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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 , 
 Sent Post v ^5 '^^"fe- 
 
 Lu^•uo^^ : trinted by wtlliam clowks and ^O^ts, mi-*.. 
 
 3TAMFOKJD STREET AND CHAUING CROSS.