron so as
to completely inclose it. This tube, which is of ebonite, is
21| inches long by 2^ inches internal diameter and \ inch
110 INDUCTION COILS AND COIL-MAKING.
thick. It can either be purchased as a tube complete or
made up by wrapping warmed sheets of thin ebonite round
a metal tube until the required thickness is obtained,
fastening the ends of each wrapping by melted shellac.
The wound core is slipped inside this tube, which it should
just fit nicely ; the ends of the primary being brought out
at two holes drilled as shown, and the two end pieces of the
tube afterwards being cemented in at each end. The two
holes through which the ends of the primary pass are tapped
to receive the ends of two short lengths of ebonite tube,
which serve to conduct the primary ends to the base, the
lower ends of the tube being also similarly fitted into the
top part of the base. The primary is thus perfectly enclosed
in the ebonite tube, the only opening being a li-inch hole at
the right-hand end through which the hammer of the con-
tact-breaker works. A little consideration will soon show the
important part played by the ebonite tube in separating the
primary and secondary windin^^s, the tendency of the sec-
ondary to spark into the primary being very great.
The Base, Bohhin-Cheehs, and Supports.
The base, which is 28 inches long by 10 wide and 5 deep, is
of polished mahogany. It is made hollow, as shown, to
contain the condenser, and has affixed to it the two supports ,
also of mahogany, which are 7J inches high, by 4 wide and
1^ thick. These supports have a hole just over 2| inches in
diameter bored in the top of each, through which pass the
ends of the ebonite tube to support the bobbin, as shown.
The bobbin-cheeks, which are of ebonite, are circular in form,
8 inches in diameter, and J inch thick. They have a hole
bored in their centre of such diameter that they fit tightly
on the ebonite tube, as shown in the figure. An ebonite
distance ring is placed outside each cheek, which, by butting
up against the supports, keep the bobbin-cheeks in place.
SPABK COILS,
111
The Secondary,
The secondary consists of 12 lbs. No. 36 silk-coverecl
copper wire, wound on in 96 sections or flat rings. It is
laid on in a curved fashion, as shown, so as to accord with
+]^ri <1ii:er;tj on,/^^ ^' ^ agnetised iron core.
New Engine.
is balanced and easily adjusted. The e:overnor i<; nf fi.^ ^^.^
type of shaft governors, has few wearing pa ts and is of
portions. .It IS sensitive, and can be adJust^/toaC^e^^^^^ '
The engine as a whole is compact, neat and strong, and is especiallv
adapted for direct-connected work. especially
A New Galvanometer.
The galvanometer is one of the oldest and most useful* of ^l.of.- i
instruments, and it has attained what was generalirsuppLed to t
permanent form. The new galvanometer, however, iLstr^ted harewUh
departs somewhat from the standard style and is s;id to be a very atfs'
factory instrument. It is made by the Utica Fire Al^rT « ^ , I
ICompany. Utica, N. Y., and possesses seveJll n^e! teatTret i^tf
,5truction that merit a brief description
^ This galvanometer consists of a ring made partly of iron and partly of
US
•st
ur
ed
en
lid
air
ets
:er,
out
ow
110 INDUCTION COILS AND COIL-MAKING.
The special 12-irich Rhumkorff coil, made by Edwards & Co., New
York, possesses some extraordinary constructional features. The sec-
ondary winding is divided by rubber discs into twelve -sections, for the
purpose of reducing as much as possible the potential difference betv/een
adjacent layers and sections of wire. In order to maintain the insulation
of the secondary coil at the highest degree, double-covered silk wure is
used as an extra precaution.
The interrupter consists of a small electric motor, desij^ned so that it
may be operated by hand.
The platinum contact points are extra heavy, and arranged so tha|
^hey may be readily renewed. All the fittings of this coil are of solid
thick. It cs\r\ piflTo-r \^ — ^..^^-u^
m
a
fai
T]
at
tu
lie
to
wl
lo^
tO]
in
th(
tac
im
pri
0I1(
of
COI
als
11.
^4
dia
enc
Th
8 ii
bor
on
disi
up .
Large Rhumkorff Coil.
Twelve-Inch Rhumkorff Coil.
bronze, audits dimensions over, all are as follows: Length, 3 feet 4 inches
width, 15^ inches; height, 20 inches. It is mounted in a heavy, hard,
rubber case, on a polished mahogany base, and weighs about 175 pounds.
The coil presents a very handsome appearance and shows skill and
care in its construction and finish.
buppurxs, Keep tne DoDDm-clieeks in place.
SPABK COILS,
111
The Secondary,
The secondary consists of 12 lbs. No. 36 silk-covered
copper wire, wound on in 96 sections or flat rings. It is
laid on in a curved fashion, as shown, so as to accord with
the direction r^f i- " ' ^ agnetised iron core.
g'l^m in Fig. 99, from
'o wards the
S ^ g induction is
^ ^* » cti -
§ 5^
110
INDUCTION COILS AND COIL-MAKING.
thick. It can eithe rjbe T)urcliag ftrl ^« a. -biL^
Large Rhumkorff Coil.
special .-inc. ^^-^'^r^rsuttiffflt^^^ T.'eTe:
York, possesses --^.-"f,:^^' discs^^^^^^^^^
• ondary winding is «3->'5^<^^J^J"^^;j^^\'^^^ potential difference between
purpose of reducing as much ^^^f'^^ ^J^^ „,intain the insulation
: Adjacent layers and sections o we- ^"^^f;,.,,^ie.,ovcred silk wire is
. of the secondary coil at the highest aeg ,
1 used as an extra precaution ■ desij^ned so that it
T--^ interrupter consists of a small eiecrr
»xtra heavy, and arranged so that
— — 4-v.iq coil are of soUa
andjl^vir iEi^L electrical papefs
years^^J had
skill and i.- i-"^^ * coi
of
cc national
a] Tropp
^- HUERSTEL.
have [^M^PP^ '^'"^no
judgm
.this work,
are sepa-
^cross.
■quires
ex-
d «;el kindly ano^^y^- Huer.
, mensions of thk L ^'
to
, be ialcl.^'^M-^^ • .
about 450 lbs • ,hi ^^'^bs
consists of No. 6 r*^^-^
,^he secondary cnnf • *
coil
S.
ICQ
finecoL^Mr JT''^- ^^e
obtaining a woHw ^^^''^^^ ^''e
tation. HeTr T'^«'"«P"-
^-"ff«meto;U:^'^«dfora
TROPP.
SFABK COILS,
111
The Secondary,
The secondary consists of 12 lbs. No. 36 silk-covered
copper wire, wound on in 96 sections or flat rings. It is
laid on in a curved fashion, as shown, so as to accord with
the direction +t^- i-- " - ^ ^ agnetised iron core.
^%-7ii in Fig. 99, from
^Dwards the
nduction is
erent sections
5^ 'H manner first
^ ^ 0) insist of four
;;^^i|sh of melted
^ » la ate, and then
g »D s: they should
' ' ze out any air
r
-< ;3 fne 4-ply sheets
c ihes in diameter,
^ lese being cut out
and the wire for the secondary got reaay to hand, we can now
pass on to
110 INDUCTION COILS AND COIL-MAKING.
thick. It can eithe r be purchaRftfi ^.s a. tiil tA nr^Tv^^^^^^ —
Large Rhumkorff Coll.
The special .2-inch Rhumkorff coil, made by Edwards & Co New
purpoL of reducing as much as possible the Dotent.nl H-ff.^-
adjacent layers a
of the secondary
used as an extra
cc natjonaJ
^^OUf 450 Jbs. .
^onsrsts of No. ,
/ ne secondary cnnf
^s. of No. 36 V '°°
'5o miles o>^-^ '
finecoiJsof S- L""^- '^^^e
obtaining a woMw "^'"^^^^ are
^°::?«'«etoS^"J^edfo?a
^- TROPP
SPABK COILS,
The Secondary,
The secondary consists of 12 lbs. No. 36 silk-covered
copper wire, wound on in 96 sections or flat rings. It is
laid on in a curved fashion, as shown, so as to accord with
the direction of the lines of force of the magnetised iron core.
The lines of force run somewhat as shown in Fig. 99, from
:owards the
V
3
3
M
^ O
<D >
^ a
la
^5
bX3'
«2
fl o
0) ;>,^
^ ^ s
S3-. Ill a
induction is
a ■
oi
y <i) d >i u
erent sections
o manner first
^ ^^2"^ I insist of four
^ 2 ^ I 1 5h of melted
. j:! Sri ^ ^te, and then
« ~S ^ ^ 5 they should
p sZ^^ any air
E
H
I j-3 % fh.e 4-ply sheets
.2 '-^jhes in diameter,
-lese being cut out
and the wire for the secondary got ready to hand, we can now
pass on to
110 INDUCTION COIL 8 AND COIL-MAKING.
thick. It can either__be_jiirchased^ oomT^lp+o
Large Rhumkorff Coil.
The special
York, possess^
ondary windin
purpose of red
adjacent layer
of the seconda
used as an ev^
1
1
1
t
L
t^
ii
tl
ta
in
G. HI
of
cc national
a] Tropp a
1 celJent qi
^ steJ kind]
^ niensions ^
r *o be ta '
about 450
consists of
The seconda..
lbs. of No. 36^
than 150 miles,
"ne coils of Mr n . , ""^
obtaining a ii^^? ""'^
Nation. HeT. r^^'^P"-
n
B. TROPp.
SPABK COILS,
111
The Secondary,
The secondary consists of 12 lbs. No. 36 silk-covered
copper wire, wound on in 96 sections or flat rings. It is
laid on in a curved fashion, as shown, so as to accord with
the direction of the lines of force of the magnetised iron core.
The lines of force run somewhat as shown in Fig. 99, from
which it will be seen that by winding more towards the
centre of the magnet the greatest amount of induction is
obtained. The insulating discs between the different sections
are formed of paraffined paper, after the manner first
employed by " Inductorium" in 1870. They consist of four
layers of some unglazed paper placed in a dish of melted
paraffin wax, and allowed to thoroughly saturate, and then
removed to drain and cool. While in the wax they should
be pressed with the end of a glass rod to squeeze out any air
bubbles that may be between them. Out of the 4-ply sheets
so formed must be cut 192 circular discs, 7 J inches in diameter,
and having a 2f-inch hole in the centre. These being cut out
and the wire for the secondary got ready to hand, we can now
pass on to
Fig. 99. — magnetic field of coil.
112 INDUCTION COILS AND COIL-MAKING.
Winding the Sections.
To wind the sections a section- winding apparatus, as
shown in Fig. 100, will be required. This consists of a
spindle a, carrying at the left-hand end a fixed metal disc h,
7 J inches in diameter, and having opposite it a similar one c,
which is movable and can be clamped up to it by the nut c?.
Between the two discs is a movable collar, of which three
Fig. 100. — section-winder.
sizes should be made, each slightly larger than the pre-
ceding one. Thus, when one of these collars is placed
between the two discs and the nut run up, a thin flat space
is formed which, if wound up with wire, will form the wire
into a thin flat ring with a hole in the centre, this hole being
of sufficient size to allow the ring to slip over the ebonite
tube. It is in this manner the sections are formed and
slipped on one after the other and joined up together. The
SPARK COILS,
113
spindle is rotated by means of a band between the pulleys
/ and g.
To wind the sections, first the coil of wire which is to feed
the section-winder a, must be fixed on some such stand as
shown at c in Fig. 101. From this the wire passes to the
tin dish 6, filled with paraffin wax, kept hot by the spirit lamp
seen beneath. The wire passes under a glass rod fixed in
the centre of the dish so as to cause the wire to pass through
the wax and become thoroughly impregnated with it. From
Fig. 101. — WINDING the sections.
the rod it passes direct to the section- winder, which is rotated
by the handle seen on the right-hand side, the wire being
guided on by letting it slide through a small piece of rag at
a point midway between the dish and the winder. It will
be noticed on looking at Fig. 98 that the sections as they
approach the bobbin-cheeks have a wider opening in the
centre, the space between the inside of the section and the
tube being filled in with paraffin wax. The reason of this is
because the tension of the secondary coil is greatest at its
1
114 INDUCTION COILS AND COIL-MAKING,
ends, and extra precautions are thus necessary at these points
to prevent the thickness of the tube being pierced by a spark.
It is to form these larger openings in some of the sections
that the three different size loose collars for the section winder
are required.
Before winding the sections it is well to pencil on the
illustration the number of each section, starting, let us say,
from the left-hand bobbin -cheek, so that we can mark each
section as it is wound with the number of the section to which
it corresponds. We then remove the top spindle of section-
winder and insert a collar that will give us the required
diameter of opening for that particular section we are going
to wind, and bolt the whole firmly together by the nut. The
movable collar prevents the two discs going up close and the
thickness of the collar is such that when screwed up together
the distance between the inside edges of the metal discs is just
under ^ih. of an inch. The end of the wire is then passed
through a hole in the disc and the space between the discs
wound up to the required diameter shown for that disc. The
handle can be turned as rapidly as possible, consistent with
feeding the wire on without breaking it, as it is unnecessary,
even if it were possible, to wind on the wire in even layers ; it
must be allowed to find its own level. As soon as the section
is wound the top spindle must be removed from the winder
and the disc arranged so as to be in a horizontal position.
The nut is then removed and the top metal disc carefully
lifted off It will now be possible with a knife to detach the
wound section from the winder and lay it fiat on the table,
the whole process being done very carefully and gently.
The section will then present the appearance of a flat ring
stuck together by reason of the paraffin wax which still
adhered to the wire when wound on. The inside end of the
section protrudes from the hole in the centre while the outside
end passes up over the top. As each section is removed from
the winder one of the paraffined-paper discs is placed each side
BPAEK COILS.
115
and smootlied on to it by ironing it with a warm flat iron ;
when one side is done, the section being turned over and
similarly ironed on the other side.
In this manner the whole 96 sections are wound, winding
those required for the centre portion of the secondary fuller,
as in the illustration. It is not necessary to alter the size of
each section quite so gradually as shown in the drawing ; if
changed every sixth section it will be less tedious and just as
efficient. When the sections are all completed they must
be joined together in pairs, the two inside ends being con-
nected as shown in Fig. 92. This will cause each pair to
form a continuous spiral, so that when all the pairs are put
on the bobbin the winding will be in the same direction
throughout its length. Great care must be exercised in
joining up these pairs not to join an inside end to an outside,
otherwise the two sections will oppose one another. A
glance at Fig. 92 will make this clear. Care must also be
taken to join such pairs as will make the secondary, when all
the sections are put on, have the form shown in Fig. 98. In
joining the sections in pairs, first lay one section on the
table and carefully place another one on the top of it.
Having cut off all superfluous wire from the two inside ends,
carefully scrape and solder them together with a small
soldering iron, using resin as a flux. Make as thin and small
a joint as possible, and when soldered it must be carefully
pushed just between the insulating paraffined paper discs
and covered with paraffin. A warm iron should then be
taken and the two sections gently smoothed together. After
all the sections have been joined in pairs each pair of sections
should be separately tested by sliding them one at a time
over the ebonite tube containing the primary, and comparing
the spark obtained from each pair when a current is sent
through the primary and the circuit is made and broken by
hand. Any pair of sections that fails to give a spark up to
the average obtained from the rest must be inspected, and if
I 2
116 INDUCTION COILS AND COIL-MAKING.
not able to be put right a new pair must be wound to re-
place it. This test should be very carefully applied as it will
quickly find out any faulty or improperly connected sections.
Presuming we have tested all the sections and obtained
good results we can pass on to
Completing the Secondanj.
Having arranged the pairs of sections (there will be 48
pairs in all) on the table in the proper order they should go
on, support the primary and tube in a vertical position so
that it rests on the left-hand ebonite bobbin-cheek, which
must be secured in its correct place along the tube by a
little shellac varnish. The right-hand bobbin-cheek is not,
of course, put on until all the sections have been passed over
the tube. The tube being properly supported in a vertical
position, the pairs of sections are then slipped on one after the
other in their correct order, as shown in Fig. 98. As each
pair of sections are put on they must be arranged centrally
over the tube and the space between the tube and inside of
the section run in with hot paraffin wax until quite full, when
it must be allowed to thoroughly set. The wax keeps the
sections centrally on the tube and makes an additional thick-
ness of insulation between the primary and secondary,
though its main object is to prevent the secondary from
sparking from section to section along the surface of
the tube by forming a continuous layer of paraffin wax.
The wax for this purpose must be very hot, and carefully
poured in so as to cause it to penetrate into every little
ci ack. When one pair of sections is cool and set, another
must be added, pressed well down against the underneath
pair, and served in the same manner, and so on until all are
put on, after which the bobbin-cheek is put on and secured in
its place.
The outside ends of all the sections must then be joined
SPARK COILS.
117
together, soldered, and the joints pushed down into one of the
sections, previously slitting down, however, for an eighth
of an inch with a sharp knife, the dividing disc between
the two sections to be joined so as to allow the wire to pass.
When all the wires are carefully joined up, the coil must be
laid on its side and a complete wrapping of thick paraffined
paper firmly tied round outside the secondary, the edges of
the paper not meeting by a quarter of an inch, but fitting
nicely between the bobbin-cheeks. Melted paraffin wax must
then be poured in along the slot till all the vacant space
above the wire in each section is filled, and a solid jacket of
paraffin wax thus cast over the secondary. This must first
be allowed to thoroughly cool and harden, after which a
wrapping or two of paraffin-paper may be put round as a
finishing layer. The surface of this may be finished ofi"
either with a wrapping of thin sheet ebonite or a layer of
black thread served afterwards with a coat of varnish.
The Contact- Br eaher.
This is of the improved form devised by Mr. Apps, and
used in his well-known coils. It consists of the upright
spring b, carrying the soft iron hammer-head a, which works
through the opening in the end of the tube as shown. This
spring is screwed to a massive brass foot i, which in turn is
bolted to the top portion of the wood base by the nut and
bolt shown. Behind the spring is the upright standard c,
carrying the heavy contact-screw e, and back-nut d. The
front of the contact-screw has a substantial platinum point
which makes contact with the platinum point on the back of
the hammer-head. Through a bone bush g, in the bottom
of the standard, passes a brass rod fitted at one end with the
milled ebonite disc h, and at the other working in a threaded
hole in the foot i. It will thus be seen that according to the
direction in which the disc h is turned, so the spring and
118 INDUCTION COILS AND COIL-MAKING,
liammer-liead are forced towards or away from the contact-
screw. The action of the contact-breaker is as follows : —
The greater the pressure there exists between the hammer-
head and the contact-screw the more completely must the
iron core of the coil be magnetised before it has sufficient
power to attract it and thus break the circuit. Moreover, as
soon as it is thoroughly magnetised the circuit is very
suddenly broken. By varying the pressure of the spring by
means of the disc ^, it will be seen that the interruptions
can be accomplished either when the core is slightly mag-
netised or very strongly magnetised only. To get the best
results from a spark coil it is necessary that the core be
fully magnetised before the circuit is broken, and when
broken it should be done instantly. This form of contact-
breaker allows, it will be seen, a ready adjustment to this
end. It is not an easy matter with an automatic contact-
breaker to get the best results from a coil of this size ; in fact
if the object is to get the longest possible sparks from the
coil, it will be found advantageous to rig up a temporary
hand-break for the occasion and cut out the automatic break,
which can easily be done by screwing up the contact-screw.
The Condenser.
The condenser, which is contained within the base (though
not shown in the figure), consists of 60 sheets of tin foil,
12 inches by 8, interleaved with sheets of paraffined paper as
previously described ; the whole is afterwards well pressed
together and clamped between two thin boards. The con-
nections to the two parts of the contact-breaker from the
condenser consist of sheet foil rolled up into a strip of six
thicknesses, soldered at one end to the foil and clamped at
the other underneath the nuts of the contact-breaker. For
those who like to ascertain for themselves experimentally
the best size condenser for the coil, this can easily be done
SPARK COILS.
119
by gradually adding more sheets to the condenser while the
coil is working, watching the spark at the terminals of the
secondary until a point is reached where the addition of
more sheets is not accompanied by any increase in the length
of the spark. It is preferable in most instances when
making a coil, to find out exjperimentally, as described above,
the best size of condenser to employ, taking care that the
normal battery-power that will be employed is used.
Finishing off the Coil.
All being completed the final mounting up may now be
done, and finishing touches given the coil. The bobbin is
placed in its supports and the supports screwed to the top
part of the base by four substantial screws. The contact-
breaker must then be correctly mounted in its place, the
condenser inserted inside the base and connected up, after
which the bottom part of the base is screwed down. The
primary terminals must be fixed at the most convenient
place, which will be found to be on the right-hand side of
the contact-breaker, looking at the coil from the contact-
breaker end. A commutator or current-reverser will be
found a great convenience, and if it is desired to add one
this can be placed close to the primary terminals. The
bobbin is not placed quite centrally of the base, but slightly
to one side so as to make room for the two ebonite dis-
charging pillars, on the top of which are mounted the
secondary terminals after the manner shown in Fig. 96.
The wires from the secondary to the terminals on the pillars
hang, it will be seen, in mid-air, but are enclosed in a piece
of indiarubber tube, the inside of which is run in with paraffin
wax, care being taken to see that a good union is made with
the wax on the coil.
With two or three large cells (Bunsen, Grove, chromic
acid, Edison-Lalande or accumulators) giving about 12
120 INDUCTION COILS AND COIL-MAKING.
amperes in the primary circuit, a spark 12 inches in length
is easily to be obtained from this coil. Should good results
not be obtained with the automatic contact-breaker after re-
peatedly trying to adjust it, a hand make-and-break should
be temporarily substituted to ascertain whether the fault is
with the contact-breaker or not. If sparks of the full length
are then obtained it is evident the contact-breaker is at fault,
which must be carefully inspected and altered.
Ajpjp's noted SparJc Coils,
The palm for constructing large spark coils must un-
doubtedly be awarded to Mr. Alfred Apps, whose mag-
nificent productions cannot fail to elicit the admiration of
all who have had the pleasure of seeing these monsters at
work. His most notable production is the famous Spottis-
woode coil, of which an illustration and description are given
on p. 123.
Another coil, also equally well known, and perhaps one
which many of my readers may have had an opportunity of
seeing, if not in the vigour of its youth no doubt in its
old age, when its sparking capabilities had somewhat fallen
off owing to a slight break-down in its insulation, is the
Polytechnic one. This coil, although it gave a magnificent
spark of 29 inches, sinks into insignificance beside the
bpottiswoode, which produced sparks 42 inches long.
Mr. Apps' method is to wind his secondary sectionally,
dividing it into a large number of small sections and fre-
quently to separate these into four large sections, as in the
case of the Spottiswoode coil. The primary is placed inside
a massive ebonite tube (J inch thick in the Polytechnic coil)
and the insulating discs between are also of ebonite. The
core and primary project beyond the ends of the secondary
while the tube projects beyond the core so as to completely
enclose it after the manner described for the coil on p. 109,
SPARK COILS. 121
Much of Mr. Apps' success in coil constructing, however, is
no doubt due to the skill he has acquired and large ex[)erience
he has gained during ■ the years he has applied himself so
assiduously to the production of these monster coils.
The Polytechnic Coil,
The core of this wonderful coil was composed of No. 16
iron wire, its total length being 5 feet, its diameter 4 inches
and weight 123 lbs. The primary consisted of 600 turns
(3770 yards) of '095 copper wire and had a resistance of
2*2 ohms. This primary was enclosed in an ebonite tube
8 feet long and ^ inch thick, centrally of which was placed
the secondary, consisting of 150 miles (606 lbs.) of -014
silk-covered copper wire, the whole secondary occupying
a space along the tube of 54 inches. When completed the
secondary bobbin was 2 feet in diameter, and 4 feet 10
inches in length, over all dimensions. With a battery of
40 large Bunsen cells the coil gave secondary sparks 29
inches long and would pierce blocks of glass 5 inches in
thickness.
Eeferring to the break-down of this coil Mr. Paul Ward,
who, under Mr. Apps' personal directions, had the con-
struction of it entrusted to him, sent to the English Mechanic
in 1886, in answer to the enquiries of a correspondent, the
following interesting particulars : —
" The Polytechnic coil broke down twice, but from no
fault in the principle of construction. The first of these
failures (and this is, I think, the only time that the causes
have been made public, and I am glad to afiford this infor-
mation in justice to its designer) was due to some experi-
ments made by Professor Pepper, in using a too small part
of the condenser, which was made in ten sections. The
extra spark, or induced current in the primary itself could
122 INDUCTION COILS AND COIL-MAKING,
not find sufficient capacity in the small condenser, at that
moment being used, and it found circuit by breaking from
one layer of the primary to another. This soon got worse
the more the coil was used, and eventually there was a
complete short circuit in the primary, caused by melted
spicula of copper bridging across the two contiguous layers.
This was repaired, and the coil worked as well as ever.
The second break-down was of a more serious nature, and
consisted in the piercing of the primary tube. It must be
remembered that this huge masterpiece weighed nearly a
ton, and it required special means and intelligent adminis-
tration in moving such a monster. But owing, I am sorry
to say, to the parsimony of the directors of the Polytechnic,
they did not avail themselves of these two most necessary
elements to success. They elected to move it themselves,
and, owing to an enormous undue strain being put on the
primary tube in so doing, it was either cracked incipiently
or so weakened that when they separated the terminals a
certain distance, through went a spark. This time, how-
ever, the damage was repaired by Mr. Apps, which, I am
sure, every one who knows anything of the almost insur-
mountable difficulties of withdrawing the old tube and
putting in a new one without damaging the ponderous and
delicate secondary, will admit was a triumph of mechanical
skill. The last time I saw the coil was in the Loan Collec-
tion at South Kensington. With regard to the proportion
of length of spark to the length of body, this is no matter
for surprise, considering that the thickness of the secondary
was not of that diameter to get the maximum length of
spark. All who witnessed the thickness of the secondary
flame, for flame it really was, could not fail to be impressed
by the enormous quantity ; and in support of this I may add
that three sparks from it were sufficient to charge twelve
9-gallon Leyden jars to saturation."
SPARK COILS.
123
The Sjpoitiswoode Coil.
In the Philosophical Magazine of January 1887 there
appeared this well-known description of Apps' monster coil
by the owner himself, the late Mr. Spottiswoode, of Seven-
oaks, which cannot be better reproduced than in his own
words.
" The general appearance of the instrument is represented
in Fig. 102, from which it is seen that the coil is supported
by two massive pillars of wood, sheathed with gutta-percha
and filled in towards their upper extremities with paraffin
wax. Besides these two main supports, a third, capable of
being raised or lowered by means of a screw, is placed in
the centre, in order to prevent any bending of the great
124 INDUCTION COILS AND COIL-MAKINO.
superincumbent mass. The whole stands on a mahogany-
frame resting on casters.
The coil is furnished with two primaries, either of
which may be used at pleasure. Either may be replaced by
the other by two men in the course of a few minutes. The
one to be used for long sparks, and indeed for most experi-
ments, has a core consisting of a bundle of iron wires, each
•032 inches thick, and forming together a solid cylinder
44 inches in length and 3*5625 inches in diameter, has a
conductivity of 93 per cent., and offers a total resistance of
2*3 ohms. It contains 1344 turns, wound singly in six
layers, has a total length of 42 inches, with an internal
diameter of 3*75 inches, and an external of 4*75 inches.
The total weight of this wire is 55 lbs.
" The other primary, which is intended to be used with
batteries of greater surface, e.g. for the production of short
thick sparks, or for spectroscopic purposes, has a core of iron
wires -032 inches thick, forming a solid cylinder 44 inches
long and 3 • 8125 inches in diameter. The weight of this core
is 92 lbs. The copper wire is similar to that in the primary
first described, but it consists of 504 yards wound in double
strand, forming three pairs of layers whose resistances are
•181, '211 ohm respectively. Its length is 42 inches, its
external diameter 5 • 5 inches, and its internal 4 inches. Its
weight is 84 lbs. By a somewhat novel arrangement these
three layers may be used either in series as a wire of •192-
inch thickness, or coupled together in threes as one of •576-
inch thickness. It should, however, be added that, owing
to the enormous strength of current which this is capable of
carrying, and to the highly insulated secondary coil being
possibly overcharged so as to fuse the wire, this larger
primary is best adapted for use with secondary condensers of
large surface, for spectrum-analysis, and for experiments
with vacuum tubes in which it is desirable to produce a
great volume of light of high intensity, as well as of long
SPARK COILS,
duration at a single discharge. The alternate discharges
and flaming sparks can also be best produced by this primary.
It has been used for high-tension sparks to 34 inches in air,
the battery being 10 cells of Grove's, witb platinum plates
6i inches by 3 inches. Great facilities for the use of dif-
ferent sets of batteries are afforded by the division of this
primary into three separate circuits, to be used together or
separately, and by a suitable arrangement of automatic
contact-breakers the primary currents may be made to follow-
in a certain order as to time, duration and strength, with
effects which, when observed in the revolving mirror, will
doubtless lead to important results in the study of striae in
vacuum tubes.
" The secondary consists of no less than 280 miles of wire,
forming a cylinder 37*5 inches in length, 20 inches in ex-
ternal and 9 • 5 inches in internal diameter. Its conductivity
is 94 per cent., and its total resistance is equal to 110,200
ohms. The whole is wound in four sections, the diameter
of the wire used for the two central sections being '0095
inch, and those of the two external being *0015 inch and
•0110 inch respectively. The object of the increased thick-
ness towards the extremities of the coil was to provide for
the accumulated charge which that portion of the wire has
to carry.
" Each of these sections was wound in flat discs, and the
average number of layers in each disc is about 200, varying,
however, with the different sizes of wire, &c. The total
number of turns in the secondary is 341,850. The great
length of wire necessary can be easily understood from the
fact that near the exterior diameter of the coil a single
t'trn exceeds 5 feet in length. The spark, it is believed, is
due to the number of turns of wire rather than to its len2:th,
suitable insulation being preserved throughout the entire
length. In order to en.Nure success the layers were care-
fully tested separately and then in sets, and the results
126 INDUCTION COILS AND COIL-MAKING.
noted for comparison. In this way it was hoped that, step
by step, safe progress would be made. As an extreme test
as many as 70 cells of Grove's have been used with no damage
whatever to the insulation.
" The condenser required for this coil proved to be much
smaller than might at first have been expected. After a
variety of experiments it appeared that the most suitable
size was that usually employed by the same maker with
a 10-inch spark coil, viz. 126 sheets of tin foil, 18 inches
by 8*25 inches in surface, separated by two thicknesses
measuring -Oil inch. The whole contains 252 sheets of
paper, 19 inches by 9 inches in surface.
" Using the smaller primary this coil gave, with five quart
cells of Grove's, a spark of 28 inches, with 10 similar cells,
one of 35 inches, and with 30 such cells one of 37*5 inches,
and, subsequently, one of 42 inches. As these sparks were
obtained without difficulty, it appears not improbable that
if the insulation of the ends of the secondary were carried
further than at present a still longer spark might be
obtained. But special adaptations would be required for
such an experiment, the spark of 42 inches already so much
exceeding the length of the secondary coil.
" When the discharging points are placed about an inch
apart, a flowing discharge is obtained both at making and
at breaking the primary circuit. The sound which accom-
panies this discharge implies that it is intermittent, the
time and current spaces of which have not as yet been
determined.
"With a 28-inch spark, produced by five quart cells, a
block of fiint glass 3 inches in thickness was in some
instances pierced, in others both pierced and fractured, the
fractured pieces being invariably flint glass. If we may
estimate from this result, the 42-inch spark would be
capable of piercing a block 6 inches in thickness.
" When used for vacuum tubes this coil gives illumination
SPARK COILS.
127
of extreme brilliancy and very long duration; with 20 to
30 cells and a slow-working mercury break, giving, say,
80 sparks per minute, the striae last long enough for their
forward and backward motion to be perceived directly by
the unassisted eye. The appearance of the striae when
observed in a revolving mirror was unprecedently vivid,
and this even when only two or three cells were employed.
" Further experiments have shown that with such large
coils only the newly-discovered effects of very high tem-
perature combustion or volatilisation can be produced. On
exciting the primary of the coil with a suitable dynamo-
electric machine or battery, and using a large Leyden jar
in the secondary circuit (according to Sir William Grove's
experiment), the electrical discharge passing between elec-
trodes placed before the slit of the spectroscope, lines and
bands may be observed to advance and recede according to
the variations made in the magnitude of the exciting dis-
charges. As the atmospheric pressure may be assumed to
remain constant, these effects are probably due to differences
of temperature arising from the action of a greater or smaller
extent of electrical effects on the electrodes in a given time."
In the English Mechanic of 1886, Mr. Paul Ward gives a
description of some further effects produced by this coil as
follows : —
" In conducting some experiments for the late Mr. Spottis-
woode on ' The Electric Discharge in Vacuo,' it occurred
simultaneously to that gentleman and myself that it would
be possible to work an induction coil without a contact-
breaker and without a condenser by means of a powerful
alternating current sent through the primary of such an in-
strument. This we found to be possible, and the effects
obtained were truly prodigious.
" The equality of discharge between the poles of the secon-
dary was remarkable, as evidenced by placing a vacuum
tube, giving well-developed striae, in the secondary circuit.
128 INDUCTION COILS AND COIL-MAKING.
The appearance was one of great beauty, the tube in question
being filled with a double set of pearl-like striae of 2 inches
diameter, devoid of all motion, for they were as steady as a
rock. This was caused by the rapidly alternating discharges
from the secondary, and was very fine as a spectacle ....
It may be of some interest to your readers that I quote
from Mr. Spottiswoode's description of the appearance pro-
duced by the excitation of an induction coil by an alternate
current, sent through the primary of such an instrument.
" ' The spark from this machine (a coil excited by a De
Meritens machine) presented an unusually thick yellow
flame, and it was accompanied by a hissing noise, different
from that commonly heard with a coil excited by a battery.
As the machine gives alternate currents, the secondary dis-
charge presents sparks of equal length in both directions,
and the general appearance to the eye is symmetrical in
respect of both terminals. The spark was observed in a
revolving mirror, first in a vertical, and secondly in a
horizontal direction ; the discharge, although apparently
continuous, was immediately seen to be intermittent, with
a period in unison with that of the exciting machine.
Tongues of flame, leading alternately from one terminal and
from the other, crossed the field of view. . . . When the
length was increased to 2 inches, flashes or bands of con-
tinuous light were seen to traverse the field of view in
diagonals of low slope (i. e. nearly horizontal), showing that
there were masses of heated material passing from time to
time at a moderate velocity between the terminals.
" * From the known period of the machine, and the number
of discharges crossed by these flashes in their passage from
terminal to terminal, it was calculated that the time of
passag^e was about '03 of a second. Occasionally there was
a still brighter flash of meteor, which similarly traversed
the field, but with a velocity apparently of about double that
of the others.'
SPABK COILS.
129
" These rapidly alternating discharges," Mr. Ward con-
tinues, " though very fine as a spectacle, were unsuited for
the investigation of the laws which Mr. Spottiswoode estab-
lished relative to the nature and behaviour of a discharge
in a rarefied atmosphere. The two sets bothered us. We
wanted only one set, and I was asked to devise an instrument
for stopping, or shunting off*, one of these alternate sets of
currents."
An ingenious piece of apparatus Mr. Ward devised for this
purpose, to which he gave the name of " Electric Valve,"
and the writer's only regret is that space will not permit its
reproduction here. Those of my readers, however, who wish
for a full description and illustrations of it are referred to the
English Mechanic of March 26, 1886.
130 INDUCTION COILS AND COIL-MAKING.
CHAPTER VL
EXPERIMENTS WITH SPARK COILS.
Perhaps no other atnnsement for a winter's evening is so
much enjoyed by the amateur coil-maker as that of experi-
menting with his newly-constructed coil. A large number
of experiments are capable of being performed with a good
spark coil, while the performing of these experiments and
endeavouring to find out fresh ones is a pastime of great
interest. In carrying out these experiments the coil should
not be less than f-inch spark, while if it is 1 J, 2 or 3 inches
the better will be the performance of the experiments and
the greater the interest attached to them. Of course these
large spark coils must not be handled carelessly, and great
caution must be exercised by the operator not to get included
in the path of the discharge, otherwise unpleasant and
perhaps fatal results may occur. The primary circuit
should always be interrupted when obliged to adjust in the
neighbourhood of the secondary terminals, and all adjust-
ments that must be made while the coil is in operation
should be done by means of a glass rod held in the hand.
If the coil has not a discharger already fixed on the base it
will be advisable to make one, as shown in Fig. 29, p. 41,
as the experiments can by this means be more conveniently
performed.
On starting the contact-breaker and adjusting the one
point of the discharger towards the other, a rapid flow of
sparks will take place between the points, the spark being
EXPERIMENTS WITH SPARK COILS. 131
short, thick and reddish when the points are close together,
and thin, bhiish-white and less frequent when far apart.
The red spark, when the two points are close together, is
the spark most suitable for igniting such substances as it ig
possible to ignite with a coil.
Vacuum Tubes.
Experiments with vacuum tubes are those most frequently
carried out with spark coils and are certainly the most
beautiful. Vacuum tubes, first devised by Giessler, of
Bonn, are thin glass tubes variously shaped and provided
Figs. 103, 104. — vacuum tubes.
with a metal connection at each end, which connections pro-
ject a short way into the tube. The tubes are then partially
exhausted or filled with different gases and hermetically
sealed. On connecting up these tubes to the ends of the
secondary coil and starting the contact-breaker a beautiful
discharge takes place, completely filling the tube with a
luminous glow. The tubes are not completely exhausted,
as the spark will not pass in a vacuum, any more than it
would if the tube was filled with air ; it is necessary to have
a rarefied medium of some description.
Figs. 103 and 104 show two of the more common forms
of vacuum tubes which, it will readily be understood, must
K 2
132 INDUCTION COILS AND COIL-MAKING.
be carefully handled to avoid breakage. A very convenient
method in which they can be connected to the coil is shown
in Fig. 105. The colour of the luminous effects obtained is
varied both by filling the tubes with different
gases and also by using glass with metallic oxides
in its composition. Thus nitrogen gas inside the
Fig. 107.
tube will give a rose colour, while if the glass has
oxide of uranium in it a bright green colour
will be the result. Compound vacuum tubes
are tubes which, owing to the intricacy of their
design, are extremely fragile and are therefore enclosed in a
second outer tube of glass, as in Figs. 106 and 107, which
Fig. 106.
Fig. 108.— compound vacuum tube. Fig. 109. — ^vacuum tube rotator.
show the vertical and horizontal form. The horizontal form
shown in Fig. 107, when it has letters inside, is known as a
EXPERIMENTS WITH SPARK COILS.
133
motto tube. Tubes are also made of the shape shown in
Fig. 108, when they are known as (J -shape or double-branch
tubes. A very fine efiect can be obtained by placing
in a vacuum rotator any of the simple form of tubes and
lighting them up while they are turning round, thus
giving the appearance of a wheel of flame. These vacuum
tube rotators are made to be operated either by hand or a
small electro-motor, one of this latter form being shown in
Fig. 109.
Altering Length and Character of the Sparh^
The length and character of the spark obtained between
the ends of the secondary can be influenced in a great
number of ways.
Hold a lighted match or candle just below the spark and
it will be found that the length of the spark can be enor-
mously increased by widening the gap, while, moreover, the
spark will assume a deep violet colour. The increased
length of spark thus obtained is due to the heated air being
a better conductor.
Again, place the candle on one side of the sparks and it
will be found that sparks will be diverted slightly out of
their course in order to pass through the flame.
Place the candle right between the points of the discharger
so that the sparks must pass through the flame and the
sparks will become of vivid brightness. Kaise the candle
slightly so that the sparks pass round the wick and they will
become of a blue character. Blacken the points of the dis-
charger so that they are covered with lamp-black, and on
bringing the points close together the black points will be
rendered incandescent.
The spark is easily pushed out of its course, as it were, by
any non-conductor, as may be seen as follows : — Separate the
points of the discharger to a medium distance, and then,
while the sparks ar@ passing slowly, insert the edge of a
134 INDUCTION COILS AND COIL-MAKING.
slieet of glass or piece of bone, wood, &c., and the sparks will
be seen to bend round the edge of the glass. If a piece of
paper is inserted the spark will be seen to pierce the paper,
and on holding the punctured part up to a strong light there
will be seen to be two holes side by side as if there had been
a double spark.
Action of Sparhs on Metal Filings^ dc.
Grind down to a fine powder some mixed steel and copper
filings and place them on a piece of wood or ebonite on
which the two points of the discharger are resting. The
sparks will then be seen to pass in all directions, fusing
particles of the metal here and there and acquiring a red
colour. Clear away the filings and substitute some fine
non-conducting powder such as crystallised gallic acid, and
as soon as the sparks commence the particles of powder will
be seen to be in motion. The particles will seem as though
blown away from one point of the discharger until a division
is made across the mass of powder to the other point.
Ignition Experiments,
Hydrogen gas can easily be ignited by the spark, as can
easily be seen by arranging the two points of the discharger
so that the sparks pass across the top of a gas-burner. 1 ^^>Hi .
Gun-cotton, gunpowder, lycopodium and phosphorus can
also similarly be ignited by the sparks, to do which they
must be placed on a glass plate or marble slab and the ends
of the discharger brought rather close together. The gun-
powder must be finely ground for this experiment and the
ycopodium dusted on some cotton wool.
A fuse, such as is used for firing charges of gunpowder or
gun-cotton, can easily be made as follows: — Make a smaL'
paper tube just large enough to slip over the ends of two
gutta-percha-covered wires, twisting the one round the other,
EXPERIMENTS WITH SPABK COILS. 135
and the ends of the wire being bared of their insulation for
-jig-th of an inch. Bend these bared ends so that they come
within |th of an inch of one another, and then fill in the
tube with fin-e gunpowder, taking care that it falls in well
between the points of the wires. The end of the fuse must
then be closed with a little sealing-wax, taking the pre-
caution, however, of inserting a wad of paper first. If the
ends of the secondary are connected to the two free ends of
the wire the fuse will go off with a loud report and fire any
gunpowder or gun-C(;tton in which it is placed. Gun-cotton
can of course be used in the fuse instead of gunpowder, or a
mixture of chlorate of potash, phosphide of copper and sul-
phide of copper, as in Abel's fuse.
Experiments with Water and other Fluids.
Sparks can be made to pass through water by taking two
gutta-percha-covered wires, as explained for the fuse, and
twisting them together so that the ends are ^ inch apart.
Plunge these ends into a glass of water, the other ends being
connected on to the discharger, and the spark will be seen
still to pass between the ends of the wire.
If one of the wires from the discharger be placed in a
glass of water and the other brought close to the surface,
sparks will be seen to pass between the point of the dis-
charger and the water. If a drop of water is placed on a
slab of marble or sheet of glass and the points of the dis-
charger brought close on each side of it the sparks will be
seen to pass over or round the edge of the water. Substitute
a drop of oil for the water and the oil will quickly appear to
work into a state of effervescence. If the drop of water be
placed on an ebonite plate and smeared over its surface, when
the two points of the discharger are made to touch the wetted
surface the spark will assume a twisted shape and be capable
of being made very much longer.
136 INDUCTION COILS AND COIL-MAKING.
Detonating Plane and Ley den Jars,
Leyden jars can readily be charged by a spark coil, to do
which one point of the discharger must be placed in contact
with the outer covering of the jar while the other is directed
towards the brass nob and sparks allowed to pass. In place
of a Leyden jar a detonating plane can be employed, which
consists of a sheet of good glass, 12 to 18 inches square, on
both sides of which are stuck, by means of some shellac
varnish, a sheet of tin foil, leaving a margin of ^ inch of glass
all round. When one point of the discharger is placed
against one sheet of tin foil and the other against the oppo-
site one, a bright spark will pass from one sheet to the other,
accompanied by a loud report as soon as the sheets become
sufficiently charged.
137
CHAPTER VII.
BATTERIES FOR COIL WORKING.
For working induction coils tlie most suitable batteries are
the Bunsen, bichromate, Edison-Lalande, Leclanche and dry
batteries. The most important points in an ideal battery
for coil working are constancy, small space, light weight and
cleanliness, but since no battery fulfils all these conditions
we must select from those above the form best suited to the
particular work we have in hand.
Batteries for Medical Coils and Galvanisation,
The batteries best suited for this purpose are dry batteries,
Leclanche, bichromate and the Edison-Lalande.
Dry Batteries,
These are chiefly employed for portable sets where, of
course, a battery that is free from the slopping and spilling
of acids is a great desideratum.
Of course it is now generally understood that all so-called
dry batteries are in reality moist, and dry only in comparison
with batteries in which liquids are freely used. The exciting
composition is in the form of a paste or jelly, the viscidity of
which varies with the temperature, and once the composition
of this jelly became generally known, innumerable forms of
dry batteries sprang into existence. The various makes are
138 INDUCTION COILS AND COIL-MAKING.
almost identical in their construction, so one make only is
shown, viz. Burnley's cell, as made by the General Electric
Company, two forms of which are figured in Figs. 110 and 111.
The containing case is of zinc, and forms the one element,
while the other consists of a carbon plate, the intervening
space between the two elements being filled with the exciting
paste. The top of the cell is run in with melted pitch, and
Figs. 110, 111. — dry batteeie
the outside of the zinc case pasted round with paper. A
wire is soldered on to the zinc case as a connection from the
one pole, the carbon pole being bored and fitted with a
terminal, as shown. Since the outer case forms one of the
poles, it is necessary to see, when setting up a battery, that
the difi'erent cells do not touch one another. For this reason
it is usual to divide up the battery box by means of wood
partitions, so that a division is provided for each cell, though
BATTERIES FOR COIL WORKING. 139
some makers supply with their batteries a special cardboard
containing box, into which the cell is slipped, and thus kept,
not only from contact with its neighbours, but also with the
ground. Fig. 110 shows the square form of this cell, which is
made only in one size, viz., 6 by 4J by 2^ inches, and owing
to its shape occupies a minimum of space consistent with its
large power. Its internal resistance is '30 of an ohm, the
E. M. F. being, of course, 1*55 for all sizes. Fig. Ill shows
the round form, of which four sizes are made, No. 1 being 8
inches in height, 3^ in diameter, and has a resistance of • 35
of an ohm, while No. 2 is 7 inches high, 2 J in diameter, with
an internal resistance of • 70 of an ohm ; No. 3 is 5f inches
high, 2 inches in diameter, and has a resistance of • 85 ohms ;
and No. 4 is 4J inches high hj 1\ in diameter, and has a
resistance of about 1 ohm.
Dry batteries, when exhausted, may be recuperated by
passing a current of 1 to 2 amperes for an hour or so through
the [cell the reverse way, though the best way is to return
them to the manufacturers who will usually exchange them
for new ones at a slight charge.
Leclanche Batteries.
Leclanche batteries are now too well known to need any
description, save to point out which forms are most suitable
for coil working. The No. 2 size is the one most frequently
employed, and the agglomerate block (see Fig. 112) having a
lower resistance than the porous pot form, is capable of giving
the most powerful results. Whether porous pot or agglo-
merate block form be used, however, they should, if required
for a portable set, be sealed across the top to prevent spilling
the liquid.
The proper charge for the No. 2 size agglomerate cell is
3 oz. of sal-ammoniac, and water must be poured in up to the
blackened part of the glass jar. Some persons prefer to use a
140 INDUCTION COILS AND COIL^MAKING.
stronger solution, but this is not recommended, as, with a
saturated solution, trouble from a creeping of the salts arises.
For working large coils where the battery is only required
for short periods with long intervals, the six-block agglom-
erate-cell, as shown in Fig. 113, will be found very convenient.
It consists of a zinc cylinder, inside which is a grooved carbon
rod, each groove being fitted with manganese blocks, and the
Figs. 112, 113. — leclanche batteries.
blocks kept in contact with the rod by a wrapping of sack-
cloth and two elastic bands. The cell has a very low resist-
ance ( • 03 of an ohm) and once set up will last for years with
occasional use.
For a more detailed description of the Leclanche battery
the reader is referred to ' Practical Electric-Bell Fitting/
where a full description will be found.
BATTEBIES FOB COIL WOBKING. 141
Bichromate Batteries.
The bottle form of bichromate battery (Fig. 114) has long
been a favourite battery for working, not only small medical
and shock coils, but also for spark coils of somewhat large
dimensions. The zinc rod being capable of being quickly
and easily removed from the solution, which has thus not to
be poured out, makes the cell a very convenient one, while,
taking into account its high E. M. F. and low resistance com-
FlGS. 114, 115. — BICHROMATE BATTERIES.
pared to its small size, it is easy to see why, where some-
what strong currents are required, this cell is always selected.
These cells are made in three sizes, the half-pint, pint and
quart.
When the battery is required to occupy a small space, such
as when fitted in a portable set, the form of bichromate cell
generally used is shown in Fig. 115. It consists of a small,
square-shaped, glass containing jar, into the top of which
142 INDUCTION COILS AND COIL-MAKING.
are fitted the two carbon and one zinc plates, after the
manner employed in the bottle form, the zinc being similarly-
removable by lifting up the knob (10) of the brass rod.
The brass pieces 8 and 9 are the connecting pieces for the
coil, and swing round so that the hooked portions fit on to the
terminals. The square ebonite top of the cell is usually let
in flush with the woodwork of the case, so that only the top
is thus visible.
Bichromate batteries are now usually charged with a
chromic acid solution, the crystals for making which can be
purchased in tins and bottles, so that all that is necessary is
to dissolve the crystals in water and the solution is ready.
For a bichromate of potash solution the proportions are as
follows : —
Bichromate of potash 6 oz.
Water (hot) 2 pts.
Sulphuric acid .. 6 oz.
The E. M. F. of the bichromate battery is 1 • 9 volt.
The Edison-Lalande Battery,
It is only quite recently that the Edison-Lalande battery
has attracted much attention in this country. It is an im-
provement by that great inventor, Thomas Alva Edison, on
the Lalande-Chaperon battery, and has so many good points
that, when better known and its action understood, it must
come largely into use. Its most important points are : —
1. Great constancy.
2. Entire freedom from local action when on open circuit.
3. Low internal resistance.
The containing jar of the Edison-Lalande battery (see
Fig. 116) is of glass, or else white glazed porcelain, and the
elements employed are zinc as the positive, and black oxide
of copper (CuO) as the negative. The exciting liquid is
simply a solution of caustic potash. The oxide of copper is
BATTERIES FOB COIL WORKING, 143
obtained by the process of roasting copper-scale, the oxide
being afterwards ground into a fine powder and com-
pressed into solid blocks. From these blocks plates of a
suitable size for the different cells are cut, which are then
reduced] on the surface to form a good conductor. These
Fig. 116. — edison-lalande battery.
plates are suspended from the cover of the containing jar by
means of a light framework of copper, one end of this frame-
work carrying the terminal for the positive pole of the
battery. On each side of the copper element in the larger
type of cells (but on one side only in the smaller types) is
144 INDUCTION COILS AND COIL-MAKING.
suspended a rolled zinc plate. . The terminal, which is
attached to the zinc plate, has an ebonite or vulcanised fibre
extension yoke, both ends of which fit closely into the
grooved sides of that portion of the copper frame above the
cover, to which they are firmly bolted. This prevents any
movement in the relative position of the elements, and does
away with the necessity of using separators to prevent any
short circuits occurring between the elements in the solution.
The zincs are amalgamated, and, as in most batteries, the
zinc is attacked more vigorously near the top than at the
lower part of the plate. The zincs for this cell are made
slightly tapering, the thick part being uppermost.
The exciting liquid employed in the battery consists, in all
types, of 25 per cent, solution of caustic potash in water ; or,
in other words, of a solution of one pound of caustic potash
in three pounds of water. When the circuit is closed and
the cell is put into action, the water is decomposed and the
oxygen forms with the zinc oxide of zinc, which in turn
combines with the potash to form an exceedingly soluble
double salt of zinc and potash that dissolves as rapidly as it
is formed, while the hydrogen, liberated by the decomposition
of the water, reduces the copper oxide to metallic copper.
This reduced copper is of great purity, and can, moreover, if
desired, be converted again into copper oxide. The potash is
manufactured in sticks, varying in size according to the
type of cell, and when the solution is exhausted a renewal
is effected by simply placing a stick in the cell and pouring
in the requisite quantity of water, A layer of heavy paraffin
oil, I inch deep, is poured on top of the solution to exclude
the air and prevent creeping. As for inspection and super-
vision, it suffices to say that, when once set up, the battery
may be left, like the Leclanche, for months together without
inspection and without trouble arising from creeping of the
solution or noxious fumes.
There are three sizes of the Edison-Lalande battery
BATTERIES FOR COIL WORKING. 145
commonly employed, size No. 1 having a capacity of 600
ampere hours, the dimensions of the containing-jar being
llj X 5J inches. Size No. 2 has a capacity of 300 ampere
hours, and the jar is X 3f inches ; while size No. 3 is of 150
ampere hours, and has a jar 5 x 2J inches.
For Galvanisation or constant current application, the
batteries employed are generally either dry batteries, chloride
of silver, or the Edison-Lalande.
Batteries for S;parh Coils,
For spark coils, of course, a very much stronger current is
required to satisfactorily work them than what is necessary
for a medical coil : first, because the primary consists of few
turns of very thick wire and has a low internal resistance ;
and, secondly, because we wish to get from the secondary
very much more powerful results. The batteries chiefly
employed to work large spark coils are the Bunsen, Grove,
bichromate and Edison-Lalande, because all these forms have
a very low internal resistance, and, with the exception of the
last named, a high electromotive force.
The Bunsen Battery.
This battery (see Fig. 117) consists of an outer glazed por-
celain or stoneware jar, inside which is a cylinder of zinc
forming the negative element, while the positive element
consists of a carbon rod or bar contained in a porous pot
placed inside the zinc cylinder. The exciting solutions are :
old method, concentrated nitric acid for the carbon, and sul-
phuric acid, 1 part to 10 in water, for the zinc ; new method,
saturated solution of nitrate of soda and nitric acid (half in
volume), a little bichromate of soda being sometimes added.
The zinc is amalgamated, and special brass terminal clamps
are provided for each element, as shown in the figure. The
L
146 INDUCTION COILS AND COIL'MAKINQ.
electromotive force of the Bunsen cell varies from 1*0 to
1 • 9 volts according to the strength of the solutions.
The Grove battery differs only from the Bunsen in the
negative element, for which a platinised silver plate is sub-
stituted for the carbon rod. It is more expensive both in
first cost and maintenance, and as its only advantage is a
slightly lower internal resistance, of the two the Bunsen cell
Fig. 117. — bunsen battery.
is generally used in preference. Both batteries give off
noxious fumes from the nitric solution in the porous pot, so
that it is advisable to place them when in use either outside
on the window sill or in a cupboard that has access to the
air.
The solutions rapidly run down after six or seven hours'
work so that it is rarely the same solutions (the positive of
which must be poured back into the bottle) are available a
BATTEEIES FOR COIL WOBKING, 147
second time, at any rate not if the full power of the cell is
required.
Bichromate cells, when employed for spark coils, should be
of the largest size and preferably of the flat shape, made up
into a battery of six cells. The medium size Edison-Lalande
battery (300 ampere hour's capacity) is an excellent cell for
working large spark coils, especially as after use it can
simply be stood on one side till next required. With the
Bunsen and Grove batteries it is advisable not only to pour
back the solutions but also to thoroughly rinse the different
parts and carefully dry the terminals.
As regards the number of cells required for working coils,
more than two will rarely be required for medical coils,
while in most instances one will be sufficient.
With spark coils from three to six cells are usually em-
ployed, and more if required to increase the length of spark,
but it must be borne in mind that with every cell added, after
the proper number are connected up, the risk of breaking
down the insulation of the secondary increases at a compound
rate.
148 INDUCTION COILS AND COIL-MAKING.
CHAPTEE VIII.
FAULTS IN MEDICAL AND SPARK COILS,
Faults or break-downs in coils may be classed under the fol-
lowing five heads. The fault may be in —
(1) Primary winding,
(2) Secondary winding,
(3) Connections underneath base,
(4) Contact-breaker and terminals,
(5) Battery,
but wherever it may be, the great thing to be borne in mind
is to alter, unwind, or unscrew nothing until perfectly sure
in which portion of the coil the fault really is. A strict ob-
servance of this rule will oftentimes save a large amount of
trouble and expense.
Faults in Primary,
Faults in tbe primary are generally confined to short
circuits or breaks in the wire, and these most frequently
occur at the two ends of the winding where they enter and
leave the bobbin. If the contact-breaker refuses to act, or
works only very feebly (and the battery is in good order), it
will usually be found that a short circuit has formed some-
where in the primary winding whereby a number of the turns
are cut out and the magnetism of the core consequently very
feeble. If on testing the core there is found to be absolutely
no magnetism and no spark when the battery wire is touched
FAULTS IN MEDICAL AND SPARK COILS, 149
momentarily on the primary terminal, it may safely be con-
jectured that the primary winding is broken somewhere. If
the break in the wire is at one of the two points where it
enters or leaves the bobbin it will be advisable to endeavour
Fig. 118. — galvanometee.
to neatly repair the break by a small bridge of solder, as it is
only possible to get at the primary winding by first unwind-
ing the secondary. In testing for faults both in the primary
and secondary windings, a galvanometer (see Pig. 118) will
be found very convenient as by this means the fault can
quickly be located between any two points.
Faults in Secondary,
Faults in the secondary are naturally much more frequent
and more difficult to cope with than those in the primary,
owing to the high tension of the current. One of the most
common secondary faults is a break-down in the insulation
between one or more layers, or in bad cases, between the
whole of the layers. This is generally caused by using too
much battery power and endeavouring to get a very long
spark, the ends of the secondary being gradually separated
more and more nntil the spark finds it easier to pass
between some two points inside the coil. Once the insula-
tion is pierced the spark will in future follow this path
unless the separation between the ends of the secondary
150 INDUCTION COILS AND COIL-MAKING.
wires is of less resistance. Breaks in the secondary wires
occur, as in the primary, "usually at the ends.
If the sparks at the ends of the secondary seem considerably
diminished while the full hattery power is on and the
primary has been found to be in perfect order, it may
generally be concluded that the insulation of the secondary
has broken down. In many instances on separating the
ends of the secondary beyond sparking distance, the sparks
passing through the leak in the coil will distinctly be heard.
This breaking down of the insulation of a coil is one of
the most serious faults, as, if it does not render the coil
useless, greatly diminishes its sparking capabilities. In
most cases the only really effectual remedy is to rewind the
secondary, but before doing this the following may be tried : —
Eemove the outer wrapping of the coil of velvet or whatever
it may be, and then get ready some very hot paraffin wax
and a hot, clean and pointed soldering iron. With the
soldering iron endeavour to get the wax insulation between
the inside of the cheek of the bobbin to run, and when this
is the case thoroughly drench the space with the hot wax
and continue to apply the wax until it soaks right through.
Let the coil stand for 24 hours for the wax to thoroughly
cool and set, when it should be tried to see if the leak has
disappeared. If not, serve the other end of the bobbin in a
like manner. By this method it is possible sometimes to
cure a bad leak, the object being to run all the wax at the
bobbin ends and ends of the layers together, as it is gener-
ally between the ends of the layers that break-downs in the
insulation occur.
Faults in Connections.
Leaks between the connections in the base of the coil
sometimes occur owing to the wires having perhaps moved,
and these faults are more frequent in medical coils with three
\
FAULTS IN MEDICAL AND SPABK COILS. 151
windings and special switch on "base to give the combined or
separate shocks. Eemedy : unscrew the bottom part of the
base, test with battery and galvanometer to find the fault,
and insulate same with paraffin wax or paper.
Faults in Contact-breaker and Terminals.
First thoroughly satisfy yourself that it is not merely a
question of adjustment, after which remove bottom part of
base and see that the connecting wires are making good
connections to the contact - pillar and spring standard.
Next inspect the platinum contacts to see that these have
not worked loose and also that the circuit is complete
through the different parts. Test also the circuit to the
primary terminals and see that these latter are making firm
connection with the connecting wires under the base.
Battery Faults.
Faults in the battery are soon found out, for if the wires
from the two poles on being momentarily connected together
do not give the customary short thick spark, the fault must be
in the battery provided the connecting wires are sound. This
latter point can be ascertained by connecting the poles by a
fresh piece of wire. If the spark occurs on breaking the
circuit the fault is in the connecting wires
The most common battery faults are exhausted exciting
solution or solutions, dirty, corroded or loose connecting
clamps, broken elements or porous pots. If the battery is a
dry one the quickest plan is to remove the cells and obtain
fresh ones from the manufacturers, who usually allow a
certain amount for the exhausted ones. With the Bunsen
battery the connecting clamps quickly corrode owing to the
fumes, or else to being splashed with the solution. These
must be cleaned up with emery paper or a file, and a little
drop of oil should be put on the threads of the set-screw
152 INDUCTION COILS AND COIL-MAKING.
to prevent the acid corroding it and causing it to stick.
Broken elements or porous pots can soon be detected on in-
spection.
Faults in Condensers,
The fault to which most condensers are liable is a
breaking down of the insulation between the different
layers of tin foil. Owing either to the use of inferior paper
or by employing too much battery power the charge in
the condenser sparks across the intervening layer of paraffin
paper, piercing a hole ; and after this has occurred it will
continue so to spark until it is repaired. Although there are
a large number of sheets of tin foil, it must be remembered
that they are connected together in two sets amounting theo-
retically to two large sheets of tin foil pasted one each side of
a sheet of paraffined paper, and that it is only necessary to
have a perforation at one point to break down the whole of the
condenser. The most prominent evidence of a break-down in
the condenser is manifested by a reduction of the length of
the secondary spark coupled with an increased brightness of
the primary spark at the contact-breaker.
If a fault in the condenser is suspected it must be discon-
nected and the different sheets removed one by one and held
up to a strong light until the perforated sheet is found.
This sheet must then be replaced by a fresh one.
153
CHAPTEE IX.
FIGURES PRODUCED BY ELECTRIC DISCHARGES ON
PHOTOGRAPHIC PLATES.
Some curious effects are to be obtained from discharges from
a spark coil passed over a photographic dry plate, and the
following description of the first discoveries in this direction
by Mr. J, Brown, of Belfast, which appeared in the PJiilo-
sopMcal Magazine for December 1888,will be found of interest.
The results shown on the frontispiece are reproductions
from figures kindly supplied by Mr. Brown, a description of
whose coil will be found in the English Mechanic for February
27 th, 1880.
" While photographing the electric discharge from an in-
duction coil it occurred to me to try what efiect would be
produced on the plate by the discharge when taking place
directly on the sensitive film itself.
" A rather rapid photographic dry plate was laid film
upwards on a piece of sheet metal connected to one terminal
of the secondary, whose ordinary discharging points were set
about 3 centim. apart to act as a by-pass to the spark and
prevent it striking over the edge of the plate. The end of a
wire from the other terminal rested on the centre of the film.
A single discharge from the coil was caused by moving its
mercury-break by hand, and the plate was then placed in
the developer.
" When the terminal wire at the centre of the plate was
negative, and particularly if no discharge passed over the
154 INDUCTION COILS AND COIL-MAKING.
edge of the plate, the result was like that represented at A
(Frontispiece), which shows the typical negative form, con-
sisting of beautiful sharply defined symmetrical palm-like
fronds on irregular stems branching out from the centre
where the wire rested, together with a mass of less distinct
irregular straggling lines also branching outwards, but not
reaching so far as the fronds.
"When the wire terminal was made positive and a dis-
charge caused under otherwise precisely similar conditions,
the figure was quite difi'erent, as in B, and consisted on the
plate of dark irregular branchings sharply defined, except
near the centre, where apparently the luminosity of the
spark has caused a nebulous edge to the branch. These
branchings are accompanied by light irregular straggling
radiations, similar to those on the negative plate, but having
a rather more distinctly centrifugal direction and extending
beyond the well-defined branches, from which they seem to
be to some extent ofishoots. The experiments were repeated
several times and gave similar characteristic results. If,
however, the metal sheet were omitted, and wires from both
positive and negative terminals brought down on a plate
insulated on a paraffin block, neither the palm-fronds on the
negative, nor the dark markings on the positive appeared,
but only the lighter irregular branchings, and these were in
much greater quantity round the positive terminal than at
the negative ; but with the poles not too far apart they
stretched across from one to the other in the form of confused
and irregular lines of flow.
" When the plate was laid, as before, on a metal sheet and
wires from both positive and negative terminals brought
down on the film, a discharge produced the characteristic
figures under their respective wires as shown at F. These
were best defined only when no spark discharge crossed be-
tween the wires on the plate ; and there was in this case no
branching out of the positive and negative markings towards
FIGURES ON PEOTOORAPHIC PLATES.
155
each other, the inductive circuit sensibly completing itself
through the metal sheet under the plate.
" When the difference of potential was made sufficient to
produce spark-discharge between the terminals on the plate,
the resulting marking depended in several respects on the
presence or absence of a metal sheet under the plate.
" With no metal sheet, and the plate insulated on a block
of paraffin, the discharge took a fairly direct course between
the terminals, with only slight crookedness, but sometimes
in a double line. On this plate the characteristic palm-
fronds of the negative and dark branchings of the positive
pole are wanting, and the lighter tracery forms a rough
indication of confused lines of flow between the poles.
" With a sheet of foil pasted on the back of the plate
(leaving a margin of about 2 centim. round the edge) and
spark-discharge between the terminals, there is exceedingly
little, if any, distinct tendency of the positive and negative
markings towards each other. The track of the main spark
is very crooked, meandering over the plate in a quite
irregular way, and making sometimes sharp distinct angles
in its course. From about half of its length from each end,
but principally from the terminals, branch off here and there
the characteristic positive or negative markings.
" When a strip only of foil was fixed on the back of the
plate so that its length crossed the line joining the pole at
an angle of about 45°, and about twenty sparks were passed,
they all took a similar S-shaped course, having been appa-
rently attracted out of the direct line to follow that of the
foil underneath the glass.
" The meandering form and general appearance of these
sparks, when acted on inductively by the foil under the
plates, remind one very much of certain kinds of lightning-
flashes, and suggest at least a possibility of some similarity
in the causes producing each.
" The question now arose as to whether the result was due
156 INDUCTION COILS AND COIL-MAKING.
to a photograpliic effect of the luminosity of the spark, or to
some more direct action of the discharge on the film — to what
might be called an ' electrographic ' action.
"As evidence for the latter view came the apparent
insufficiency of the light actually visible when the discharge
took place to produce the effect. The lighter branchings
were not to me visible at all in either positive or negative, and
only an indication of the frond formations in the negative.
" There are also in the positive plate, B, several intervals, as
if the spark had not been in immediate contact with the
film, but had passed over it, leaving only a foggy mark
instead of a sharply defined black line. The break or
interval would scarcely have been so marked if the whole
effect were photographic only.
"However, to further investigate this question the dis^
charge was taken with the two terminals on the back or
uncoated side of the plate. The figures which now appeared
on the film were quite different from those described above.
In each case there was impressed on the hack of the film
(next the glass) the cloudy photographic effect of the
branching discharge on the back of the plate.
" On the front or outer side of the film under the positive
terminal the figure reminded one of a photograph of a
maiden-hair fern out of focus. That under the negative is a
collection of peculiar tadpole-like markings, whose general
arrangements correspond in size and shape to the fronds
formed by the discharge on the back of the plate. These
figures would appear to be due to electricity induced in the film.
" To try further the effect of induction on the film, I cut
my initials in tin foil after the manner of a stencil plate,
placed the foil on the film, a piece of gutta-percha tissue on
the foil, and pressed all together in an ordinary photographic
printing frame. A second piece of foil was placed on the
back of the plate, leaving a margin all round, and the foils
were joined to opposite poles of the coil with its terminals
FIOUREB ON PHOTOGRAPHIC PLATES. 157
giving a by-pass of about 1 centim., so as not to have any
spark-discharge over tbe plate. The result with the poles
connected in either sense, and the coil working for about a
minute was, with the stencil foil either positive or negative,
an irregular blackening all round the edge of the foil,
including the edges of the cut-out parts, as if a discharge
had passed out from the edges. In some places the character-
istic markings of positive or negative, as the case might be,
were visible. There was, however, a slight visible discharge
from the edge of the foil on the back of the plate, and pro-
bably therefore the same from that on the film to which the
edge-marking may be due. There was also on the plate
much blotchy marking, apparently corresponding to the
wrinkling of the foil in contact with the film.
" With a piece of gutta-percha tissue between the stencil-
plate foil and the film, the result was similar, only the line
round the edges was narrower and the blotching less marked,
hence the initials came out more distinctly. When four
thicknesses of gutta-percha were interposed, there appeared
only blotchy markings on the part under the foil.
" These results would go to show that actual disruptive
discharge over or in the film is not needed to produce an
effect visible on development, but that the figures are pro-
duced partly, at least, by direct electric action on the sensitive
film without the intervention of a visibly luminous action, or
what would be usually understood as a purely photo-chemical
cause. Possibly further investigation may show that we
have here a new kind of experimental evidence on the relation
of electricity to light.
*' I may add that it is necessary, especially in the experi-
ments with the terminals on the back of the plate, to use
rather sensitive plates ; * 60 times ' plates do very well,
while slow plates give imperfect figures in all cases and show
almost nothing with the terminals on the back."
INDEX.
A
A 2-INCH spark coil, 105, 107
Accessory appliances of medical
coils, 81
Action of condenser, 43, 44
— of regulating tube, 22
Adjusting contact breakers, 38
Amount of primary wire, 14
— of secondary wire, 15
Ampere-turns, 5
An inch spark coil, 93
Apparatus for observing induction, 6
Apps* contact breaker, 117
— noted coils, 120
B
Bases for coils, 41
Bath coils, 59
another form of, 65
Batteries for coil working, 137
— bichromate, 141
— Bunsen's, 145
— dry, 138
— Edison-Lalande, 142
— Leclanche, 139
Bobbin ends, 18
Bobbins, 17
— building up, 19
— size of, 18
— core for, 20
— cheeks of, 18
C
Coil winder, 23
— wound in 2 sections, 100
in 4 sections, 100
in 8 sections, 100
Collectors, 89
Commutator or current reverser, 97
Condenser, 43
— action of, 43
— building up, 43
— connections of, 96
Conducting cords, 84
Connections of bath coil, 62
— of spark coil, 96
Contact-breakers, 30
action of, 38
adjusting, 38
Apps', 117
horizontal, 32
separate, 33
variable, 35
vertical, 31
Continental method of winding
cheap coils, 29
Current reversers, 90
D
Determining size of windings, 13
Detonating plane, 134
Dischargers, 41
Dubois-Keymond coil, 66
160 INDUCTION COILS
AND COIL-MAKING.
E
Effects of induced current, 9
Electro cautery, 82
Electrodes, 84
— brush, 87
— double pole, 85
— eye, 86
— needle, 87
— roller, 87
— sponge, 85
Electrolysis, 82
— battery for, 82
— burners for, 82
— needles for, 82
Electro-magnet, 4
Electro-medical appliances, 84
Electro-medical cabinet, 63
Experiments with spark coils, 130
F
Faradisation, 81
Faults in battery, 151
— in coils, 148
— in connections, 150
— in contact-breaker, 151
— in primary winding, 148
— in secondary, 149
Franklinisation, 81
G
Galvanisation, 81
Galvanometers, 88
Gauge of wire for primary, 14
for secondary, 15
H
Handles, 84
— for medical coil, 56
— for street coil, 80
I
Induction, 6
" Inductorium's,*' metLod of wind-
ing, 99
Iron cores, 20
building up, 21
movable, 22
wires for, 20
— filings, 2
L
Leyden jar, 134
Lines of force, 2
Liquid resistances, 91
M
Magnetic field, 3
Medical coils, 55
primary of, 14
secondary of, 15
Methods of regulation, 46
for primary coil, 49
sledge method, 48
switch method, 48
tube method, 47
Milliampere-meters, 88
O
Oersted's discovery, 3
P
Parts of coil, 12
Polytechnic coil, 121
Portable coils, 70
— sledge coil, 72
Primary shock coils, 50
— terminals, 40
INDEX,
161
Primary winding, 23
amount of, 14
Primary winding, size of, 14
Putting coils together, 42
K
Regulating tube of coil, 22
construction of, 22
Repairing coils, 148
Residual magnetism, 4
Resistance, 90
Resistances of different sized copper
wires, 16
Rheophores, 84
Rheostats, 90
Rotator for vacuum tubes, 132
S
Secondary terminals, 40
— winding, 15
amount of, 15
faults in wire for, 27
foreign method, 29
joins in, 27
size of, 15
Sectionally wound coils, 98
Self-induction, 13
Separate contact-breakers, 33
Shock coils, 46
— from primary coil, 11
— from secondary coil, 9
Siemens & Halske's coil, 99
Size of bobbins, 20
Sledge coils, 66
bobbin for, 67
connections of, 69
contact-breaker of, 68
Sledge coils, guides fur, 67
primary of, 68
secondary of, 67
Spark coils, 92
Sparks, altering length and cha-
racter of, 133
— experiments with, 133
— firing gunpowder with, 134
— on metal filings, 134
— on photographic plates, 153
— passing through water, 135
Spottiswoode coil, 123
Street coils, 75
battery for, 80
bobbin for, 76
contact breaker for, li\
dial for, 76
handles for, 79
important points of, 76
Switch for bath coil, 60
T
Table of proportions for different
size coils, 98
— of resistances of copper, 16
Terminals, 40
Tertiary winding, 61
Turning up a bobbin, 19
Twelve-inch spark coil, 108
base bobbin-cheeks, 110
completing secondary, 116
condenser for, 118
contact-breaker for, 117
core of, 108
finishing off, 1 1 9
insulating tube, 109
primary winding, 109
secondary of, 111
section winder for, 115
winding the sections of, 114
162 'INDUCTION COILS AND COIL-MAKING.
V
Vacuum tubes, 131
compound, 132
horizontal, 132
fixing in coil, 131
rotator for, 132 .
— tubes, vortical, 132
Variable contact-breakers, 35
Vertical contact-breakers, 31
W
Winding primary, 23
— the secondary, 20
LONDOM : I'RINTED BY WILLIAM CLOWKS AND SONS, LIMlTliU,
3TAMF0KI> STREET AND CHARING CROSS.
THE HANDY
SKETCHING- BOOK.
For the use of engineers and draughtsmen, for
rough sketching, with three useful tables.
The paper is square lined to exact ei^^hths of
an inch in blue ink, and bound in stiff boards.
Size, 8 in. X 5 in.
25 cents each. Per doz., $2.50.
PRESS OPINION.
" This handy little book has earned a well merited
success. . The lines are printed from plates and are as
nearly mathematically correct as mechanical appliances
ran !)roduce. Every engineer and draughtsman should
have one."
THE HANDY
Sectional lined paper to eighths of an inch.
On paper sufficiently transparent to enable
good blue prints to be taken. Size, 8 in. x 10 in.
25 cents each. $2,50 per doz.
162
INDUCTION COILS AND COIL-MAKING.
Vacuum tubeis, 131
compound, 132
horizontal, 132
fixing in coil, 131
rotator for, 132 .
— tubes, vertical, 132
Variable contact-breakers, 35
Vertical contact-breakers, 31
W
Winding primary, 23
— the secondary, 2(J
""CCJits, ^
Connections a i
Phonographs, Ph^; u "'"''Piones, w'/ ^
Telephone ' ^^"*°Piones, Storage
, T'l's little work •
'arge class of <; addressed to fK
'35 pages, x2mo. doth ,
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