■(M^u^ 
 
 UNIVERSITY 
 
 (J^ 
 
 • UBRARY 
 
 Q>^^ 
 
 
 Hii 
 
 
- CIRCULATJOH 
 
 Cornell University Library 
 QC 645.H18 
 The Tesla high frequency coH, its constr 
 
 3 1924 012 334 706 
 
Cornell University 
 Library 
 
 The original of tliis book is in 
 tine Cornell University Library. 
 
 There are no known copyright restrictions in 
 the United States on the use of the text. 
 
 http://www.archive.org/details/cu31924012334706 
 
THE TESLA HIGH FREQUENCY COIL 
 
Plate 1. — Complete 12" Apparatus. Frontispiece. 
 
THE TESLA HIGH 
 FREQUENCY COIL 
 
 ITS CONSTRUCTION AND USES 
 
 BY 
 
 GEORGE F. HALLER 
 
 AND 
 
 ELMER TILING CUNNINGHAM 
 
 S6 ILLUSTRATIONS 
 
 NEW YORK 
 D. VAN NOSTRAND COMPANY 
 
 23 Murray and 1910 27 Warren Sts. 
 
Copyright, 1910, by 
 D. Van Nosteand Company 
 
 X 
 
 The Plimpioii Press Norwood Mass. U.S.A. 
 
INTRODUCTION 
 
 In presenting this book on the Tesla coil to the pubhc the 
 authors hope that they have filled a long felt vacancy in the 
 practical library of science. No attempt has been made to 
 give a mathematical explanation of the oscillation transformer, 
 and other parts of the high-frequency apparatus, for the 
 simple reason that the theory is too complex, and when 
 obtained of no practical use. Neither have the authors 
 tried to lead the amateur, who is just learning how to string 
 bells and connect batteries, from the elements of the galvanic 
 cell up to the working of a high-potential, alternating cur- 
 rent, but have merely made an effort to place in the hands of 
 advanced amateurs in electrical science a practical working 
 manual on the construction of high-frequency coils, now so 
 useful in scientific investigation. 
 
 The attention of the authors was first called to the Tesla 
 coil when they were fortunate enough to be given the use of 
 the 7" standard coil described in the last chapter of this 
 book. A systematic line of experiments was carried on with 
 it, in order to study the effects of a change in the constants 
 of the various circuits. All the mechanical and electrical 
 details of construction were carefully worked out, and the 
 authors finally decided to design and construct a larger coil. 
 The coil, as first constructed, was a decided failure, due to 
 
VI Introduction 
 
 too small a condenser capacity. For about five months they 
 further experimented on the details of construction and 
 finally arrived at the 12" coil described in this book. This 
 coil they feel assured is as efficient as can be made. It is 
 especially designed to give a high-frequency discharge of 
 great volume. This latter fact makes it useful for wireless 
 telegraphy. 
 
 In conclusion they have to thank Mr. G. O. Mitchell for 
 many suggestions and for the kindly interest he has taken in 
 this work. They feel that without his help the writing of. 
 this htde book would have been impossible. 
 
 G. F. H. 
 
 E. T. C. 
 
CONTENTS 
 
 CHAPTER PAGE 
 
 I. General Survey . i 
 
 II. The Transformer . 4 
 
 III. The Condenser . . 20 
 
 IV. The Oscillation Transformer 24 
 V. The Interrupter . . . 32 
 
 VI. The Construction of the Boxes 60 
 
 VII, Assembling . . 64 
 
 VIII. Theory of the Coil . 72 
 
 IX. Uses of the Coil . . 84 
 
 X. Dimensions of 7" Standard Coil . 97 
 
 Appendix . . ....... iii 
 
LIST OF PLATES 
 
 FACING 
 
 PLArE PAGE 
 
 I. Complete 12" Apparatus Frontispiece 
 
 II. Transformer for 12" Apparatus . 20 
 
 III. Oscillation Transformer and Glass for Condenser of 12' Coil . 20 
 
 I\'. Motor-driven Interrupter .... 52 
 
 \'. The Electrolytic Rectifier . . . 52 
 
 VI. Discharge from the 12' Coil . 96 
 
 VII. The 7' Standard Apparatus . .... ... 96 
 
LIST OF FIGURES 
 
 FIG. PAGE 
 
 1. Method of Fastening Primary Terminals — Completed Primary 8 
 
 2. Secondary Bobbin of Transformer ii 
 
 3. Hand Winder 13 
 
 4. Wire-spool Holder . 14 
 
 5. Frame for Secondary of Transformer 18 
 
 6. Section of Completed Transformer . 18 
 
 7. Condenser Frame and Brass Condenser Sheet 22 
 
 8. End Support for Secondary of Oscillation Transformer 25 
 
 9. Fibre Strip 25 
 
 10. Centre Rod 25 
 
 11. End Support for Primary 28 
 
 12. Primary of Oscillation Transformer . 29 
 
 13. Completed Secondary of Oscillation Transformer 29 
 
 14. Bushings for Support of Oscillator Standards . . 30 
 
 15. Hard Rubber Block on Oscillation Transformer 30 
 
 16. Simple Primary Air-gap 34 
 
 17. Magnetic Interrupter 37 
 
 18. Motor Interrupter Fan . 38 
 
 19. Brass Angle Piece 39 
 
 20. Hard Rubber Block 40 
 
 21. Section of the Motor Interrupter . 41 
 
 22. Patterns of Base 42 
 
 23. Patterns of Yoke 44 
 
 24. Section of Completed Molor 45 
 
 25. Rotor Disc - . 46 
 
 26. Rotor and Clamp Nut 48 
 
 27. Stator Disc 49 
 
 28. Frame for Stator Coils . 51 
 
 29. Self-starting Device . . . 52 
 
 30. Rectifier Plates and \\'iring Diagram 58 
 
 31. Transformer Box 61 
 
 32. High-tension Box . 63 
 
 33. Connections for Primary of Transformer 65 
 
xil List of Figures 
 
 FIG. PAGE 
 
 34. High-tension Bushing ^ 
 
 35. Oscillators and Standards ..... ^7 
 
 36. Wiring Diagram . - 08 
 
 37. Waves on Wires . 92 
 
 38. Primary and Core of Transformer of 7' Coil 98 
 39- Secondary Bobbin of Transformer of 7" Coil 99 
 
 40. Plate and Frame of Condenser loi 
 
 41. Oscillation Transformer of 7' Apparatus . 104 
 
 42. Box for 7" Apparatus 106 
 
 43. Wiring Diagram . 107 
 
 44. Oscillators and Standards for 7' Apparatus . 109 
 
 45. Oscillation Transformer of Small Coil 113 
 
 46. Completed Transformer of Small Coil 115 
 
 47. Primary Spark-gap . . 117 
 
 48. Wiring Diagram . 118 
 
 49. Wiring Diagram . 118 
 
THE TESLA COIL 
 
 CHAPTER I 
 
 GENERAL SURVEY 
 
 By far the largest and most interesting branch of science 
 is electricity, for Maxwell has proven mathematically, and 
 Hertz verified experimentally, that light is an ejectromagnetic 
 disturbance in the ether, and thus added that subject to the 
 realm of electricity. Amongst the various phenomena of 
 electricity, those of the high-tension current are the most 
 interesting and instructive. With such a current all the 
 wonders of the Geissler and Crookes tubes may be seen. 
 With it waves for wireless messages may be sent out into 
 space, and a great number of other experiments carried out. 
 
 It is the purpose of this book to show how a satisfactory 
 apparatus for producing these currents may be constructed, 
 and also to describe a few of the uses for such a coil. 
 
 The apparatus, as described in this book, is most commonly 
 known as the Tesla High-Frequency Coil, and consists, in 
 general, of four parts : i. The Step-Up Transformer; 2. The 
 Interrupter; 3. The Condenser; 4. The Oscillation Trans- 
 former. Each of these will be fully considered in subsequent 
 chapters. 
 
 Before entering upon the description of the Tesla high- 
 frequency apparatus, however, it would be well to make a 
 few general remarks which are of the greatest importance. 
 
2 The Tesla Coil 
 
 Throughout the whole work of construction the most exact- 
 ing care must be given to the matter of insulation. All the 
 wire used must be carefully tested, and each layer of wire in 
 the transformer must be thoroughly shellacked, and then in- 
 sulated from the next layer, by two turns of carefully oiled 
 paper. In the condenser, which is really the vital part of 
 the apparatus, the glass should be of the best grade obtain- 
 able. It must also be free from all air bubbles. It is in 
 the high-frequency apparatus, howe^'er, that the greatest 
 care as regards both construction and insulation must be 
 taken. The secondary consists of one layer only of No. 32 
 B. & S. gauge, double cotton-covered wire, wound on an 
 octagonal frame, formed of strips of vulcanized fibre fastened 
 to two end pieces of wood. When winding the wire, care 
 must be taken that no two adjacent wires touch, for that 
 would cause a short circuit. When the wire is completely 
 wound, it is given about five coats of shellac, not only to act 
 as an insulator, but also to prevent any slipping of the wires, 
 The primary consists of a thin band of copper, making two 
 and a half turns around a circular frame surrounding the 
 secondary. The frames on which the primary and secondarj 
 are wound must be \'ery firm and sulDstantial, so that ai 
 occasional jar will not displace any of the wires on the 
 secondary. 
 
 All connections must be soldered, and the connecting 
 wires run through glass tubes. 
 
 When the apparatus is finished, two carefully made box6 
 must be constructed. These must be oil tight. This iii 
 
General Survey 3 
 
 accomplished by mortising all joints, and then giving the 
 boxes, especially the joints, about four or five coats of 
 shellac. Into one box the transformer fits, and into the 
 other the condenser and oscillation transformer. Then the 
 boxes are filled with pure paraffine oil, which is the only 
 efficient insulator for these high-tension currents. 
 
 Some who intend to build this coil will think that all these 
 precautions regarding insulation are extreme, but it will be 
 found that, in dealing with high-frequency, high potential 
 currents, too much care cannot be taken, for "Good insu- 
 lation is the key to success in high tension work." 
 
CHAPTER II 
 THE TRANSFORMER 
 
 The transformer — sometimes called a converter — is 
 merely an induction coil that is connected directly to the 
 alternating-current mains, without the use of an interrupter, 
 and is used to raise or lower the voltage. In a transformer 
 the number of watts in the primary equals approximately 
 the number of watts in the secondary. 
 
 In the case of any step-up transformer, the ratio of the 
 number of volts in the primary to those set up in the sec- 
 ondary is nearly the same as the number of turns of wire 
 in the primary to the number in the secondary; but the 
 amperes decrease in the inverse ratio. 
 
 The transformer used in the coil described in this book is 
 of the common induction-coil type, oil-immersed, step-up 
 transformer. It takes the alternating current from the mains 
 at no volts or 55 volts, and steps it up to about 10,000 volts. 
 
 The efficient working of a transformer depends largely ■ 
 upon the design of the core. The iron used must be of high 
 permeability and should have little retentivity. A straight 
 core is always best to use; for, on the fall of the current from 
 its maximum value to zero, the magnetic flux falls from its 
 maximum value, not to zero, but to a value which depends 
 
The Transformer 5 
 
 on the residual magnetism. The residual magnetism in an 
 open circuit is much less than in a closed magnetic circuit, 
 so that when the current suddenly becomes zero, the mag- 
 netic flux drops lower in an open circuit than in a closed one. 
 As the electromotive force in the secondary is proportional 
 to the fall in the magnetic field, it is greater with a straight 
 core than with a closed circuit of iron. 
 
 The coil designer is obliged to determine the length of 
 the iron core from the experience of others, as the mathe- 
 matics for calculating it is too complex, although simple and 
 useful in the case of closed circuit transformers. If the core 
 is made too long the primary magnetizing current will be 
 too large, while if made too short the secondary coils would 
 have to be made of too large a diameter to be efficient. There 
 is, therefore, a certain length which will give the best results. 
 
 In the case of this transformer the length of the core was 
 determined after having gained all possible information 
 from certain eminent men who had made a life study of these 
 matters; in fact, all the dimensions of the transformer for 
 this special use were determined in this way. 
 
 The iron core is made up of pieces of No. 20 or 22 B. & S. 
 gauge iron wire 18" long. The wire is first cut nearly to 
 size with a pair of pliers, and,' when assembled, the ends of 
 the bundle are sawed off square with a hack saw. An ordi- 
 nary piece of iron pipe, a little less than 18" long, and having 
 an internal diameter slightly less than 2", is tightly filled 
 with these wires. When putting the wires in, stand the pipe 
 on end on a smooth surface, and force in each wire until 
 
6 The Tesla Coil 
 
 it hits this surface. When the bundle is finished, the upper 
 end is sawed off «-ith a hack saw to exactly i8." 
 
 The tube containing the iron wires is now placed in a coke 
 or coal fire and left there until the fire burns itself out, thus 
 insuring slow cooling. This heating and subsequent slow 
 cooling so softens the iron wires that their retentivity is re- 
 duced to a minimum. When cool, the wires are taken out 
 and sandpapered to ^emo^'e any superfluous oxide. They are 
 then, one by one, dipped into boiling water, wiped dry, and 
 while still warm are coated with thin shellac varnish. When 
 the shellac is dry they are again packed, as tightly as possible, 
 in the pipe, to hold them in the desired shape. Then, while 
 still packed closely together, they are forced slowly out of 
 the pipe; starting at the end thus released, they are tightly 
 bound with a narrow cotton bandage, which can be obtained 
 from any surgical supply house. The bandage should be 
 between one and two inches wide, but no more. When the 
 entire core is wrapped with this cloth, the cloth should be 
 hea\ily shellacked. The ends of the core are now filed flat 
 and smooth; after this it is put in a warm place to dry thor- 
 oughly, when it will be ready for the primary winding. The 
 use of the insulating varnish on the iron wires is to arrest 
 eddy currents as much as possible, thus preventing the iron 
 wire from becoming heated and energy wasted, which would 
 lower the transformers efficiently. 
 
 The primary is wound in two sections of two layers each, 
 one above the other. No. 12 B. &l S. gauge, double cotton- 
 covered copper wire is used. About 2\ pounds will be 
 
The Transformer 7 
 
 required. The primary may be wound by hand, by erecting 
 two wooden supports 17" apart, and having a 2" hole 
 bored in each, to receive the iron core. Then, by turning 
 the core by hand the wire may be wound fairly well. But 
 as it is rather difScult to wind the wire tightly in this way, 
 it would be more satisfactory to wind it in a lathe, if the 
 amateur has access to one. To mount it, cut a half-inch 
 piece from the end of the pipe in which the core was formed, 
 and slip it over the extreme end of the core. Make the 
 ring fit as tightly as possible by placing between it and the 
 core a few strips of tin or other thin sheet-metal. Now clamp 
 it firmly in the chuck. The other end of the core should also 
 be fitted with a half-inch piece of pipe and supported at this 
 place in the steady rest. The one piece of pipe is used to 
 prevent any of the wires from being forced in unequally at 
 the points where the chuck clamps it, and the other to afford 
 a smooth bearing surface for the steady rest. If there is 
 any tendency for the core to slip out of the chuck, the tail 
 stock, with the centre removed, may be pressed up against it. 
 About I ft. from the end of the copper wire take a couple 
 of turns of tape around it. At this point bind the wire to 
 the iron core, about i" from its end, by taking several turns 
 of tape around it. Proceed now to wind the wire tightly and 
 closely to within i" of the other end. Here the winding of 
 the primary is stopped for a short time in order to give the 
 wire a good coat of shellac. After the shellac has dried, 
 another coating is given it, and then the second layer is 
 wound on while the wire is still wet. When the winding 
 
8 The Tcsla Coil 
 
 has come to within about six turns of the starting point, a 
 piece of tape doubled back on itself is laid on the first layer, 
 with its ends projecting beyond the unwound portion of the 
 second layer. The looped end of the tape must be on the 
 outer side of the winding. See Fig. i. 
 
 /I kf''') ' 
 
 Sji-ix^rams showing rnar^rter oC CasientTn^ la^t "tuTYt. 
 
 Fig I. — Method of Fastening Primary Terminals - 
 
 Primary. 
 
 ■ Completed 
 
 The winding of the second layer is finished over the piece 
 of tape, the last turn being brought through the loop in the 
 tape. The loop is drawn tight by pulling on the other pro- 
 jecting ends. In this way the last turn is kept from slipping 
 off. By using this method or fastening it is unnecessary to 
 use any bobbin heads for the primary; this is a decided 
 advantage, as, with a removable primary, bobbins are always 
 getting loose. The wire is cut off about 2' from this ending 
 
The Transformer g 
 
 in order to allow plenty of wire for making the various 
 connections, which will be described in a later chapter. 
 When this layer is thoroughly shellacked, the first section 
 of the primary is complete. 
 
 The second section is wound directly on top of the first, 
 starting at the same end, and being sure to wind in the same 
 direction. Each layer when wound is thoroughly shellacked, 
 and the last turn is fastened in the same manner as before. 
 If the wire has been put on carefully 164 turns can be wound 
 in the 16" and the total diameter will be 2|". 
 
 The secondary is wound in four sections. It will first be 
 necessary to procure two micanite tubes, the one fitting 
 tightly within the other. The inner diameter of the smaller 
 tube is a trifle greater than 2!", the external diameter of the 
 larger one being 35". The length of the tubes is 18" and their 
 thickness -J". Now turn out a wooden rod so that the larger 
 tube will fit around it tightly. Mount the rod in the lathe 
 with the tube on it, clamping one end of the wood in the 
 chuck, and supporting the other end on a centre. With a 
 thin parting tool, cut off seven rings, three i" wide, and four, 
 3^" wide. If the amateur has no lathe the rings may be cut 
 off in a mitre box. Out of some quarter-inch sheet-fibre, 
 cut eight circular pieces, 6" in diameter and having a 3" hole 
 in the centre. Slip one of the 1" rings on the smaller tube, 
 and with Le Page's glue fasten it to the extreme end of the 
 tube. Next slip on one of the circular discs of fibre, and 
 then one of the 3J" rings, fastening them with glue. Two 
 more discs are put on, and then another 3^" ring. After 
 
10 The I'rsld Coil 
 
 this comes another disc and a i" ring, followed by a disc 
 and a 3,{" ring. Then put on two more discs and the remain- 
 ing 3 1" ring. This is followed by the remaining disc and i'' 
 ring. Be sure that each ring is carefully glued in place. 
 Before putting on the discs, small holes should be drilled in 
 them, through which to carry the wires. The romijlcted 
 bobbin for the secondary is seen in Fig. 2. The discs num- 
 bered 2, 3, 6, 7 ha\'e the holes fcjr the connecting wires 
 drilled on their inner edge, while the others have them drilled 
 about \" from their outer edge. Obtain a wooden rod upon 
 which the secondary bobbin will lit tightly. It should be 
 i8i" long. 
 
 If the coil builder is skilled in winding wire in the lathe, 
 the winding may be done there much more rapidly than by 
 hand; but for an amateur, who has had but little experience 
 with lathe windirig, or for one who does not possess a lathe, 
 the following method is given. In winding in the lathe, 
 great care must be taken that the wire is not snapped off when 
 the end of the layer is reached, and while the jjaper is being 
 wrapped on before the next layer is wound. 
 
 For the hand winder, the wooden rod, on which the second- 
 ary bobbin fits tightly, is drilled in at both ends for about 
 4" with a litde less than a {" hole. Pieces of |" iron are then 
 driven into these holes, to serve as an axh,-. They should 
 fit tightly, so as to turn with the cylinder. About 6" should 
 project at one end, which is bent into a handle. 1 !/' at the 
 other end is sufficient for a bearing. 
 
 The standards are made of '(" oak, fastened jg|" apart, 
 
^ 
 
 The Transformer 
 „9 
 
 II 
 
 r 
 
 -I* 
 
 
 -Id- 
 
 
 f ^ 
 
 -I* 
 "1 
 
 
 
 4e- 
 
 -^ 
 
 1-^ 
 
 :,f^i^,^^-J^.?/-^ 
 
 <5^ 
 
 ^ 
 
12 Tlie Tesla Coil 
 
 to a baseboard 2' long. A piece of oak f" square and 2" 
 long is fastened with two screws to the top of each standard, 
 to serve as a cap. A I" hole is then bored with its centre 
 on the joint. This allows the cylinder to be taken out of 
 its bearings when necessary. Two iron washers are slipped 
 over the shaft at the short end to act as a thrust bearing, 
 and two washers, with an open, steel-wire spring between 
 them, are put on the other end. This will give the friction 
 required to enable the amateur to stop the winding at any 
 time, and still be sure that the cylinder will not rotate and so 
 loosen the turns of wire. The dimensions of the winder are 
 seen in Fig. 3. 
 
 As the wire must be wound under some tension, and as 
 it is tiresome to give the required tension by letting the wire 
 run through the hand, the holder shown in Fig. 4 was 
 devised. 
 
 It consists of an axle which fits the spool tightly, and which 
 is 4" longer than the spool. There is a thread cut on one 
 end of this axle for about 2" It is then mounted in two 
 wooden standards fastened to a baseboard. Iron washers 
 are put between the spool and the standards for the spool 
 to bear on. An open spring made of piano wire is slipped 
 up on the threaded end of the shaft, outside of the standards. 
 A washer and a nut are now put on to give the required 
 tension to the spring. A lock nut is put on to keep this 
 nut from turning. 
 
 Care must be taken to detect any breaks that may occur 
 in the wire. When winding the wire it quite frequently 
 
The Transformer 
 
 13 
 
 
 o "n 
 
 F\ 
 
 c 
 -a 
 
 I 
 
 M 
 O 
 
 
 C5 
 
14 
 
 The Tesla Coil 
 
 happens that a little kink will cause a break; but because 
 it is covered by the cotton insulation, it will be wound on the 
 bobbin, unknown to the coil builder. To detect these breaks 
 immediately, the authors used the following method. A ring 
 is cut out of a piece of sheet brass or copper. It is }" wide 
 and 3" in diameter. This is fastened by several flat-headed 
 brass screws to one side of the spool on which the wire is 
 
 / 
 
 ^ 8 
 
 Fig. 4. — \\'ire-Spool Holder. 
 
 bought. If the wire has been bought from a reliable dealer, 
 the inner end will be found projecting outside of the reel. 
 This wire is soldered to the ring on the outside of the spool. 
 A strip of sheet copper or brass, which is of such a length 
 that it will bear on the ring, is fastened to the upper end of 
 the standard, on the side on which the ring is. From here 
 a wire is led to one pole of a dry cell. 
 
The Transformer 15 
 
 On the winder a strip of sheet metal is fastened to one of 
 the standards. It is best to fasten it to the one farthest up 
 from the handle. It is bent so that it presses firmly on the 
 projecting axle, which has been polished to make good elec- 
 trical contact. A wire is then led from the brush on the 
 standard to a binding post on the baseboard. A telephone 
 receiver is now connected in series with the binding-post and 
 the other pole of the cell. A watch-case receiver, with a 
 head attachment, is the best to use. If the amateur has only 
 the Bell receiver, an attachment to hold it to his head can 
 easily be arranged. 
 
 If the amateur prefers he may use a sensitive galva- 
 nometer. 
 
 Everything is now ready for the winding of the secondary. 
 To begin, pass about i' of the wire through the hole in the 
 bobbin heads numbered 2, 3, from the side on which bobbin 
 head 2 is. The insulation should be scraped off of the end 
 of the wire for about 2", and then this bare part should be 
 tightly wrapped on the axle between the washer and the 
 wooden cylinder. It will now be seen that there is a complete 
 circuit through all the wire on the spool. The diaphragm 
 in the telephone receiver is drawn down or if a galvanometer 
 is used, the needle will be deflected. If the wire should 
 break, the diaphragm will return to its normal position and 
 a click will be heard, or in the case of the galvanometer the 
 needle will return to the zero position. When this happens 
 the break should be located and the wire soldered. Acid 
 should not be used in soldering, as a little left on the wire 
 
1 6 The Tesla Coil 
 
 will corrode it and spoil the electrical connection. Rosin 
 is the best thing to use as a flux. 
 
 The first layer in the section between the bobbin heads 
 I, 2, is wound from 2 to i, and after it is wound it is given 
 a good coating of shellac. Before winding the next layer, 
 a little over a turn of paper is taken around the pre\ious 
 one. The edge of the paper can be held down with a little 
 shellac. Parafiine wax must not be used to increase the in- 
 sulation, as the transformer when finished is immersed in 
 paraffine oil, which acts as a partial solvent to paraffine wax, 
 thus spoling its insulating properties. All the layers after 
 the first should start about \" from the inner face of the discs 
 and stop the same distance from them. Be sure to shellac 
 each layer after it is wound and then take a turn of paper 
 around it. Continue winding until 61 layers are in place. 
 The last layer should be wrapped over with a narrow cotton 
 bandage which is thoroughly shellacked to keep it in place 
 About two feet of wire should be left projecting from the 
 section for making the various connections. This wire is 
 then brought through the hole in disc i, and its end is 
 connected to the axle. 
 
 Unwind the other wire from the axle and, after polishing 
 the uninsulated part with a piece of emery cloth, t^vist it 
 around the end of the wire from the spool, which has also 
 been polished, and then solder the connection. \\'rap the 
 bare part of the wire with some silk thread, so as to thorou'^hly 
 insulate it. 
 
 The section between the bobbin heads 3, 4, is now wound. 
 
The Transformer 17 
 
 ; The first layer is wound from j towards 4, the winder being 
 turned in such a direction that the direction of the current 
 in the wire of this section will be the same as in the one just 
 wound. That means that the winder must be rotated in 
 the opposite direction. For convenience, howe\er, in the 
 winding the secondary bobbin can be taken off of the wooden 
 rod and put back in a reversed position, ^^■hen this is done 
 the winder is rotated the same as predously. The same 
 instructions hold for this section as for the pre\"iou5 one. 
 Each layer must be shellacked and wrapped with paper and 
 the winding must stop before getting to the bobbin heads. 
 In this section 71 layers are wound on. About 2' of wire 
 should be brought through the hole in disc 4 to allow for 
 
 ; connections. 
 
 The two remaining sections are now wound in the same 
 
 - manner, there being 71 layers in the section between the 
 bobbin heads 5, 6, and only 61 la\ers in the section between 
 the discs 7, 8. Remember to keep the direction of the wire 
 the same as in the previous coils. This method of winding 
 has several advantages, one of them being that it relieves 
 static strains. A practical reason is that all the leading out 
 wires are from the outer layers, thus making it ahvavs easy 
 
 ; to bring out a new piece of wire if an}- are ever broken off. 
 
 The reason for ha^'ing a greater number of la}-ers in the 
 
 middle coils w-ill readily be seen from a consideration of the 
 
 . direction and intensity of the lines of magnetic force around 
 a solenoid. 
 Leave the completed secondary in a warm place to thor- 
 
The Tesla Coil 
 
 <t'V----7f" ^'V 7i ff 
 
 Fig. 5. — Frame for Secondary of Transformer. 
 
 oughly dry, and in the meantime construct the frame for the 
 secondary. Working drawings are given in Fig. 5. The 
 base is made of a piece of i"x 9" pine 20^" long. The comers 
 and edges are rounded to give it a better apgearance. Three 
 supports of i"x7" pine, 4V' high, are erected at the points 
 
 1! ,.r,r..— .nm-rrrr-g^ 
 
 !lbi'Anm(Trn-F-n-3T^"'T:::r; 
 
 m. 
 
 Fig. 6. — Section of Completed Transformer. 
 
The Transformer 19 
 
 shown in the figure. At the top of each a half-round hole 
 3j" in diameter is cut. Into these fits the tube on which 
 the secondary is wound. The distance between the supports 
 
 is 7r. 
 
 The two terminals from the middle coils of the secondary 
 are now soldered together, and the connection wrapped with 
 silk thread to insulate it. The other two terminals are left 
 alone at present as their connections are described in a later 
 chapter. 
 
 The reason for using a cotton cloth instead of tape in the 
 construction of the transformer is that oil almost immedi- 
 ately spoils all the adhesive qualities of the type. 
 
CHAPTER III 
 
 THE CONDENSER 
 
 A CONDENSER is an apparatus for accumulating a large 
 quantity of electricity on a small surface. The form may 
 vary considerably, but in all cases it consists essentially of 
 two conductors separated by a non-conductor or dielectric, 
 and its action depends entirely upon induction. 
 
 The thinner the dielectric and the greater its specific 
 inductive capacity, the greater is the capacity of the con- 
 denser. A thin dielectric, however, cannot withstand a high 
 potential. Besides the thickness, the dielectric strength de- 
 pends on the character of the material. 
 
 The condenser used in this apparatus is especially designed 
 for long continued use on high voltages. The dielectric 
 used is glass and the plates are made of sheet brass. When 
 finished the condenser is immersed in pure parafi&ne oil. 
 
 For the dielectric 95 sheets of glass, yV thick and io"xi2" 
 in size, should be obtained. These sheets may be had cut 
 to size for about nine or ten cents apiece. In purchasing 
 them each sheet should be examined carefully to see if there 
 are any air bulAles. If any are found in a sheet of glass 
 it should be rejected. 
 
 The brass used is number 32 or 34. Forty-six sheets 
 
Plate II. — Transformer for 12" Apparatus. 
 
 Plate III. — Oscillation Transformer and Glass for Condenser of 
 12" Coil. 
 
The Condenser 21 
 
 8"xio" are required. In one of the corners of the shorter 
 side is a tongue 2" wide and i j" long. A j" hp is bent across 
 the top of this tongue. If the brass can be had in rolls 8" 
 wide, a little more should be obtained and the tongues cut 
 out of it. They should be cut 3"xi|". The extra i" is for 
 soldering them to the plates. Rosin, not acid, should be used 
 as a flux. These tongues should always be soldered in the 
 position shown in the figure. 
 
 As a rule 12" is the only width that can be obtained in 
 most places. When this width is used the tongues are cut 
 right on the sheets. 
 
 The condenser occupies the part that has been constructed 
 for it in the oscillation transformer box. The frame in 
 which the condenser is built up is made out of well dried 
 pine. The base is made of a piece of ]" x 11" pine, iif" 
 long. The sides are cutout of y'xi2" material lof" long. 
 The ends are also cut from j"xi2" wood and are 11" long. 
 The sides and ends should be planed up smooth on both 
 sides, so as to make them a little less than i" thick. The 
 completed frame is seen in Fig. 7. 
 
 Place the condenser frame on a table or some other flat 
 surface, with one of the ends down. Before putting any of 
 the glass sheets in the frame, they should be carefully wiped 
 clean so as to remove any dust or moisture. Commence 
 by putting two glass sheets in the frame so that they reach 
 the bottom of the frame. Place a brass sheet on top of these 
 glass plates so that there is a i" margin of glass all around the 
 sheet, except were the tongue comes out. If the lip on the 
 
22 
 
 The Tesla Coil 
 
 tongue has been bent carefully it will just fit up against the 
 sheets of glass. Without displacing the brass lay two sheets 
 of glass on top of it. A brass sheet is next put in, but in this 
 case the tongue comes out on the reverse side. There should 
 be a i" margin around the brass as in the previous case. After 
 the brass come two more sheets of glass. This process is 
 
 A. 
 
 n." "Frame for Condenser 
 
 B."Sha|3= o^ brass jheet 
 Dotted lines show size op 
 tort4ue w^dn so\<ierG6 on. 
 
 Fig, 7. — Condenser Frame and Brass Coxdexser Sheet. 
 
 kept up until the 46 sheets of brass have been put in place. 
 Three sheets of glass are placed on top of the last brass plate. 
 If the glass and the brass used are the size called for in this 
 book the last sheet will just go m tightly. In forcing in the 
 last few sheets it is a good idea to lay a cloth between the last 
 two, to take up any excess pressure, which would otherwise 
 crack the glass. 
 
The Condenser 23 
 
 Set the condenser upright, when finished, and solder a 
 piece of No. 16 bare copper wire about 3' long to each of 
 the lips in turn, down the one side, and the same is done on 
 the other side. 
 
 Two leather straps should be fastened to the sides of the 
 frame to lift it by when lowering into the oscillation trans- 
 former box. The object for building the condenser in a 
 separate frame, instead of in the division in the oscillation 
 transformer box, is to facilitate moving should the condenser 
 ever require rebuilding, due to the rupturing of the glass 
 shet . 
 
 AA'hen the condenser is placed in the box, the end which 
 has the three glass sheets should be placed against the parti- 
 tion, that is, nearest the oscillation transformer. This is to 
 pre\-ent the spark from the oscillation transformer breaking 
 through into the condenser, instead of following its air path. 
 
CHAPTER I\' 
 THE OSCILLATION TRANSFORMER 
 
 It is this part of the apparatus, so simple in construction, 
 in which the most care as regards insulation must be taken. 
 The success of the whole apparatus depends on the care 
 with which this part is constructed. The least fault, such 
 as two wires touching, or many other small similar mistakes, 
 may cause a short circuit and require its reconstruction. 
 It is not wise to hurry the work, as it will be necessary to 
 reconstruct it if careless. 
 
 The end supports are made out of any suitable piece of 
 wood. The two supports for the secondary are 8" in diam- 
 eter, and from f " to i" in thickness. Eight equidistant points 
 are marked off on the periphery and slots J" deep and \" 
 wide cut at these points. See Fig. 8. These slots are for 
 the fibre strips, on which the secondary is wound, to fit into. 
 These strips are 17" long and \" square and are cut from the 
 best vulcanized fibre obtainable, eight being required. In 
 each end of the strips a hole is drilled and countersunk to 
 recei^'e a small brass screw, which is to fasten them to the 
 end pieces. A wooden rod about i" in diameter is now 
 obtained and a shoulder turned on each end. The diameter 
 of the shoulder is \" and its length just equals the width of 
 
 24 
 
The Oscillation Transformer 
 
 25 
 
 the supports, i.e., |" if f" wood is used, or i" if i" wood is 
 used for the ends. The length of the rod over all must be 
 
 Fig. S. — End Support for 
 Secondary of Oscillation 
 Transformer. 
 
 only 17". See Fig. 10. A h" hole is now drilled in the centre 
 of each of the end pieces. These pieces are now slipped on, 
 
 E 
 
 IT- 
 
 Fig. 9. — Fibre Strip. 
 
 one on each end of the rod. Screw the fibre strips on the 
 supports, after fitting them in the slots and seeing that they 
 are parallel to the rod in the centre. If any of the strips are 
 
 
 "TT 
 
 77" 
 
 
 Fig. 10. — Centre Rod. 
 
20 The Tcsia Coll 
 
 bent or warped they should be straightened before being 
 fastened in place. A good way to straighten them is to lay 
 them between two boards, placing some heavy weight on 
 the top board, and lea\c them thus over night. 
 
 About 4 or 5 ounces of Xo. 28 B. &: S. gauge double cotton 
 covered copper wire is required for the secondary. There 
 are several equally good \\'a\-s in which the wire can be wound 
 on the frame. Two of these methods will be described, as 
 the authors have found them both satisfactory. 
 
 The first method is intended for those that have a lathe 
 at their disposal. A cylinder of wood 4" in diameter and 
 18" long is first turned out, and on it are screwed the eight 
 strips of fibre, so that their ends are in line, and that they 
 strike one another. If the amateur has a couple of clamps 
 made of strips of sheet iron with a bolt through the ends, it 
 will greatly help matters by clamping them around the strips, 
 about 6" apart, and moving them as needed. A light cut is 
 first taken off the strips, and then they are polished with a 
 file and sandpaper. A No. 18 thread is now cut, starting 
 about i" from the end of the strips to within an inch of the 
 other end. It should be cut just as deep as possible. In 
 order to make a clean cut the tool must be \cry sharp and 
 several light cuts should be taken instead of one. When the 
 strips are mounted on the frame again it will be seen that 
 there is a continuous groove in which the wires will lie with- 
 out touching one another. 
 
 The secondary frame is now supported between centres 
 in the lathe so that it just turns easily. Around one end 
 
Till' Oscillalion Transformer 
 
 ~i 
 
 of one of the strips wrap about i' of Xo. 2S wire. This is 
 to be used for connections. Starting at this point wind the 
 wire tightly on the frame, always keeping the wire in the 
 groove cut for it. About i' extra should be left at the end 
 for making the connections. When finished, a hea\y coat 
 of shellac is gi\'en to the wires where they rest on the fibre 
 strips. When this is dry all the wire is heavily shellacked. 
 A soft brush should be used so as not to displace any of 
 the wires. The secondary should now be placed in a warm 
 place to dry. 
 
 The following method can be used in case a lathe is not 
 available. A spool of silk thread and some silk are re- 
 quired. There must be enough silk to make two turns 
 around the secondary frame. It is wrapped on tightl}', and 
 smoothl}'. and shellacked in place. It might be mentioned 
 here that shellac dissoh'ed in wood alcohol should not 
 be used. The wire is then wound on, starting about i" 
 from the end up to within an inch of the other end. A 
 silk thread is wound on at the same time between the 
 turns, to keep them apart, ^^'hen it is all wound the wire 
 is heavily shellacked and the frame put in a warm place to 
 thoroughly dry. 
 
 Get t\vo pieces of J"xi2" pine 12" long and find the centre 
 of each by the intersection of their diagonals. With these 
 points as centres describe two circles, with radii 4I'' and 5^' 
 on each board. Di\"ide the circles ha\-ing the radius 5}" 
 into 36 equal parts. These points can be located with a 
 protractor or mariner's compass, there being one e\ery 10°. 
 
28 
 
 Tlie Tesla Coil 
 
 At each of these points drill a \" hole. In each corner of these 
 boards cut out a piece i"x i". See Fig. ii. 
 
 At a planing mill buy eighteen -]" dowels, 36" long. These 
 may be had for one cent apiece. Each one should be cut 
 in two, which will gwe you thirty-six \" rods, 18" long. 
 Fasten one of the 12" end pieces to each of the ends of the 
 
 Ai 
 
 Sb- ^ holes o 
 
 
 \ 
 
 o 
 
 -5i- 
 
 o 
 \ 
 
 o 
 
 \ 
 
 o 
 I 
 o 
 
 v 
 
 \/, 
 
 
 ^-o' 
 
 ,0^ 
 
 /^' 
 
 
 Fig, II. — End Support for Primary. 
 
 completed secondary frame, by four flat-headed brass screws. 
 See that the circular end pieces of the secondary just fit on 
 the smaller circles drawn on the boards. The ^" rods are 
 now driven into place, thus forming a circular cage on which 
 the primary is wound. 
 
 For the primary get an 8' piece of No. 36 copper ribbon 
 i" wide. AAYap the end of the ribbon once around the end 
 
The Oscillation Transformer 
 
 29 
 
 of the dowel shown in Fig. 12 and solder it in place. 
 A piece of copper wire should also be soldered on here. 
 Starting from here take two and a half turns around 
 
 Fig. 12. — Primary of Oscil- 
 lation TRANSrORMER. 
 
 the secondary frame. This will bring you to the end of 
 the dowel diametrically opposite the dowel from which 
 you started. Wrap the ribbon once around this dowel 
 and solder it in place. A piece of wire should also be 
 
 Fig. 13. — Completed Sec- 
 ondary or Oscillation 
 Transformer. 
 
 soldered on. The turns of the ribbon should be equally 
 spaced. Get four pieces of i"xi" pine 183" long and 
 fasten them as braces from the one end of the frame to 
 the other, the pieces fitting in the corners that were cut 
 out for them. 
 
3° 
 
 The Tcsla Coil 
 
 I I J I I 
 
 - Bushings for 
 Support of Oscillator 
 Standards. 
 
 ^ 
 
 / ~^ z "¥" -e "^ 
 
 4- 
 
 
 Fig. 15. — Hard Rubber Block on Oscillation Transformer. 
 
The Oscillation Transformer 31 
 
 Turn two bushings out of a piece of f" brass or copper rod. 
 They are shown in Fig. 14. The length over all is V' and 
 the shoulder is I" thick. A §" hole is drilled down the 
 centre, and the shoulder is drilled and slightly countersunk 
 at two opposite points to recei\"e two small brass screws. 
 Two pieces of vulcanized fibre or ebonite J"xi'', 2" long, with 
 a 5" hole drilled h" from the end, are fastened to the middle 
 of the top side of the primary frame. The bushings just 
 made are fitted into the holes and screwed in place. The 
 wires from the secondary are soldered onto these bushings. 
 ^^'hen this is done the oscillation transformer is finished and 
 all that needs to be done is to connect it up properly. The 
 completed oscillation transformer is seen in plate III. 
 
CHAPTER V 
 
 THE INTERRUPTER 
 
 Let us now consider that important part of the apparatus, 
 the primary spark-gap. The function of this, as previously 
 stated, is to provide a path of high resistance until the con- 
 denser is charged to its full capacity. Then it suddenly 
 breaks down, and allows the current to surge back and forth 
 across it, until the current is damped out by resistance and 
 other factors in the circuit. After the oscillations have 
 ceased, the ideal spark-gap should return to its maximum 
 disruptive strength before the condenser can be charged by 
 the next cycle from the secondary of the transformer. 
 
 In practice this is far from being the case. The air 
 between the discharging balls becomes heated, and offers 
 a comparatively low-resistance path for the current. This 
 results in an arc being formed, which prevents the condenser 
 from performing its function. The mechanical problem 
 which confronts the designer is to find some way to get rid 
 of this heated air and thus prevent the arc being formed. 
 The object of this chapter is to show several ways of par- 
 tially accomplishing this result, but in no case perfectly. 
 
 The simplest form of spark-gap for the primary of the 
 oscillation transformer, one which has given fair results 
 
 32 
 
Tlte Interrupter ^li 
 
 with the writers, consists of merely two adjustable brass 
 balls. No provision is made for blowing out the arc that 
 forms, so that considerable of the energy of the transformer 
 is wasted. Nevertheless sufficient oscillations are set up 
 to bring the coil to within i" or 2" of its maximum length of 
 spark discharge. 
 
 Most amateur coil builders will, at this point in the con- 
 struction, be very anxious to see what the possibilities of 
 their work are. A good idea of the working value of the 
 apparatus may be obtained with the simple air-gap. When 
 a designer is more acquainted with the workings of a Tesla 
 coil he can construct that one of the spark wipe-outs which 
 is best suited to the current that he has at his disposal. 
 
 THE SIMPLE AIR-GAP 
 
 Two standards of brass J" in diameter and 4" long are 
 mounted on a piece of hard rubber. They should be about 
 5" apart. A -^^" hole is drilled J" from the upper end of 
 each rod and is taped with a standard \" machine screw tap. 
 These two holes must be in line. 
 
 Next two pieces of \" brass rod 3!" in length are threaded 
 for the whole of their length to fit the holes tapped in the 
 standards. Two balls Y in diameter are turned out of 
 brass or are procured from a dealer in physical supplies. 
 These balls are drilled and threaded to fit in the brass rods. 
 Two vulcanized fiber rods are turned out of f" rod, 3" in 
 length, and a \" shield 2" in diameter is screwed on one 
 end of each handle. The handle is drilled and tapped for 
 
34 
 
 The Tcsla Coil 
 
 i" to fit the brass rod on which it is screwed. The shield 
 is to safeguard the operator's hand from sliding off the 
 fibre handle and coming into contact with the transformer 
 current, which would probably be fatal. Connection can 
 be made to the brass standard near the bottom by drilling 
 
 -6 
 
 -*Af- 
 
 an 
 
 !_ 
 
 cl*' 
 
 
 ^ 
 
 e 
 
 -^ J <- 
 
 .f 
 
 c 
 
 IEHIImQ) '• 0/-/'A/-)'/'/-, 
 
 Fig. i6. — Simple Primary Air-cap. 
 
 a small hole through them, and then drilling and tapping 
 another hole at right angles to the first, for a thumb screw 
 to bind the wire. 
 
 A set screw at the top of the standards to clamp the rods 
 in place after they have been adjusted will be a convenience 
 to the operator. See Fig. i6. 
 
The Interrupter 35 
 
 THE AIR-BLAST INTERRUPTER 
 
 In order to make the air-gap more efficient, getting rid 
 of much of the heated gases between the spark terminals, a 
 mechanical means can be used of forcing in cold air, thus 
 driving out the heated gases and keeping the resistance 
 much higher. To efficiently accomplish this a piece of f" 
 hard glass tubing is drawn out into a nozzle having an 
 opening ^\" in diameter. This is mounted on a brass stand- 
 ard and is connected by means of a rubber tube to a foot 
 bellows such as is used in the laboratory to operate a blast 
 lamp or any other suitable supply of compressed air. A 
 good blast of air will effectually wipe out any arc that tends 
 to form, thereby increasing the disruptive length of the bright 
 oscillation transformer discharge. The operator will find, 
 however, that it is a tedious task to pump a foot bellows, 
 occupying so much of his time as to handicap him in per- 
 forming experiments with the high-frequency discharge, and 
 he will soon decide that the best policy is to construct either 
 a magnetic wipe-out or a motor-driven interrupter. 
 
 THE MAGNETIC INTERRUPTER 
 
 The magnetic blow-out is well suited for those who have a 
 source of direct current at their disposal; either the no- volt 
 lighting circuit or a suitable battery current. For those who 
 have only the alternating current and who wish to use the 
 magnetic wipe-out the writers have added to this chapter 
 a simple home-made electrolytic rectifier of their own design 
 
36 The Tesla Coil 
 
 which will give a current suitable for magnetizing the magnet 
 of this interrupter. 
 
 Two standards of the same form as those used in the simple 
 interrupter, but 6" in length and having spark balls I" instead 
 of J" in diameter, are mounted on a hard rubber base 10" x 7". 
 The fibre handles and shields are also necessary for this in- 
 terrupter. Two electromagnet bobbins 5^" long and having 
 an iron core |" in diameter with fibre heads 3" in diameter 
 are procured. An iron yoke made from i"xj" iron 7" long 
 has 2 holes drilled in it in the middle i^" from both ends, and 
 the bobbins are fastened to it by a screw in the core. Two 
 polar pieces of f " square iron, filed into an egg-shaped point 
 are screwed to the upper ends of the core. The bobbins 
 are wound full of No. 14 B. & S. gauge cotton-covered magnet 
 wire, if they are to be operated on a battery current of from 
 eight to ten volts such as a plunge battery. If a direct cur- 
 rent of 1 10 volts is available No. 24 should be used. If the 
 rectifier described at the end of this chapter is used No. 22 
 wire should be used as the voltage of the rectified current on 
 no volts alternating is about 90 volts. The magnets must 
 be thoroughly saturated in order to give the best results. 
 
 After the magnets are finished they should be fastened to 
 the hard rubber base, at right angles to the rods carrying 
 the spark balls, by two screws through the iron yoke. The 
 balls of the spark-gap should be between the projections 
 of the magnet. A sheet of mica is bent around the polar 
 projections of the magnets in order to prevent the spark 
 from jumping to the cores of the magnets. Fig. 17. 
 
The Interrupter 37 
 
 The ends of the coils are so connected that the current 
 will traverse them in opposite directions. The outer ter- 
 minals are brought out to suitable binding-posts and the 
 interrupter is finished. 
 
 Fig. 17. — ^Maonitic Inieehupter. 
 
 The principle of this piece of apparatus is based on Da\'y's 
 experiment in which he found that the electric arc is extin- 
 guished upon the approach of a magnet. 
 
 THE MOTOR-DRIVEN INTERRUPTER 
 
 This interrupter is the one the writers used in their earlier 
 experiments with the 7" standard coil described in the latter 
 part of this book. It consists essentially of a fan motor 
 run on the alternating current at no volts, driving a brass 
 disc having a number of projections bolted around its 
 face, and a brass oscillator so mounted that the distance 
 separating it from the projections on the disc can be varied 
 at will. The motor may be of any suitable design that the 
 builder may possess. A small battery motor rimning on 
 direct current can be pressed into service if the amateur does 
 
38 
 
 The Tesla Coil 
 
 not care to go to the expense of purchasing a fan motor or 
 has not the facihties for building one. The directions for 
 building a suitable induction motor are given at the end of 
 this chapter. 
 
 /bbei" 
 
 Fig. i8. — Motor Interrupter Fan. 
 
 To make the disc for this interrupter turn out of \" sheet 
 brass a circular piece lo" in diameter. If no lathe is avail- 
 able it may be procured at a model maker's quite reasonably. 
 Lay off on its face two concentric circles, 8" and 9}" in diam- 
 eter respectively. Di\ide the inner of these into thirty 
 equal divisions and draw radial lines from the centre of the 
 disc through each of the points marked o£F, thus dividing 
 
The Interrupter 
 
 39 
 
 the outer circle into the same number of equal divisions. 
 Drill a hole through each of the points laid off on both 
 circles and tap them to jQt a standard 4-36 machine screw. 
 A number of brass angle pieces made by bending ■j'g" brass 
 
 I, coontersunk 
 
 ->i^ 
 
 Brass Angle Piece. 
 
 into the form shown in the figure are procured. Two holes 
 are drilled and tapped in each one to fit a standard 4-36 
 flat'-headed machine screw. These pieces are screwed to 
 the brass disc with j" screws. 
 
 A \" hole is drilled in the centre of the disc and three ig" 
 holes are drilled on a circle having a radius of |". Next 
 
40 
 
 The Tesla Coil 
 
 turn out a circular block of hard rubber 2" in diameter and 
 the same shape as in Fig. 20. The brass disc is screwed 
 to it with three brass wood screws {" long and the whole 
 is fastened to the shaft of the motor so as to be well 
 insulated from it. To mal<:e electrical contact with the brass 
 
 & % I 
 
 Fig. 20, — Hard Rubber Block. 
 
 plate a brush to bear on the back near the centre is cut out 
 of a piece of y^" sheet spring brass. This piece should be 
 10" long and Y wide. It is mounted on a piece of hard 
 rubber with a suitable binding-post, so as to press against 
 the back of the disc. 
 
 For the other side of the spark-gap a standard mounted 
 on hard rubber similar to the one described for the simple 
 spark-gap is used, but instead of being 4" long it is 6" in length 
 
The Interrupter 
 
 41 
 
 and the brass ball is best if it is slightly less than \" in 
 diameter. The fibre handle and shield are necessary in 
 order to adjust the length of spark-gap while the coil is in 
 operation. 
 
 r 
 
 — 
 
 1 
 
 
 ^ f"^ 
 
 Vl'BE'R BLOCK 
 
 A 
 
 ^ 
 
 ^ 
 
 =rs S^-^^J 
 
 ^a ir-dSJ 
 
 Z^_ 
 
 A 
 
 COM e.K 
 
 Fig. 21, — Section of the Motor Interrupter. 
 
 The spark takes place between the brass ball and the pro- 
 jections on the disc. As there is a considerable air current 
 set up by the rapid rotation of the disc, very little arc will 
 form across the spark-gap. 
 

 
The Interrupter 43 
 
 A SiLVLL SELF-ST.\RTIXG SDsGLE-PHASE IXDUCTIOX 
 MOTOR 
 
 To build a motor, to run on the single-phase alternating 
 current of no volts, suitable for running a fan interrupter, 
 is perhaps ver}' difficult. The builder will require more 
 tools and a much greater knowledge of machine shop prac- 
 tice to construct an efficient motor than to build all of the 
 parts of Tesla apparatus combined. For those who have 
 had but litde experience in motor construction, the writers 
 suggest that the amateur purchase an alternating-current fan 
 motor or a suitable direct -current batter}- motor. The 
 following description of the building of this motor is given 
 in order that this book may be complete in itself and so that 
 the coil builder will have all the necessary data to build the 
 complete apparatus without reference to other works. 
 
 The first step in the construction is to make the necessary 
 patterns for the base and yokes. There are two castings 
 required. The base supporting the punchinss for the stator 
 is cast directly on the standard which supports the motor. 
 The drawint;. Fig. 23. will give the required dimensions. 
 
 It is assumed that the amateur pattern-maker is aware 
 that an iron casting is smaller than the pattern from which 
 the mould was made, therefore shrinkage must be allowed for 
 in the pattern in order to be sure that the casting will be large 
 enough. One eighth of an inch to the foot is about the proper 
 amount. 
 
 Turn out a circular piece 5 " in diameter and i" in thick- 
 
44 
 
 77/r 'Icslii ('nil 
 
 ncss. Then a rod i" in diameter, and 6" long, swelling into 
 a graceful enlargement at the lower enrl, is turned out and 
 fastened with glue and nails to the centre of the < in iilar disc. 
 
 
 
 1- -,'^ - 
 
 -^ 
 
 D-^ 
 
 for jiUUifr heads 
 
 I'lO, 23, — PAn;.i':.'s oi' YoKE. 
 
 A piece of wood is cut out of ;" stock to the form shown in 
 the figure. The radius of curvature of the arr must be 2\" 
 so that the stator will fit it accurately. This jjiere is glued 
 and nailed to the top of the upright. The whole pattern is 
 
The Inlcmipter 
 
 iliilllllllilMI'Hliii 
 
 45 
 
 A 
 
 Fig. ;:4. — Section of Completed Motor. 
 
46 Tlic Tcsla Coil 
 
 gi\cn two coals of l)csl shellac \'arnisii, containing suOicicnt 
 lampblack to make it jet black. 'I'his tompktt's llic pattern 
 for the base. In order to pro\i(le a su|)porl for (he bearing 
 of the rotor shaft we must make a pattern lor a yoke. 
 
 To make this pattern we turn out of \\" sloik a circular 
 disc 5I" in diameter and of the same form as in the drawing. 
 Two castings are made from this palleni, one to lit each end 
 of the stator. They not only ser\e to furnish bearings for 
 the rotor, but also to enclose the entire motor and thus keep 
 out moisture and dust. 
 
 Vie. 25, — Rotor Disc, 
 
 After the patterns arc shellacked they should be sent to a 
 foundry where the castings can be obtained quite reasonably. 
 It requires one casting of the base and two of the yoke or 
 journal. When the castings arc obtained they should be 
 chipped and all the roughness filed off. 
 
 The rotor consists of a number of iron discs 2\" in diameter 
 
The Interrupter 47 
 
 and having twelve f " holes drilled around the edge and a 
 j^g" hole in the centre. They should be made of iron about 
 j^|§Q-" in thickness; about forty of them making a pile i" 
 high. These discs can be made by the coil builder wfith the 
 help of a lathe and drill press, or they can be obtained already 
 stamped out from any of the large dealers in electrical sup- 
 plies. A sufficient number of them are mounted on a shaft, 
 turned down from a f" rod of cold rolled steel to the size 
 shown in the figure, to make a pile 2v" in height. As the 
 motor will not have a very heavy load thrown on it, it will 
 not be necessary to key them to the shaft. A good driving 
 fit is sufficient to keep them from turning. They can be 
 clamped in position by the nut shown in the figure. The 
 conductors consist of twelve s" copper rods 2f" long. One 
 of these rods should be driven in each of the holes around 
 the edge of the discs and should project J" beyond them on 
 both sides. To short-circuit them, two heavy rings made 
 by bending two pieces of \" copper rod into a circle having 
 an outside diameter of if" are soldered to the ends of the 
 rods. Use sufficient solder in making these connections 
 in order to prevent heating at the connections by the induced 
 current in the rotor. This completes the rotor. 
 
 The next thing to consider is the stator punchings. In 
 this case they will not be punched out, but will be cut out 
 on either a shaper or milling machine, or cut out by hand 
 after as much metal has been removed as is possible by 
 drilling. The diameter of these discs is given in the draw- 
 ing on Fig. 27. To make them, cut roughly out of tHtt" 
 
48 
 
 The Tesla Coil 
 
 hi 
 
 %k 
 
 -%%. 
 
 "T 
 
 «)i» 
 
 I s ■ 
 
 o 
 
 o 
 
 ^ 
 
The Interrupter 
 
 49 
 
JO The Tesla Coil 
 
 iron about loo pieces 6" in diameter, with a pair of snips. 
 A I" hole is drilled in the centre of them and the whole number 
 are clamped on a mandrel between two nuts and turned 
 down in a lathe to si". Next four Y' holes are drilled as in 
 the figure, and a fibre tube i" in external diameter and J" 
 internal diameter is driven in each hole. A J" stud 3I" in 
 length, with hexagonal nuts and f" iron washers, binds the 
 discs together. After tightening up the nuts the bolts can 
 be slightly riveted to guard against possible loosening. The 
 whole is clamped in a chuck and the centre is bored out to 
 2f" in diameter. Next the slots are cut as in the figure, 
 either on a milling machine or shaper or by the use of a hack 
 saw and file. A large bulk of the metal can be removed by 
 drilling. 
 
 When the stator is complete it is mounted on the pedestal 
 with four cap screws which screw into the bottom edge. 
 The two ends are fastened to the stator with four l" fillister 
 screws i" long. The holes for these screws are drilled mid- 
 way between the nuts binding the stator discs together. In 
 order that the heads fit up against the stator four holes should 
 be drilled to allow the nuts to project into the heads. 
 
 The rotor is next wrapped with paper until it just fits 
 into the stator and the heads are bolted on in the way they 
 are to be permanently. It is well to mark them so that they 
 can always be put back in the same place. Then the space 
 between the shaft and the journal is filled with the best 
 grade of Babbit metal obtainable. Cardboard washers 
 slipped over the shaft prevent the babbit from running out. 
 
The Interruptef 
 
 51 
 
 The next thing to do is to wind the stator coils. The wire 
 used is No. 22. The coils are wound in a wooden frame of 
 the size shown in Fig. 28. After the coils are wound they 
 are wrapped with tape, shellacked, and allowed to dry by 
 thoroughly baking. Before the coils can be put into place, 
 means must be provided for making the motor self-starting. 
 This is accomplished by means of a short-circuited copper 
 
 ' 
 
 
 
 
 ~] 
 
 i 
 
 32 
 
 
 
 
 
 ^e 
 
 .1. 1 
 
 e 
 
 \ 
 
 
 •4 
 
 
 
 
 
 T 
 
 
 I 
 
 V 
 
 ■;tf -I, 
 
 ^ 
 
 tape 
 
 1 1 1 1 (( / 1 1 rj 
 
 7 layers 14 turns 
 each of No. 22 B&S 
 D.C.C. Cupper wire 
 
 ZTHunrs 
 
 + 
 
 /J? Finished stator coil showing outside dimensions 
 
 f'^^^'^-i Note:- About 65 feet of No. 22 B & S guage 
 
 ^^tapered 
 
 ± 
 
 ± 
 
 D. C. C. copper wire is needed for each stator coil. 
 
 ■^18 
 
 Frame on which stator coils are wound. 
 
 Fig. 28. — Fkame for Stator Coils. 
 
 conductor lying in the grooves marked "A" in the drawing 
 of the stator punchings, Fig. 29. This conductor consists 
 of a piece of No. 14 bare copper wire bent into the form of 
 a rectangle, so as to fit around the one half of the polar 
 projections of the stator. The two ends are soldered 
 together. A glance at the figure will make this clear. 
 
 The coils are next slipped into place over these short- 
 circuited conductors. The terminals of the stator coils are 
 so connected as to induce opposite poles in adjacent polar 
 
52 
 
 The Tesla Coil 
 
 pieces. The six coils are in series, the end terminals being 
 brought out to suitable binding-posts fastened to the end 
 pieces and suitably insulated from it. A coating of black 
 paint completes the motor. 
 
 Although the induction motor is a constant-speed motor 
 at varying loads, we can secure some slight speed regulation, 
 
 4- 
 
 Short circuited cortduc/br 
 of Wo, 14- Bare Cof>(>er Wi>«, 
 showing €haf]e of wire 
 bff[ore alijj^in^ into slots 
 
 Fig. 29 — Self-starting Device. 
 
 which is a great advantage in operating the coil, by inter- 
 posing a liquid resistance in series with the motor. This 
 resistance consists essentially of any suitable glass jar, such 
 as is used in a Daniell or Gra\'ity cell, ha\'ing two metal 
 plates suspended in an electrolyte, so that the distance be- 
 tween them can be varied at will. Copper sulphate is gen- 
 erally used as the electrolyte. 
 
Plate I\'. — Motor-driven Interrupter. 
 
 Plate \'. — The Electrohnic Rectifier. 
 
The Interrupter 53 
 
 AN ELECTROLYTIC CURRENT RECTIFIER 
 
 It was only at the last moment that the authors decided 
 to make public the results of their experiments on an electro- 
 lytic current rectifier, which has proven highly satisfactory. 
 Its advantages are that it is easily and cheaply built, it re- 
 quires only slight attention, its efficiency is very high, and 
 the current which a small set will rectify is very large. 
 
 To a great many it may seem out of place in putting in 
 this description of a rectifier, which is entirely foreign to 
 the Tesla apparatus. The reasons for so doing, however 
 appeared, to the authors at least, sufficiently great, for if 
 the amateur constructs the magnetic wipe-out he will need 
 a source of direct current at about 80 or go volts pressure, 
 since this current can hardly be obtained from the lighting 
 circuits which are generally alternating or from batteries. 
 Then having this source of direct current he will be able 
 to substitute a D. C. motor for the induction motor de- 
 scribed in this chapter. 
 
 The greater number of rectifiers now on the market use 
 the method of choking out the one half of the alternating- 
 current wave and it is to this fact that their low efficiency 
 is due. The high efficiency of the apparatus devised by the 
 authors depends on what might be called the alternate path 
 connection or method; that is, there are two paths for the 
 current to traverse, one of enormous resistance and one of 
 very low resistance. 
 
 The idea of this form of rectifier came to the authors in 
 
54 The Tesla Coil 
 
 the following way. They were experimenting on some 
 cathode tubes of peculiar construction, using a 12" induction 
 coil. The current from the secondary of this coil is oscil- 
 latory in character, of course. It was observed that the dis- 
 charge through the tube was ^-ery unsteady, especially when 
 the interruptions were not very rapid. A line of experi- 
 ments was carried out to determine the cause of this unusual' 
 effect, with the result that the resistance was found to be 
 enormously greater for currents in the one direction through 
 the tube than in the opposite direction, due entirely to the 
 difference in the forms of the two electrodes. 
 
 After discovering this fact they wondered if some electro- 
 lytic cell might not be made which would possess the same 
 properties and could be used to rectify the ordinary alter- 
 nating currents. From a previous study of the effects of 
 various electrodes on the electrolysis of certain solutions we 
 arrived at se\'eral cells which exhibited these properties to 
 a marked extent. 
 
 It was found that an aluminium electrode was the essen- 
 tial thing in every cell, together with some acid salt capable 
 of forming an oxide with aluminium. The other electrode 
 might be any conductor unaffected by the solution. 
 
 Some of the conductors suitable for the other electrode 
 are iron, carbon, and lead, and the following solutions all 
 gave more or less satisfactory results. Acid sodium car- 
 bonate, acid sodium phosphate, acid potassium tartrate, 
 potassium alum, and in fact most of the ionizable, slightly 
 acid sulphates, carbonates, tartrates, and phosphates. 
 
The Interrupter 55 
 
 By merely putting one of these cells in the circuit, the one 
 half of the alternating current wave may be choked out. 
 But this method gives an efficiency of less than 50%. Thus 
 the authors were led to devise the alternate-path method. 
 
 Before describing this method, however, we will take up 
 in detail the properties of a single cell. After a current is 
 passed for a few minutes through one of the cells, a coating 
 of oxide is formed on the aluminium electrode which is prac- 
 tically a non-conductor. While this does not prevent the 
 difference of potential from being maintained across the cell, 
 it does prevent the ions from giving up their charge and in 
 this way it acts like a polarized copper plate in a single gal- 
 vanic cell. This condition of enormous resistance exists 
 when the aluminium is the anode. When on the reversal 
 of the current the aluminium becomes the cathode there is 
 merely the resistance of the electrolyte encountered. Any 
 cell possessing this property is called asymmetric. 
 
 As stated before, a single cell by being merely put in the 
 circuit would choke out the one half of the alternating wave, 
 but as this gives an intermittent current, the following is 
 the method devised by the authors. Three cells are needed 
 in all. Two of these consist of one electrode of aluminium 
 and one of iron, with a solution of sodium acid carbonate. 
 The third has two aluminium plates and one iron plate 
 between them. The same solution is used. 
 
 On looking at Fig. 30 it will be seen that when E is positive 
 the current can flow from either plate 2 or 3 across the elec- 
 trolyte to plates I or 4. The path from 3 to 4 is of enormous 
 
56 Tlic Tcsla Coil 
 
 resistance, as the aluminium is the anode, but the path from 
 I to 2 is of low resistance and hence the current takes this 
 path. AA'hen H becomes positive the current can flow from 
 6 to 7 or from s to 4. It takes the path from 6 to 7 as this 
 is of low resistance. In this way both waves of the alternat- 
 ing current are used and the only loss is due to the resistance 
 of the electrolyte. 
 
 Thus the direct current from this set has a sine wave form, 
 in which all the negative values in the alternating have been 
 made positive. 
 
 The following are plans for a rectifier suitable for use 
 directly on the no- volt-alternating-current light mains. The 
 rectifying cells have glass containing jars. The jars are all 
 7" X 6" X 4" inside measurements. The aluminium plates 
 are cut out of -J" sheet and are all the same size and shape. 
 They are 5" x 7" and four are required. The aluminium 
 should be comparatively pure to prevent deterioration of 
 the plates due to local action. If the plates are to any 
 extent impure the cells may fail to work, and if they do 
 rectify it will be at a very low efficiency. The iron plates 
 are cut from I" sheet. Two of them are the same size as 
 the aluminium plate and the third is 8" x 5". This larger 
 plate is to be used in the middle cell. The necessity for 
 making it larger is that it goes between two aluminium 
 plates and the extra length is required to fasten the bind- 
 ing-post to. The plates are held three eighths of an inch 
 apart in the following manner: Out of some f" sheet vul- 
 canite or hard rubber (fibre must not be used as it swells in 
 
TJic Interrupter 57 
 
 water) cut four strips .1" x 61". Also cut out four washers 
 about V' in diameter and a number of pieces h" square. 
 These latter pieces are drilled and tapped to fit a standard 
 \" thread. The washers have a -}" hole drilled in them. 
 Some \" vulcanite rod is cut up into about 2" lengths and 
 threaded to fit a \" nut. With these strips and washers the 
 plates are held the required distance apart and the bolts 
 firmly fasten them together. The strips of rubber are used 
 across the tops of the plates and the washers at the bottom. 
 See Fig. 30. This method of using hard rubber bolts and 
 nuts is far superior to using iron ones and fitting them with 
 an insulating bushing. A binding-post is fastened at the top 
 of each one of the plates. As the strips across the top of 
 the plates are longer than the jars are wide; when the 
 electrodes are put in place they will be suspended in the 
 solution by the strips resting on the edges of the jars. 
 
 The electrolyte, if sodium acid carbonate is used, should 
 be a saturated solution. Other solutions than this can be 
 used, although the authors obtained the best results with 
 this one. Besides it is about the cheapest of all the possible 
 electrolytes. 
 
 In selecting an electrolyte the following factors must be 
 taken into consideration. It must have low resistance, it 
 must be a stable compound, and when no current is flowing 
 it must not attack the aluminium plate and only slightly 
 attack it when current is passing. 
 
 To make the set con\'enient to handle the jars should be 
 mounted in a wooden frame, with the cell containing the 
 
58 
 
 The Tcsla Coil 
 
 A.c. :; 
 
 -->fi'bai* wojW 
 
 
 D.C, 
 
 r lauRE. ehowr'tig faos'ition o£ 1fia ce)/5 and eledrtdcs 
 
 Fig 30. — Rectifier Plates and Wiring Diagram. 
 
The Interrupter 59 
 
 two aluminium electrodes in the middle. The connections 
 are shown in the figure. 
 
 With this electrolyte the aluminium plates will form in a 
 few minutes, it being merely necessary to short circuit the 
 D. C. taps with a resistance and allowing the rectifier to 
 take a full load current. 
 
 The efficiency of the apparatus will be somewhat increased 
 by using a cooling worm, as the electrolyte when cool has 
 the greatest current density. 
 
 It will be necessary to renew the electrolyte at intervals, 
 depending on the use the set is given 
 
CHAPTER \T 
 THE CONSTRUCTION OF THE BOXES 
 
 The next thing to consider in the building of a Tcsla coil 
 is the boxes which contain the transformer and high-tension 
 coil. One box for transformer, condenser, and high-tension 
 coil might be used, but for a coil of this size the weight would 
 be objectionable. Two separate boxes give the ideal result. 
 They ha\'e the advantage of not being too bulky to handle, 
 and the transformer in this form can be used separately, if so 
 desired. A single box, however, has the advantage of taking 
 up less room and of having all the high-potential connections 
 inside where they are safe, except those which lead to the 
 interrupter which is placed on top of the box. 
 
 Oak makes the most substantial box, but it is harder to 
 
 make tight, owing to the fact that the shellac \'arnish which 
 
 is used for filling up the pores in the wood does not sink 
 
 into oak with the same readiness as it does in a softer wood. 
 
 Pine is the best material to use as the joints will require 
 
 considerable filling up to make them imper\-ious to paraffine 
 
 oil, which will soak through almost anything in time. Sugar 
 
 pine may be readily stained and looks very neat when \-ar- 
 
 nished. 
 
 For the sides and ends of the transformer box it requires 
 
 60 
 
The Conslniclion of the Boxes 
 
 6i 
 
 a piece of straight-grained pine free from knots, ij" x lo", 
 6" long. The bottom should be made of a piece of i J" x 14", 
 26" long, and the top of a piece of i"x 11", 2' long. Cut the 
 pieces to the size shown in Fig. 31, and plane the edges true. 
 The end pieces must be mortised into the sides i" from the 
 end. These tongues and grooves may be cut with a saw 
 and chisel if a rabbeting plane is not at hand. After the sides 
 
 TOP l/IEW-Tiivy rm^vtj 
 
 Zoi" 
 
 -i'i' 
 
 iA 
 
 -iSif' 
 
 COVER 
 
 '®l 
 l@| 
 I® 
 I® 
 
 I® 
 
 W 
 
 I® I 
 
 zsi- 
 
 I®! 
 i®| 
 I©' 
 i@| 
 i@| 
 
 T 
 
 0000 
 
 SIDE VIEW ' END VIEW 
 
 Fig. 31. — Teansfokmer Box. 
 
 and ends are finished the tongues and grooves are given a 
 heavy coat of shellac, which has been dissolved in grain alco- 
 hol, and while still wet are put together. Six long brass 
 screws with round heads are to be used in each board to hold 
 the sides. A brass washer Y l^i diameter should be placed 
 on the screw to prevent the head from sinking into the wood. 
 Next the edges should be gone over with a plane if neces- 
 sary so that the bottom board will fit flush in all places. The 
 
62 TJic Tcsla Coil 
 
 bottom board is to be i" wider than the width of the box, so 
 that it laps over V' on each side. \\'\\tn the bottom board 
 is cut to size, the edges are rounded off with a plane to gi\-e 
 a finish, and then it is fastened to the box with long ilat- 
 headed brass screws, placed every four inches along the sides 
 and ends. A coating of shellac is given to the edges of the 
 box just before putting the bottom on, to help make it tight. 
 The screws must be forced in until they are flush with the 
 wood. 
 
 Next the inside should be given five or six coats of shellac, 
 paying especial care to get it into the joints, and allowing 
 each coat to dry before applying the next. 
 
 A small brass cock in the end near the bottom is a conven- 
 ience in emptying the box of its oil, but the labor of putting 
 it in so that the box will not leak is such that a siphon is 
 quick enough for an occasional emptying of the oil. 
 
 The box for the high-frequency coil and condenser must 
 have the same care taken in its construction as in the case 
 of the transformer box. The dimensions are given in the 
 working drawings in Fig. 32. A partition is put in between 
 the condenser and oscillation transformer, but several holes 
 should be bored in it near the bottom to allow of the free 
 circulation of the oil. This box must also have several 
 coats of shellac, as the insulating oil used will leak through 
 in spite of all the precaution taken. 
 
 After the boxes are finished they should be stained or 
 varnished to suit the taste of the builder. Walnut stain 
 looks well, and as it is dark it covers up a multitude of faults 
 
The Construction of the Boxes 
 
 63 
 
 in the wood working. If the boxes are well made a good 
 ciling followed by several coats of shellac makes a very good 
 finish. 
 
 TOV VIEW COVER REMOVED 
 
 .1 
 
 h— /?■ 
 
 "?+= 
 
 COVER 
 
 ii 
 
 J6i 
 
 :=F^ 
 
 :^i 
 
 -36?- 
 
 
 
 000 
 
 
 SIDE VIEW 
 
 END VIEW 
 
 Fig. 32. — High-tension Box. 
 
 Everything is now ready for the assembling of the parts, 
 which will be taken up in the next chapter. 
 
CHAPTER VII 
 
 ASSEMBLING 
 
 It is not wise to hurry when assembling the apparatus, 
 for if the high-tension wires are not properly insulated, 
 brush-discharge effects will be noticed on operating. In 
 nine cases out of ten poor insulation will result in punc- 
 turing his condenser and probably burning out his trans- 
 former. Care should be taken to follow these directions. 
 
 First mount the transformer in its box. After lowering 
 the transformer into its box bring its four primary leads to 
 four heavy binding-posts on the end of the box. The two 
 inner terminals of the two sections are brought to two 
 adjacent binding-posts, and the two outer ones to the other 
 two, in such a manner that connecting electrically the two 
 middle binding-posts puts the sections in series and short- 
 circuiting the two outer pairs throws the sections in parallel. 
 See the diagram. This is accomplished by means of a 
 piece of fiat brass with slots filed in it so that it just fits 
 across two binding-posts, or by a short piece of brass rod 
 which fits in the holes of the binding-posts. 
 
 The secondary terminals are soldered directly to two brass 
 
 rods f" in diameter and 3" long, which extend through the 
 
 opposite end of the box for ij". These rods are insulated 
 
 64 
 
Assembling 
 
 65 
 
 by two heavy hard rubber or fibre bushings made as fol- 
 lows: A 2" piece of hard rubber or fibre rod 2" in diameter 
 is turned down to iV', except for a i^" flange on one end the 
 full diameter of the rod. A §" hole is drilled down the centre 
 of the bushing. The bushinsrs should be a driving fit both 
 
 
 Fig. 33. — Connections for Primary of Transformer. 
 
 in the end of the box and over the brass rod. The rods 
 are 6" apart and 8" up from the bottom of the box. This 
 brings them well above the level of the oil, thus assuring no 
 leakage at this point. 
 
 The leads from the sections should be led to the brass 
 rods through glass tubing bent in the desired shape. A hole 
 is drilled and tapped in the end of each rod to fit a standard 
 
66 
 
 The Tesla Coil 
 
 brass set screw. Another hole is drilled at right angles to 
 the first about \" from the end of the rod to meet the first 
 hole. This makes an efficient binding-post to hold the con- 
 ductor. 
 
 This finishes the connections on the transformer, which 
 can now be placed in the position which it is to occupy. The 
 box is filled with enough pure paraffine oil to cover the trans- 
 former. This oil should be of the best quality obtainable, 
 free from moisture and impurities, such as is used for insu- 
 lating purposes. It should be allowed to soak into all the 
 sections for 24 hours before using. The primary terminals 
 are connected to a source of alternating current by means 
 of a suitable switch and fuse capable of carrying 30 amperes. 
 
 The next step is putting in the connections and terminals 
 in the high-tension box. 
 
 I 
 
 FiG. 34 
 
 - - - h- - - 
 
 ■ High-tension Bushing, 
 
 I 
 
 Three brass rods similar to those used in the transformer 
 box, with the same form of bushings, are driven through holes 
 in the end of the high-tension box next to one end of the 
 oscillation transformer. These holes should be about 6" 
 from the bottom and 3" apart. The brass rods project 
 
Assembling 67 
 
 I" williin the box. The bushings and rods can be made oil 
 tight by giving them a good coating of Le Page's glue before 
 driving them into place. 
 
 A 2" strip of wood is glued on in the lower inside end of the 
 box below these rods, to prevent the end of the oscillation 
 transformer from coming in contact with them. 
 
 i!.;.c: 
 
 <J} 
 
 Fig. .35, — Oscillators and Standards. 
 
 The condenser is first lowered into place in the box and a 
 
 wire is run from one terminal to the nearest outer brass rod, 
 
 to which it is soldered. The wire should be enclosed in a 
 
 ' *^ glass tube suitably bent and should follow the lower edge of 
 
 the box. A wire is run from the other terminal of the con- 
 
68 
 
 The Tesla Coil 
 
 denser in a similar manner to the other outer brass rod. It 
 follows the other lower edge of the box. A tap wire is led 
 from this conductor at a point opposite to where one terminal 
 of the primary band of the oscillation transformer is to be. 
 A wire is also soldered to the middle brass rod. The oscilla- 
 tion transformer is now lowered into place. Tlie tap wire 
 from the condenser lead is soldered to one end of the primary 
 band and the wire from the middle brass rod to the other end 
 of the band. These leads should be run in glass tubes and 
 as directly as possible. They should also be kept under the 
 oil. The connections are shown in Fig. 36. 
 
 o 
 o 
 
 Fig. 36. — Wiring Diagram. 
 
 After all connections have been securely soldered the box 
 is filled with oil, so that the entire apparatus is completely 
 immersed. 
 
 The oscillators and standards can now be constructed. 
 Two fibre or hard rubber bushings 2]" in diameter and i.\" 
 in length, and having a flange ]" thick and 3" in diameter 
 turned on one end, are set in two holes cut in the co\'er 
 directly above the holes in the brass bushings on the oscil- 
 lation transformer. A J" hole is drilled through the centre 
 
Assemhling 69 
 
 of each bushing. Two J" brass rods 10" long are enclosed 
 in fibre tubes f" in outside diameter and gY' lo^ig- The 
 tubes should fit the rods tightly. The ends of the brass 
 rods project from the fibre and can be slightly tapered to fit 
 the bushings on the oscillation transformer. 
 
 The oscillators consist of two brass balls i" in diameter 
 screwed on the end of two ^f/' brass rods 12" long, which 
 are to slide easily in two holes drilled \" from the top of the 
 standards through both the fibre and the rod. A set screw 
 at the top of each standard will be of convenience in clamping 
 the rods in any desired position. 
 
 In order that the discharge gap may be adjusted while 
 the coil is in operation two vulcanite handles f" in diameter 
 are screwed on the ends of the rods, carrying the oscillators, 
 for about li". 
 
 Slide the standards through the bushings in the cover 
 until the rods make good contact with the bushings on the 
 oscillation transformer. This completes the connections in 
 the second box. It should now be placed in its final 
 position at the high-tension end of the transformer, leav- 
 ing a space of about i^' for the interrupter between the 
 two boxes. 
 
 The particular form of interrupter which has already been 
 built is connected to the binding-posts of the high-tension 
 coil as in the wiring diagram. These leads and those from 
 the transformer to the high-tension box should be of No. 
 12 B. & S. gauge hard copper wire and enclosed in the heavi- 
 est glass tubing obtainable. They should be as straight and 
 
yo The Tesla Coil 
 
 as short as is consistent with safety in operating the pri- 
 mary spark-gap. 
 
 A suitable panel switchboard, with the necessary fuses, 
 transformer and interrupter sv/itches, makes a desirable 
 acquisition to the apparatus, but it is not essential. This 
 matter is left to the taste of the individual worker. 
 
 When the connections have all been made and the oil 
 has dri\en out all the air that it can, open the interrupter 
 gap about i" and cautiously close the transformer switch. 
 If no excessi\'e load is taken by the transformer as manifested 
 by the 30 ampere fuses or an alternating-current ammeter 
 in series, if one is obtainable, the spark-gap can be slowly 
 closed until the condenser discharges across it. Then, if 
 the directions have been carefully followed in building the 
 apparatus, a heavy, bluish white, snapping discharge of over 
 12" in length will pass between the oscillators, upon the 
 further adjustment of the interrupter gap. 
 
 When the transformer is used on 55 volts the primary 
 sections should be connected in parallel and for no volts 
 in series. If the sections on the primary were connected in 
 parallel on no volts, the voltage output of the secondary 
 would be about double what it ought to, and hence the con- 
 denser may puncture. 
 
 If by any chance a discharge should fail to take place, the 
 fault may be due to several things. In most cases it will 
 be due to the fact that the sections on the primary or secondary 
 of the transformer are connected in opposition. To deter- 
 mine whether the transformer is working satisfactorily or 
 
Assembling -ji 
 
 not, disconnect it from the rest of the apparatus and see if 
 an arc discharge of at least 6 or 8 inches can be drawn out 
 bet\\"een two electrodes. This arc is generally of a yellow 
 color and easily extinguished by any draught of air. If 
 you do not obtain a 6 or 8 inch arc test the sections to see if 
 they are connected in the right manner, and if they are and 
 no arc still results, which is highly improbable, then some 
 error has been made in the construction. 
 
 If the fault does not lie in the transformer, it is most likely 
 that it lies in the condenser; To test this connect the trans- 
 former up with the condenser and s;e if a condenser dis- 
 charge, determined by its bluish white color, can be obtained 
 in the primary spark-gap. If there is none obtained, your 
 condenser is most likely short-circuited or even punctured, 
 which can only be remedied by its reconstruction. 
 
 The next place to look for trouble is in the oscillation 
 transformer. Ring out by means of a magneto the primary 
 and secondary circuits to see that there are no open circuits, 
 and then see if there is any short circuit bet«'een the primary 
 and secondar}-. If these parts are all right the fault may be 
 due to poor insulation, in having the turns of the secondary 
 touching. The remedy is ob^■ious. 
 
 Finally look and see that aU the electrical connections are 
 as they should be, then the apparatus cannot fail to discharge 
 over a 15" air-gap. 
 
CHAPTER VIII 
 THE THEORY OF THE TESLA COIL 
 
 Although, in the introduction, the authors stated that 
 they would not attempt to give a mathematical explanation 
 of the coil, still they feel that a few facts regarding the theory 
 would not be out of place here, in that it may suggest certain 
 improvements to the reader. It will also be of assistance if 
 the amateur wishes to construct a coil of his own design. 
 
 The first thing to consider is the transformer. Its action, 
 as is well known to almost everybody, depends on electro- 
 magnetic induction. The alternating current flowing in the 
 primary sets up an alternating magnetic field, which being 
 linked with the secondary induces an electromotive force 
 in it. When the secondary is open there is theoretically no 
 current passing through the primary, due to its high self- 
 induction, except that necessary to magnetize the core. 
 As a load is thrown on to the secondary, the current through 
 the primary automatically adjusts itself as the self-induction 
 is decreased by the opposing ampere turns of the second- 
 ary, that is, if the transformer is self-regulating for vary- 
 ing loads. 
 
 The normal current through the primary of the transformer 
 
 used in the 12" coil is from 22 to 25 amperes, the secondary 
 
 72 
 
Theory of the Coil 73 
 
 voltage being about seventy-five hundred, and thus the amper- 
 age in the secondary is about -3 oT- 
 
 To get the required voltage of seventy-five hundred in the 
 secondary on fifty-five volts in the primary it is necessary 
 to connect the two sections of the primary in parallel, as 
 this has the effect of cutting the .primary turns in two. On 
 one hundred and ten volts the sections are connected in series. 
 
 The use of the transformer in the Tesla apparatus is 
 merely to charge a condenser, and thus it is seen that an ordi- 
 nary induction coil or even a static machine of the proper 
 dimensions could be used, but they are not nearly as handy. 
 
 Another important matter in connection with the construc- 
 tion of a transformer to be used for creating electric oscilla- 
 tions is to secure a sufficiently small resistance in the second- 
 ary. The reason for this is that the transformer is used to 
 charge a condenser. 
 
 When an electromotive force is applied to the terminals of 
 a condenser, the full difference of potential is not created 
 between the terminals of the condenser immediately, but 
 rises gradually. The time required to charge the condenser 
 depends on its capacity (C) and the resistance {R) of the 
 charging circuit. The product CR is called the time constant 
 of the condenser, and practically the condenser is charged 
 in a time equal to ten times the time constant. The time 
 constant is to be reckoned as the product of the capacity (Q 
 in microfarads and the resistance {R) of the charging circuit 
 in megohms.- The time is gi\en in fractions of a second. 
 
 The condenser, more than anything else, constitutes the 
 
74 The Tesla Coil 
 
 essential part of the Tesla coil. It plays the same part as 
 the mechanical interrupter in the ordinary induction coil. 
 Its action, however, is purely electrical and its great advantage 
 lies in setting up the currents of enormous frequency. 
 
 When any condenser is discharged, the discharge may take 
 one of several forms, depending only on the three electrical 
 constants of the discharging circuit — inductance, capacity, 
 and resistance. The discharge may be either oscillatory or 
 entirely unidirectional, consisting only of a gradual equali- 
 zation of the potentials on the two plates. 
 
 This may be made clear by the following mechanical illus- 
 tration. Suppose a glass U-tube to be partly fdled with 
 mercury, and the mercury to be displaced so that the level 
 in one side of the tube is higher than in the other. There 
 is then a force due to the difference of level, tending to cause 
 the liquid to return to an equal height in both limbs. If 
 the mercury is now allowed to return, but is constrained, 
 so that it is released slowly, it goes back to its original 
 position without oscillations. If, however, the constraint is 
 suddenly removed, then owing to the inertia of the mer- 
 cury it overshoots the position of equilibrium and oscilla- 
 tions are created. If the tube is rough in the interior, or 
 the liquid viscous, these oscillations will quickly subside, 
 being damped out by friction. 
 
 What we call inertia in material substances corresponds 
 with the inductance of an electric circuit and the frictional 
 resistance experienced by a liquid moving in the tube, with 
 the electrical resistance of a circuit. If we suppose the 
 
Theory of the Coil 75 
 
 U-tube to include air above tiie mercury and to be closed up 
 at its ends, the compressibility of the enclosed air would 
 correspond to the electrical capacity in a circuit. 
 
 The necessary conditions for the creation of mechanical 
 oscillations in a material system or substance are that there 
 must be a self-recovering displaceability of some kind, and 
 the matter displaced must possess inertia; in other words, 
 the thing moved must tend to go back to its original position 
 when the restraining force is removed, and must overshoot 
 the position of equilibrium in so doing. Frictional resist- 
 ance causes decay in the amplitude of the oscillations by 
 dissipating their energy as heat. 
 
 In the same way the conditions for establishing electrical 
 oscillations in a circuit is that it must connect two bodies 
 having electrical capacity with respect to each other, such 
 as the plates of a condenser, and the circuit itself must 
 possess inductance and low resistance. Under these condi- 
 tions, the sudden release of the electric strain results in the 
 production of an oscillatory electric current in the circuit, 
 provided the resistance of the circuit is less than a certain 
 critical value. We have these conditions present when the 
 two coatings of a Leyden jar are connected by a heavy 
 copper wire. 
 
 Professor William Thomson, titled Lord Kelvin, published 
 in 1853 a paper on "Transient Electric Currents" in which 
 the discharge of the Leyden jar was mathematically treated 
 in a manner that elucidated important facts. 
 
 If we consider the case of a Leyden jar or condenser charged 
 
76 The Tesla Coil 
 
 through a circuit having inductance and resistance, then in 
 the act of discharge the electrostatic energy stored up in 
 the condenser is con\'erted into electric current energy and 
 dissipated as heat in the connecting circuit. At any moment 
 the rate of decrease of the energy in the jar is equal to the 
 rate of dissipation of the energy in the discharging circuit 
 plus the rate of change of the kinetic or magnetic energy 
 associated with the circuit. 
 
 From these facts Lord Kelvin sets up an equation of 
 energy, vi^hich leads to a certain class of differential equation 
 ha\ing two solutions. The solutions in this case depend on 
 the relation between the constants inductance, resistance, 
 and capacity. 
 
 If Z = inductance, C = capacity, R = resistance, then the 
 
 solutions are determined by the relative values of — and LC. 
 
 If is greater than — , that is, if R is greater thanl/lr:, 
 
 4U LC V (J 
 
 J^C T 
 
 or if — is greater than — , the charge in the jar dies away 
 4 R 
 
 gradually as the time increases, in such a manner that the 
 discharge current is always in one direction. 
 
 The ratio — is called the time constant (Z) of the dis- 
 R 
 
 charge circuit, and the product CR is called the time con- 
 stant {T') of the condenser circuit. Hence the discharge 
 is unidirectional when the time constant of the inductive 
 circuit is less than half the geometric mean of the time con- 
 stants of the inductive circuit and condenser circuit: 
 
Theory of the Coil 77 
 
 (^ J? T 
 
 If, however, — is less than — the discharge current will 
 4 R 
 
 be oscillatory, the current decaying in accordance with the 
 law of a damped oscillation train. 
 
 When the discharge is so highly oscillatory that the cur- 
 rent is not uniformly distributed through the cross-section 
 of the conductor, then the ordinary resistance (R) and in- 
 ductance (L) must be replaced by the high-frequency resist- 
 ance and inductance of the circuit. 
 
 AA'hen the discharge takes the oscillatory form the fre- 
 quency is given by the expression, 
 
 2- > LC 4L- 
 
 If R is very small, then can be neglected in compari- 
 
 son with , and then the frequenc}- is given by the expres- 
 
 sion, 
 
 In this equation both the quantities C and L must be measured 
 in electromagnetic units or both in practical units, viz., in 
 henrys and farads. 
 
 In the majority of cases in which electric oscillations 
 are practically used, the resistance of the oscillatory cir- 
 cuit is negligible, and the inductance is small and hence 
 easily measured in centimeters or absolute C. G. S. units, 
 one milli-henry being equal to a million centimeters (10°). 
 
78 The Tesla Coil 
 
 Also the capacity is best measured in microfarads; one 
 microfarad being the one millionth part of a farad or 
 IO-" of an absolute C. G. S., unit (electromagnetic) of 
 capacity. 
 
 Hence when L is expressed in centimeters and C in micro- 
 farads, the expression for the natural frequency of the circuit 
 
 becomes 5.033 X 10" 
 
 n = ■ 1= 
 
 VCL 
 
 The energy storing capacity of a condenser is given by 
 
 the expression - CV, where C is the capacity of the con- 
 2 
 
 denser and V the charging voltage. 
 
 The main thing in constructing condensers to be used on 
 high charging voltages is the solid dielectric. There are in 
 all only a few dielectrics suitable for high-tension work, and 
 this number is reduced when cost and internal energy loss 
 in the dielectric are considered. Glass of certain composi- 
 tions, ebonite, mica, and micanite are practically all that 
 are suitable, and of these flint glass is the best, as its dielectric 
 constant is high, being from 5 to 10, and its dielectric strength 
 is also great. Glass is brittle, however, and liable to have 
 flaws which sooner or later give way. 
 
 The capacity of a condenser depends on the area of the 
 plates, their distance apart and the constant of the dielectric 
 used, and is expressed by the following formula in micro- 
 farads, where K is the dielectric constant, 5 the total area of 
 the plates expressed in square centimeters, and D the distance 
 apart in centimeters, 
 
Tlieory of the Coil 79 
 
 ^_ KS 
 
 The constant 9 x lo'^ comes from the fact that one micro- 
 farad equals 900,000 electrostatic units of capacity. 
 
 The oscillation transformer is nothing but a modified trans- 
 former with an air core. The only important facts about 
 its construction are that it should be built to withstand great 
 voltage differences between the turns, and that the primary 
 should have as small an inductance as is practicable, in order 
 to make the frequency as great as possible. No advantage 
 is gained by having many close turns in the primary, because 
 the increase of inductive effect on the secondary, due to an 
 increase in the number of primary turns, is about exactly 
 annulled by the decreased current through the primary due 
 to its own greater inductance. 
 
 The function of the interrupter is to destroy any arc that 
 may be formed across the terminals of the primary spark- 
 gap, for if this arc is not wiped out there will be no true 
 oscillatory discharge in the condenser circuit or only a feeble 
 one. The reason for this is that as long as the arc discharge 
 continues, the secondary terminals of the transformer are 
 reduced to nearly the same potential, or at most differ only 
 by a few hundred volts. 
 
 The function of the primary spark-gap is to regulate the 
 voltage to which to charge the condenser. Since the poten- 
 tial difference between the spark balls is almost equal to the 
 potential difference across the condenser, the condenser will 
 discharge at a voltage determined by the length of the air- 
 
8o The Tcsla Coil 
 
 gap. Now there is a certain length of spark-gap which is best 
 suited for each coil and it can easily be determined by trial. 
 As a rule it is best to start with a rather short spark-gap, 
 gradually lengthening it out until a point is ahnost reached, 
 when opening it out any further would cause it to cease pass- 
 ing. This spark length almost always gives the best results. 
 
 In the earlier part of this chapter it was stated that the high- 
 frequency resistance and inductance should be substituted 
 for the ordinary resistance and inductance, when dealing 
 with circuits which are subject to the action of electric oscil- 
 lations. The processes and means used for the measure- 
 ment of low-frequency alternating currents and potentials 
 are not always applicable or correct either when applied to 
 high-frequency measurements. The main reason for the 
 difference between the two cases is to be found in the fact 
 that a high-frequency current does not penetrate into the 
 interior of a thick solid conductor of good conductivity, but 
 is merely a surface or skin effect. 
 
 When tra^'ersed by an alternating current, there are five 
 qualities of a circuit to be considered. 
 
 1. The resistance of the conductor, which is always greater 
 for high-frequency currents than for the ordinary currents; 
 that is, direct currents and alternating currents up to about a 
 frequency of loo per second. 
 
 2. The inductance of the conductor depends on its geo- 
 metrical form, material, and the nature of the surrounding 
 insulator. The greater the frequency, the smaller the induc- 
 tance becomes. 
 
Theory of the Coil 8i 
 
 3. The capacity of the conductor, depending on its posi- 
 tion with regard to the return circuit and other circuits and 
 on the dielectric constant of the surrounding insulator. 
 
 4. The dielectric conductance of the insulator surrounding 
 the conductor. 
 
 5. The energy dissipating power, due to other causes than 
 conductance, such as dielectric hysteresis, which exist in 
 the dielectric. Under this heading comes the loss of energy 
 from the brush discharges through the air between the con- 
 ductors. 
 
 If the constants of a circuit for low-frequency currents are 
 known, the values of the constants for high frequencies can 
 be calculated fairly correct. The high-frequency constants 
 can, however, be measured directly, but the apparatus is rather 
 delicate and inconvenient for use and besides not always sat- 
 isfactory. If the coil builder cares to measure the constants 
 of a circuit for himself, he will find the description of the 
 necessary instruments in other books as it is beyond the 
 scope of this work. 
 
 Ha\-ing now briefly treated theoretically on all four of 
 the principal parts, the authors will try to show how these 
 parts work together to form the Tesla high-frequency appara- 
 tus. 
 
 The condenser is connected in series with the secondary 
 of the transformer and thus is being continually charged. 
 \Vhen the potential difference betweeen the plates of the con- 
 denser reaches a certain critical value determined by the 
 length of the primary spark-gap, the diameter of the spark 
 
82 The Tcsla Coil 
 
 balls, etc., a discharge takes place which oscillates through 
 the primary of the oscillation transformer and back and 
 forth across the primary spark-gap. The frequency of the 
 current depends entirely, as shown before, on the constants 
 of the circuit. On first thought, one would think that the 
 condenser would discharge through the closed circuit in the 
 transformer secondary rather than jump the air-gap, but a 
 little consideration of the matter will show that the induc- 
 tance of this circuit to electric oscillations of this nature is 
 so great that no discharge can take place. Another matter 
 that might be touched on here is the resistance of the spark- 
 gap. Before any discharge has passed and under normal 
 conditions the resistance of the spark-gap is very great: the 
 voltage required to break down one centimeter of air being 
 about 10,000. After the initial discharge has passed and the 
 air becomes heated and ionized the resistance may drop as 
 low as two or three ohms. This fact plays an important 
 part in the damping of the oscillation trains. 
 
 The discharge from the condenser which oscillates through 
 the primary of the oscillation transformer sets up a rapidly 
 alternating magnetic field, which being linked with the sec- 
 ondary induces an electromotive force in it. The law for the 
 induction in this case is not nearly as simple as in the case of 
 the ordinary transformer, the capacities of the circuits playing 
 an important part. If the capacity of the circuits is below a 
 certain critical value, the induction is in the ratio of the 
 capacities of the circuits, while if greater the induction de- 
 pends on the relation between the number of turns in the 
 
Theory of the Coil 83 
 
 primary and secondary. The formulae for calculating the 
 voltage difference across the secondary in either case are 
 extremely complex, involving the damping factor, the capaci- 
 ties of the circuits, and other constants. Drude and Bjerknes 
 have treated the subject of the oscillation transformer analyt- 
 ically in an admirable maimer. 
 
 The frequency of the spark in the large spark-gap is not a 
 simple one but consists of se\eral, one being the natural 
 period of vibration of the secondary and one a forced vibra- 
 tion of the secondary, due to the fact that the primary and 
 secondary are never exactly in tune. There is also a certain 
 small current of a high frequency, due entirely to the con- 
 stants of the spark balls and connectors, which act as a 
 condenser. 
 
 The Tesla coil in its present form is still very crude leaving 
 much to be improved upon and wished for. The problem 
 that presents itself in the construction of Tesla coils is prac- 
 tically the same one that presents itself in selective wireless 
 telegraphy, so that the solution of the one will solve the other. 
 
CHAPTER IX 
 USES OF THE COIL 
 
 The Tesla coil readily lends itself to a great number of 
 experiments, some interesting in their effect, others useful 
 in scientific research. Waves for wireless messages may 
 be sent out into space, X-Ray tubes excited, Geissler tubes 
 illuminated, beautiful brush effects shown, and a great 
 number of other things done. 
 
 The high potential current obtained from this coil pos- 
 sesses certain interesting properties due to its high frequency 
 that are not possessed by either the Ruhmkorff induction 
 coil or a static machine. 
 
 These properties are best seen in the beautiful brush 
 effects, which may be obtained even with the coil described 
 in the Appendix. All of these experiments on the brush 
 discharge should be performed in the dark, as they then 
 show to the best advantage. 
 
 These effects, besides affording a pleasing sight, are of 
 great scientific \alue. It is a known fact that the phenome- 
 non is due to the agitation of the molecules near the terminal, 
 and it is thought, since the brush is hot, that some heat must 
 be developed by the impact of the molecules against the ter- 
 minal or each other. A little consideration of the matter 
 
 84 
 
Uses of the Coil 85 
 
 leads us to the conclusion that if we could but reach suffi- 
 ciently high enough frequencies, we could produce a brush 
 which would give intense light and heat. But this is stray- 
 ing too much from the practical nature of this book; the only 
 reason for putting it in being, that it might suggest to the 
 amateur new lines for experiment. 
 
 The following experiments on the brush discharge 
 have been taken from Nikola Tesla's "Experiments 
 with Alternate Currents of High Potential and High Fre- 
 quency." 
 
 There is practically nothing except some wire and a few 
 supports required for all these experiments on the brush 
 discharge, and thus they can be performed by every one. In 
 the first experiment two insulated wires about 10 feet long 
 are stretched across the room. They are supported at dis- 
 tances of a foot and a half by insulating cords. One of the 
 wires is attached to each terminal of the coil. When the 
 coil is put in action the wires are seen to be strongly illumi- 
 nated by the streams issuing abundantly from their whole 
 surface. (This experiment must be shown in the dark, of 
 course.) The cotton covering on the wire, although it may 
 be very thick, does not affect the result. 
 
 To produce the best effect the primary gap and the length 
 of the wires must be carefully adjusted. It is best to take 
 the wires at the start very long and then adjust them by 
 cutting off first long pieces and then shorter and shorter 
 lengths until the correct length is reached. When this 
 adjustment has been obtained and the wires are fed by either 
 
86 The Tesla Coil 
 
 the 12" or 7" coil, the hght from them will be sufficient to 
 distinguish objects in the troom. 
 
 Another way of easily exhibiting the brush effect is by 
 attaching the end of 10 or 20 feet of No. 36 insulated copper 
 ' wire to the one terminal of the coil and the opposite end to 
 an insulating support, lea\-ing the wire hanging clear. Upon 
 touching the remaining terminal with a bit of metal held in 
 the hand, the wire will break forth into numberless streams 
 or threads of light palpitating in unison with the discharge 
 of the condenser. 
 
 The luminous intensity of the streams can be considerably 
 increased by focusing them upon a small surface. This is 
 illustrated by the following experiment. To one of the ter- 
 minals of the coil a wire bent into a circle about one foot in 
 diameter is attached and to the other terminal a small brass 
 sphere. The centre of the sphere should be in a line at right 
 angles to the plane of the circle at its centre. When the 
 discharge is set up, a luminous hollow cone is formed, and 
 in the dark one half of the brass sphere is seen strongly illu- 
 minated. To get the best results possible with this experi- 
 ment, the area of the sphere should be equal to the area of 
 the wire. 
 
 Another way in which the luminous effect of the discharge 
 may be shown is as follows: two circles of rather stout wire, 
 one being about 32" in diameter and the other 12", are formed, 
 and to each of the terminals of the coil one of the circles is 
 attached. The two circles must be concentric and in the 
 same plane. When the coil is turned on the whole space 
 
Uses of the Coil 87 
 
 between the wires is uniformly tilled with streams. The 
 intensity of the streams forming this luminous disc is such 
 that objects in the room can be plainly distinguished even 
 though at a considerable distance. 
 
 By this time the experimenter will realize that to pass 
 ordinary luminous discharges through gases, no particular 
 degree of exhaustion is necessary, but a very high frequency 
 is essential, and of course a fairly high potential. This 
 shows us that the attempts to produce hght by the agitation 
 of the molecules or atoms of a gas need not be limited to the 
 vacuum tube, but the time is to be looked forward to, and 
 that in the near future, when light will be produced without 
 the use of any vessel whatever and with air at ordinary pres- 
 sure. When light is obtained this way, there will be no 
 chemical process nor consumption of material, but merely 
 a transfer of energy, and the probability is that such a light 
 would have an efficiency far exceeding that of even the best 
 of the present incandescent lights, which waste so much of 
 the energy in heat. 
 
 The Geissler effect can be readily shown by using only a 
 bui^nt-out incandescent globe, in which the vacuum has not 
 been destroyed. To show it the bulb should be suspended 
 from an insulating knob, so that the discharge will pass 
 through the centre of the bulb. The room should be dark 
 in order to see the changes that take place in the globe. On 
 putting the coil in action the bulb lights up with a gently 
 pulsating, delicate purple hue. This color in a few minutes 
 generally turns to a lovely pale green, and then sometimes, 
 
88 TJie Tesla Coil 
 
 but rarely, this changes to a delicate white light. The in- 
 tensity of the light is not very great, but the delicateness 
 of the colors is something to be admired. The discharge 
 must not be continued too long through the tube as it is 
 liable to pierce the glass. A peculiar thing always happens 
 if a bulb with a good filament is used, namely, that in a few 
 minutes the filament is found to be completely shattered. 
 With the 12" coil it is only necessary to hold the globe in the 
 hand, without any connections to the coil, for it to light up. 
 By moving the bulb around in the vicinity of the spark, 
 various changes in the intensity of the light will be seen. 
 
 If the amateur is fortunate enough to possess some Geissler 
 tubes, these may be lit up by connecting them, in series with 
 an adjustable spark-gap, to the coil. In the case of the large 
 coil tubes will light up when merely brought into the vicinity 
 of the discharge. 
 
 The best X-ray tube to use with this Tesla coil is what 
 is known as a double-focus tube, although any other tube 
 may be used with not quite as good results. As the terminals 
 of the X-ray tube are alternately cathode and anode, when 
 an alternating current is used the pulsating or rather va:ry- 
 ing light, if X-rays may be called light, would be objection- 
 able, but due to the high frequency of the current from this 
 coil the pulsations are not noticeable. When the double- 
 focus tube is used the rays are reflected first from one re- 
 flector and then from the other in rapid succession, and thus 
 double the space is filled with the rays than if a single-focus 
 tube had been used. 
 
Uses of the Coil 89 
 
 If the tube is a small one and is used on either the 7" or 
 12" coils, it should be connected in series with an adjustable 
 spark-gap, to prevent any injury to it. On starting the coil 
 this spark-gap should be open as far as possible and then 
 gradually closed until the best result is obtained. The tube 
 should be felt now and then to see if it has become hot, and if 
 it becomes too hot the coil should be stopped and the spark- 
 gap lengthened. 
 
 For those amateurs who may desire to use this coil for 
 wireless telegraphy, the authors merely state that it is suit- 
 able, with a few minor changes, in most of the systems using 
 a step-up transformer. For detail information they are 
 referred to a large number of admirable books, especially 
 devoted to this subject, written by very eminent men in this 
 field. 
 
 But after all the great field of experimental investigation 
 opens itself to the possessors of one of these coils when they 
 enter the realm first mathematically investigated by Maxwell; 
 that is, the field in which Hertz made himself famous, the 
 field of electric waves. The subject itself is still in its infancy 
 and great results are expected when the laws of electromag- 
 netic disturbances are verified in a convincing manner. 
 
 It is not the easiest subject to experiment in by any means, 
 on the other hand it is the most difficult. 
 
 In order to carry out any of the experiments at all and get 
 satisfactory results a large room free from all metallic objects 
 and electric wiring is required. A good place is in the loft 
 of a large barn, provided no better place can be had. It 
 
QO The Tesla Coil 
 
 was in a large barn that the authors verified some of Hertz's 
 experiments on electric waves. In dealing with electric 
 waves there are two disturbances to be taken into account, 
 the electromagnetic and the electrostatic. There are also 
 two classes of waves, the electromagnetic waves and the sta- 
 tionary waves on wires. A method will be given for obtain- 
 ing both the electromagnetic and the stationary waves, 
 which is all that can consistently be brought within the 
 scope of this work. 
 
 D. E. Jones has made a translation from the German 
 of the original papers of Heinrich Hertz, dealing with the 
 experiments and results which have made his name famous. 
 
 The radiator or oscillator used by Hertz consists of two 
 metallic plates, having attached to them short rods ending 
 in knobs a short distance apart. These knobs are connected 
 to the secondary of the coil. Hence, as the secondary electro- 
 motive force accumulates, the plates are brought to a differ- 
 ence of potential and lines of electrostatic displacement 
 stretch out from the positive side of the oscillator to corre- 
 sponding points on the negative. We have thus a strong 
 electrostatic displacement created along the lines of force. 
 When the potential dilTerence reaches a certain point de- 
 pending on the length of the spark-gap, the air insulation 
 breaks down and a current flows from the one plate to the 
 other across the spark-gap. It is merely the discharge of 
 a condenser, for the two plates on the oscillator form a con- 
 denser, with air as a dielectric. This creates in the space 
 around a magnetic flux, the direction of which is everywhere 
 
Uses of the Coil 91 
 
 normal to the direction of the electric displacement. The 
 electrostatic energy is thus transformed into electrokinetic 
 energy. 
 
 If the electric oscillation is started sufficiently sudden 
 some of the energy is thrown off as a displacement wa\e. 
 As the coil is continually in operation we have groups of 
 intermittent oscillations and therefore trains of electric 
 waves thrown off which spread out through the dielectric. 
 In this way the electromagnetic waves are set up in the air. 
 
 In order to have the breaking down of the air sufficiently 
 sharp to obtain the oscillations, at least three things are 
 necessary: the spark-ball surfaces must be bright and clean, 
 no ultra-violet light must fall on the balls, and the balls must 
 be a certain distance apart, best determined by experience. 
 
 The form of resonator de\ised by Hertz consists merely 
 of a nearly closed ring or rectangle of wire the ends of which 
 end in metallic balls placed very close together. The one 
 ball is adjustable by the use of a micrometer screw. There 
 have been many modifications of this resonator, but the one 
 the authors found most satisfactory was where a neon or 
 carbon dioxide Geissler tube of the spectrum variety was 
 connected directly across the spark-gap. Instead of getting 
 a spark then the tube becomes illuminated. 
 
 By using this resonator and the before-meintioned oscilla- 
 tor, the magnitude of the electric displacements in the space 
 surrounding the oscillator can be mapped out. 
 
 Hertz's great work was in setting up stationary waves in 
 air. This is accomplished by having a large metal plate 
 
92 
 
 The Tcsla Coil 
 
 set up in the room to act as a reflector. The coil with its 
 radiator is set up in front of it, at some distance from it, so 
 that the plane of the plates on the oscillator is parallel to the 
 reflector. By holding the resonator parallel to the metallic 
 reflector the nodes and antinodes can be easily traced out. 
 The size of the plates on the radiator used by Hertz in 
 this experiment was i6" square. The resonator was a ring 
 of about No. 6 copper wire, 14" in diameter. 
 
 m 
 
 ■n 
 
 a^ 
 
 w 
 
 Fig. 37. — Waves on Wires. 
 
 To set up stationary waves on wires the same radiator is 
 used. Two similar plates to those on the oscillator are gotten 
 
Uses of tlie Coil 93 
 
 and mounted a short distance from the plates of the radiator. 
 The plates are all parallel and have two parallel wires led 
 from them across the room as shown in the figure. 
 
 Thus the wires are electrostatically connected to the 
 Hertz radiator and the plates rapidly alternate in potential, 
 and this applies to the ends of the wires an alternating 
 electromotive force. This EMF creates electric waves of 
 potential which travel along the wires with the velocity of 
 light. If the length of the wire is suitably adjusted, sta- 
 tionary waves of electric current and potential are set up 
 by the interference of the direct and reflected waves. 
 
 To try the experiments on reflection, refraction polariza- 
 tion and interference of electric waves, a small oscillator 
 consisting of two brass rods ending in balls and mounted 
 in a zinc box of parabolic form is required. The resonator 
 must be very sensitive, for otherwise no results will be ob- 
 tained. The prisms and lenses used are moulded from either 
 paraffine or pitch. 
 
 As before stated the authors said that they would merely 
 outline the experiments possible with the Tesla coil, and for 
 the detailed information the translations of Hertz's original 
 works must be consulted, unless the amateur has sufficient 
 knowledge of German to read the originals, which will of 
 course be far more satisfactory. 
 
 There are several experiments which can be readily per- 
 formed with this apparatus and which would appeal to those 
 having no knowledge of electric phenomena. These experi- 
 ments have but little practical value, the only reason for 
 
94 The Tesla Coil 
 
 citing them being that they may furnish entertainment to 
 some friend who should chance into his shop. The writers 
 first saw them performed on the vaudeville stage with an 
 Oudin resonator which is far inferior to the Tesla coil. 
 
 To begin the performance the operator should make a 
 few general statements as to the voltage required to leap 
 different air-gaps, such as 30,000 for i", about 55,000 for 
 2", etc. Then show the discharge across the 6" or 12" gap 
 and let the spectator imagine what voltage that represents. 
 After he has become somewhat impressed with the intensity 
 of the discharge, the operator can approach one of the oscil- 
 lators and allow the spark to play on his bare hand. To 
 do this without severe burning of the hand, he should keep 
 his hand in constant motion to prevent the spark from playing 
 on one point. There is only a very slight sensation felt 
 when this current is traversing your body and no injurious 
 effects result so far as the authors are aware. The effect 
 is best shown in the dark. 
 
 The next experiment is to grasp one oscillator with the 
 hand when the coil is in operation and to have an assistant 
 touch the operator's bare elbow with a cotton cloth dipped 
 in alcohol. The handkerchief immediately bursts in flame. 
 To get the result without any uncertainty the cloth is wrapped 
 rather tightly around the assistant's hand. An amusing 
 modification of the same experiment is to touch the cloth to 
 the hair of the operator, showing that the hair is not ignited. 
 
 If any of the audience are inclined to smoke, a suggestion 
 as to lighting cigars on a windy night without the annoyance 
 
Uses of the Coil 95 
 
 of matches blowing out will be greatly appreciated. The 
 operator has merely to bring a piece of metal, such as a nail, 
 held in the hand to within h" of one of the spark terminals 
 and light the cigar from the spark. 
 
 A gas flame can be lit with the bare fingers by grasping 
 one oscillator with the bare hand and approaching the burner 
 with the finger of the other hand so that a spark will jump to 
 the metal tip of the burner. The A^riters have made this 
 more spectacular by letting the spark jump from the tip of 
 the tongue. 
 
 In these experiments an assistant should adjust the spark- 
 gap so that no more current passes than is necessary. This 
 is to prevent the spark from burning the operator. 
 
 To convince the audience of the tremendous voltage 
 passing through the operator's body he has merely to bring 
 one hand up to a lighted incandescent globe, while he grasps 
 one terminal of the coil with the other hand. On the near 
 approach of the hand the filament will violently vibrate and 
 then shatter, blackening the bulb and of course extinguishing 
 the lamp. 
 
 Lighting Geissler tubes held in the hand or even in the 
 mouth by approaching them to the oscillators is an experi- 
 ment that never fails to bring forth the applause of those 
 present. 
 
 Perhaps the most spectacular experiment, one which is 
 unaccountable for by the every-day electrician who does 
 house wiring and has never been brought in touch with 
 high-frequency- currents is the lighting of an ordinary incan- 
 
96 The Tesla Coil 
 
 descent lamp with the current traversing the operator's 
 body. 
 
 Before performing this experiment some few remarks 
 on the quantity of current necessary to bring the filament 
 to full brightness on the no- volt circuits should be made, 
 if those present are ignorant of electrical matters. They can 
 thus see that the energy required to light an ordinary 16 
 C. P. lamp is equivalent to 55 watts, and that this amount 
 is therefore taken through the operator's body. At no 
 \olts this means approximately \ ampere, while according 
 to the best authorities ^^ of an ampere is fatal to the aver- 
 age human. The reason why this amount of energy can be 
 taken with impunity is not definitely known, but it is thought 
 to be due to the fact that the high-frequency current does 
 not penetrate into the interior of a solid conductor, but 
 follows the surface. This is known as the skin effect. 
 
 The operator, to get the best results, should stand on an 
 insulated stool and grasp one terminal of the coil with one 
 hand, and approach with a piece of metal held tightly in the 
 hand or mouth one lead of a lamp, the other lead of which 
 has been previously grounded. The lamp will come up 
 to bright red and if an assistant adjusts the primary spark- 
 gap to its best working distance, the lamp may be brought 
 up to full brightness. 
 
Plate VI. — Discharge from the 12" Coil. 
 
 Plate VII. — The 7" Standard Apparatus. 
 
CHAPTER X 
 
 DIMENSIONS OF 7" STANDARD COIL 
 
 For those amateurs who, ha\'ing read the previous 
 chapters, think that an apparatus giving a twelve-inch 
 spark is too large for their limited uses, this chapter has 
 been added. 
 
 This coil is by no means to be thought of as a toy, for the 
 authors themselves used the very apparatus described in 
 this chapter in carrying out their first experiments with the 
 X-ray and Geissler tubes. Wireless messages were also 
 sent successfully over a distance of three miles in wet weather. 
 This was the greatest available distance over which the 
 authors could try the coil, so that three miles should not be 
 considered the maximum transmitting distance. In clear 
 weather messages could easily be sent a distance of about 
 fifty miles, provided your antennae is well insulated from 
 grounds. 
 
 Because this apparatus is not as powerful as the other, 
 does not mean that any less care should be taken as 
 regards insulation and mechanical construction, for it depends 
 entirely on this whether the coil builder is to get a thin, inter- 
 mittent spark or a fat crackling one. The only difference 
 between this apparatus and the larger one, besides that of 
 
 97 
 
98 
 
 The Tesla Coil 
 
 size, is in the construction of the transformer and condenser 
 and then they are only trivial. 
 
 The core of the transformer is 2f" in diameter and is 
 built up of pieces of No. 32 B. & S. gauge iron wire 13" 
 long after the manner described in Chapter II. The same 
 care should be taken in annealing and insulating the iron 
 wires as was done before. 
 
 The primary ■ is wound in two sections adjacent to each 
 other, as seen in Fig. 38. Each section is wound towards 
 
 Fig 38. — Primary and Core of Transpoemer of 7" Coil. 
 
 the centre, starting i" from the ends of the core, for a distance 
 of 5". There are six layers of No. 16 B. & S. gauge double 
 cotton covered copper wire in each section. Each layer is 
 thoroughly shellacked when put on and the terminal wires 
 are held by the same method as described in Chapter II. 
 At least two feet should be left for bringing out the terminals 
 to the binding-posts. 
 
 The secondary is wound in two sections. No. 32 B. & S. 
 gauge double cotton covered copper wire is used. The tube 
 
Dimensions of 7" Standard Coil 
 
 99 
 
 on which it is wound has an internal diameter of 4" and the 
 thickness of the wall is ^^". It is 11" long and made from 
 the best vulcanized fibre. The bobbin heads are cut out 
 of j" sheet fibre. They are 6" in diameter and have a hole 
 4g" in diameter cut out of the centre. Four of these are 
 
 w^ 
 
 II 
 
 ! J 
 
 ^^--^^—^4^11 
 
 Fig. 39, 
 
 -4^ 
 
 -^i- 
 
 ->'i^ 
 
 - Secondary Bobbin of Transformer of 7" Coil, 
 
 required. They are slipped on the tube to the positions 
 shown in the figure. The distance between the bobbin 
 heads of each section is 3 J", and the distance between the 
 two sections 2}/'. The bobbin heads can be held in place 
 in the same manner as described in Chapter II by cutting 
 the rings out of a very thin fibre tube, or in this case it will 
 be sufficient to wrap some heavy brown paper several times 
 around the tube between the bobbin heads. After winding 
 a layer on the secondary, shellac it and wrap it with one 
 turn of paper. In this way build up the secondary to within 
 \" of the bobbin heads. When the last layer is put on it 
 
loo The Tesla Coil 
 
 is wrapped with several turns of paper which is shellacked 
 in place. This completes the construction of the transformer. 
 \A'hen finished it should be left for some time in a warm dry 
 place as behind the stove, to thoroughly dry the shellac. 
 The reason for this is that green shellac is a fairly good con- 
 ductor. 
 
 For the condenser, twenty-eight sheets of brass 6" x lo" 
 are required. No. 32 or 34 soft sheet brass is used. Each 
 sheet has a lug i^' long and i]" wide either cut direcdy 
 on it or soldered on in the upper corner. Whether they 
 are cut directly on the sheet or are soldered on will depend 
 on the width of the brass sheet used. \Mth 12" brass they 
 are cut on the sheet or soldered on with 8" brass. A i" 
 lip is bent across the top of each lug. See Fig. 40. Thirty 
 sheets of glass -^q" thick and 7" x 12" are required. Any 
 sheet of glass that has an air bubble in it should be rejected 
 as it is liable sooner or later to give way, thus causing the 
 reconstruction of the condenser. Wipe each sheet of glass 
 clean before putting it in to the condenser. 
 
 The frame is constructed from well seasoned pine. The 
 two sides are made from J" x 12" pine 4" long. The base 
 is made of a piece of I" x 4" pine 8" long. The base is fas- 
 tened to the sides by some flat-headed brass screws. The 
 heads of the screws should be sunk flush with the wood. 
 On the one side of the frame two strips J" x i" are fastened; 
 one at the top and the other at the bottom of the frame. 
 See figure. The frame is now laid on a flat surface with 
 the side on which the strips have just been fastened down. 
 
Dimensions of 7" S/andard Coil 
 
 lOI 
 
 I 
 I 
 
 I 
 I 
 I 
 I 
 I 
 
 
 t:: 
 
 f^l" Sirijit 
 
 ^it'^l'strik 
 
 ^ 
 
 i^Cstrl^ 
 
 I > 
 
 --3" ^ 
 
 . ^ 
 
 
 Fig, 40. — Plate and Frame of Condensek. 
 
102 The Tesla Coil 
 
 A glass plate is placed in the frame after having been wiped 
 dry and clean so that it touches the bottom of the frame. 
 Then a brass sheet is laid in so that there is i" margin of 
 glass at the bottom and a V' margin on the sides. The lip 
 should just fit against the upper edge of the glass. Without 
 displacing the brass sheet place a sheet of glass on top of 
 it. This is followed with a sheet of brass, but in this case 
 the lug on the brass is brought out on the opposite side 
 to the previous one. Continue this process until the 28 
 sheets of glass have been put in place. Two glass sheets 
 are placed on top of the last brass sheet. Also remember 
 to bring the lugs from the brass sheets out on alternate sides. 
 
 Mortices should be cut in the top and bottom of the upper 
 ends of the sides a little deeper than the point to which the 
 last glass sheet reaches. These are to receive two strips 
 of pine l" X i" similar to those on tjie other side. These 
 should be screwed down so that they press firmly against 
 the glass. A piece of paper or cloth placed between the 
 strip and the glass will prevent the breaking of the latter. 
 
 Set the condenser upright and solder a piece of copper 
 wire, which has already been tinned, to each of the lips 
 in turn down the one side and another wire to all the lips 
 on the other side. About No. 16 bare wire will do. Enough 
 extra wire should be left to make all necessary connections. 
 
 The oscillation transformer is constructed in the same 
 manner as the one for the 12" coil. The circular supports 
 for the secondary are 6" in diameter and are turned out 
 of i" material. Eight equidistant slots are cut in the per- 
 
Dimensions of f' Standard Coil 103 
 
 iphery Y square. The fibre strips are A" square and 11" 
 long. A rod 11" long is turned out to the size shown in 
 the figure 41. It is i" in diameter and has a A" shoulder 
 turned on each end. This rod holds the two supports for 
 the secondary apart. If the method of winding the wire in 
 grooves is to be used, the thread should be cut on the fibre 
 strips before mounting them on the supports. 
 
 In the original coil the wire was merely wound on three 
 fibre supports 3" wide mounted on a hexagonal end piece. 
 The wire was wound so that no adjacent turns were in 
 contact and the whole was thoroughly shellacked. Although 
 this method of winding gave good results while the coil was 
 new, it was found after some usage that the wires became 
 loosened, thereby reducing the effecti\-e sparking distance. 
 
 A better way howe\-er, is to use one of the methods de- 
 scribed in Chapter 1\. The wire used is Xo. 28 B. & S. 
 gauge double cotton covered copper wire and was wound 
 18 turns to the inch. 
 
 The end pieces for the primary are cut out of h" material 
 and are 9" square. The diameter of the circle in which the 
 dowels fit is 8". There are twenty -four \" maple dowels 
 used in all. After the secondary is wound these end pieces 
 are screwed to the secondary frame, and the dowels slipped 
 in place. The length of the frame o^'e^ all is 12" The 
 primary winding consists of one and a half turns of a copper 
 ribbon -i" wide. The turns should be equally spaced and 
 the ^^'inding stretch from the one end of the frame to the 
 other, A copper wire is soldered on to each end of the 
 
104 
 
 The Tesla Coil 
 
 -*IT'-| 
 
 I r 
 
 f„0 
 
 =-i 
 
 o 
 
 DC 
 
 (- 
 
 
 UJ 
 
 T.OC 
 
 — T- 
 
 I 
 
 , I- 
 
 il-jt 
 
 1 1^ 
 
 I to 
 
 ' I ° 
 
 I '^ 
 
 I "-• 
 
 I "^ 
 
 U ' K 
 
 - IP 
 
 I — -^ (Jj 
 
 i^; CO 
 
 -^J' 
 
 Ti 
 
 ii 
 
 --./ 
 
 4 
 
 g^ 
 
 ^ 
 
 t3 
 
 « 
 o 
 
 i4 
 
 is! 
 H 
 
 z 
 o 
 
 
Dimensions of f Standard Coil 105 
 
 primary band for making tiie connections. A hard rubber 
 strip i"x|", 12" long, is screwed across the top of the 
 completed oscillation transformer. Two holes are drilled 
 in it 8-y apart. Into these two bushings, similar to those 
 used in the 12" coil and described in Chapter IV, are fitted. 
 When the secondary terminals are soldered to them the 
 oscillation transformer is complete. 
 
 All the parts of the 7" coil are mounted in one box. The 
 dimensions of this box are given in Fig. 42. The box is 
 built out of f" well seasoned oak. All the directions given 
 in Chapter VI apply to the construction of this box. The 
 cover is divided into two halves, one carrying the interrupter 
 and the other the discharge oscillators. 
 
 The connections from the interrupter to the condenser are 
 made through the hinges so that the cover may be swung back 
 without disturbing the connections. A partition is put in be- 
 tween the transformer and the condenser. It has a number 
 of holes drilled in it to allow of the free circulation of the oil. 
 Suitable handles are put at each end of the box for carrying it. 
 
 The transformer is now set on end in the smaller division 
 of the box. It is held in place by two yoke-shaped, wooden 
 supports fastened to the inside of the box and encircling the 
 core between the two sections of the secondary. The primary 
 terminals are brought to four heavy binding-posts at the upper 
 end of the box. They should be soldered on in the same 
 order as for the transformer on the large coil, that is, so that 
 shorting, the two middle posts puts the sections in series and 
 shorting the two outer pairs gives a parallel connection. 
 
roo 
 
 Tlie Tesla Coil 
 
 K > 
 
 
 <vi 
 
 / N 
 
 
 ^c 
 
 
 ^ 
 
 (. Q <J 
 
 -. -Is* Q 
 
 ■O* .. IK 
 
 
 
 <i^ 
 
 ,//- 
 
 -^j^ 
 
 TIL 
 
 "tt- 
 
 ^ ■,;* 
 
 oo 
 
 A 
 
 -la- 
 st- 
 
 .H^ 
 
 •^ ^ 
 
 '"I.. 
 _1 
 
 ■i.1 
 
 
 JCl 
 
 W 
 
 Z^ i _®_ ® ^B'^ ^_1.~^_ 
 
 i 
 
 
 
 lU 
 
 
 
 
 
 i 
 
 > 
 
 2 
 
 5 
 
 I. 
 
 
 t 
 
 c 
 
 
 n. 
 
 « 
 
 
 ^ 
 
 li 
 
 - 
 
 ^ 
 
 s 
 
 
 a 
 
 " , V 
 
 
 
 
 
 »• 
 
 "^ Ol v> 
 
 
 ^ 
 
 c; ^ 5 
 
 
 
 ■^ S ? 
 
 5 
 
 
 
 
 
 ~ ■»-> -kw 
 
 > 
 
 K 
 
 s s ^ 
 
 ul 
 
 a 
 
 5 
 
 i^ 
 
 V) 
 
 H 
 
 li ^ 
 
 
 
 
 J 5 f 
 
 
 PQ 
 
 Vj U ^> 
 
 -.-f/- 
 
Dimensions of 7" Standard Coil 
 
 107 
 
 The condenser and oscillation transformer are now put 
 in place, the condenser being between the two. The sec- 
 ondary terminals from the transformer are led in glass tubes, 
 suitably bent, directly to the condenser. From one side of 
 the condenser a wire is led to an end of the primary band 
 on the oscillation transformer. The remaining end of the 
 copper band and the other side of the condenser are directly 
 connected to the two hinges of the cover carrying the inter- 
 rupter. All connections should be carefully soldered. They 
 should be of about No. 20 B. & S. gauge copper wire, enclosed 
 in glass tubes and kept under the oil as much as possible. 
 
 o 
 o 
 
 Fig. 43. — Wiring Diagram. 
 
 Any one of the forms of interrupters described in Chapter 
 V can be used with the coil; the coil in question being equipped 
 with the motor interrupter. The connections between the 
 primary spark-gap and hinges can be run in glass tubes lying 
 in grooves cut in the under side of the cover. A piece of 
 I" hard rubber sheet should be screwed over the grooves 
 wherever there is any danger of shorting to the core of the 
 transformer or primary terminals. The connections are 
 shown in the wiring diagram. 
 
io8 TJic Tcsla Coll 
 
 The oscillators consist of two brass balls J" in diameter 
 screwed on the end of two -^^" brass rods 7" long, which are 
 to slide easily in two holes drilled \" from the top of the 
 standards, through both the fibre and the rod. A set screw 
 at the top of each standard will be of convenience in clamping 
 the rods in any desired position. 
 
 The standards are constructed as follows. Two fibre or 
 hard rubber bushings 2" in diameter and iV' in length and 
 having a flange \" thick and 2^" in diameter turned on one 
 end are set in two holes cut in the cover directly abo^•e the 
 holes in the brass bushings on the oscillation transformer. 
 A I" hole is drilled through the centre of each bushing. Two 
 f " brass rods 8" long are enclosed in fibre tubes f " in outside 
 diameter and 7 V' long. The tubes should fit the rods 
 tightly. The ends of the brass rods project from the fibre 
 and should be slightly tapered to fit the bushings on the 
 oscillation transformer. 
 
 In order that the discharge gap may be adjusted while 
 the coil is in operation, two ^'ulcanite handles I" in diameter 
 are screwed on the ends of the rods, carrying the oscillators, 
 for about i\". 
 
 The standards are now slid through the bushings in the 
 cover until they make good contact with the bushings on 
 the oscillation transformer. When the coil is now connected 
 up to the alternating current mains, it will break forth in 
 a beautiful 7" discharge. 
 
 If everything is not as it ought to be, the trouble may be 
 found in the manner described in Chapter VII. 
 
Dimensions of 7" Standard Coil 
 
 log 
 
 i 7 
 
 f - 
 
 m§$$^^^ 
 
 \- 
 
 5 i 
 
 Oscillator ■**« %5tandar.i> 
 7 Coil 
 
 s" hrasfi rod 
 
 BUSHINQ Foil 7 COIL 
 
 FiG, 44, — Oscillators and Standards for 7" Apparatus. 
 
APPENDIX 
 
 For those of our readers who have limited means at their 
 disposal, and who desire to carry on some of the many experi- 
 ments possible with high-tension currents, this Appendix 
 has been added. Besides many are not situated in cities, 
 where an alternating-current lighting supply is available, 
 but who possess an ordinary induction coil, giving a two or 
 three inch spark, which they may substitute for the trans- 
 former to be described in the present article. 
 
 This coil is not oil immersed, hence no boxes will be 
 required, as it is simply mounted on a base in a place free 
 from dust and moisture. A large amount of the precautions 
 regarding insulation and other things can be dispensed with, 
 thereby reducing the cost of the materials to within the reach 
 of almost every one. While speaking of cost, let us state 
 that to purchase a coil giving a 12" spark from the regular 
 dealers would mean an outlay of about $300, while the 7" 
 coil in a single box is worth S165. The cost of construc- 
 tion by the amateur, not considering his time, should not 
 exceed S50 for the 12" coil and $25 for the 7" coil. 
 
 This piece of apparatus giving about a 3'' spark should 
 not exceed Sio to build at home. It is large enough for 
 most of the experiments on Roentgen and Geisler tubes and 
 for wireless work o\-er short distances. 
 
112 The Tesla Coil 
 
 The above sums include the simple interrupter. The 
 others will bring the price up in proportion. 
 
 The high-frequency coil is made as follows: Cut out two 
 end pieces of i" wood lo" square and describe on each one 
 two concentric circles, having diameters of 9 and 7 inches 
 respectively. On these circles bore a number of \" holes i" 
 apart as in the figure. Next procure from a planing mill 
 about twenty Y dowels. These are made of hard wood and 
 come 36" long. Cut each dowel into 12" lengths and fit one 
 in each of the holes on the smaller circle of one of the boards. 
 When they are all in place the other board is put on the 
 other end of the dowels. The outer circle of holes is left 
 empty until the secondary is wound. 
 
 The secondary winding consists of one layer of No. 32 
 B. & S. gauge double cotton covered copper wire. Begin the 
 winding about Y from the ends. Shellac the wire with 
 several coats of the best orange shellac when the winding is 
 finished. 
 
 The dowels for the primary are next put in place by push- 
 ing them through the holes from one end. If they fit too 
 tightly the holes may be reamed out. Next six turns of No. 
 18 bare wire are wound on the outer dowels, each turn being 
 over an inch from the one next to it. 
 
 The whole coil is then mounted on a base. The ends of 
 the primary are connected to two binding-posts mounted on a 
 piece of hard rubber. Two oscillators with standards are 
 provided for the terminals of the secondary. This com- 
 pletes the high-tension coil. 
 
A ppendix 
 
 "3 
 
 ^- 
 
 ^-- 
 
 1 
 
 
 
 
 
 ( 
 
 
 o 
 
 o 
 
 ® 
 
 o 
 
 o 
 
 o 
 
 
 o 
 
 ? 
 
 o 
 
 o 
 
 o 
 
 o 
 
 
 
 o o 
 
 o 
 
 
 
 -I'li 
 
 
 o 
 
 o 
 
 
 
 y- 
 
 
 o 
 
 o 
 
 
 
 1 
 
 
 ^ o 
 
 o 
 
 
 
 1 
 
 
 ^ o 
 
 o 
 o 
 
 
 
 { 
 
 
 ^ o 
 
 o 
 
 
 
 1 
 
 
 .- o 
 
 o 
 
 
 
 1 
 
 
 ~.;^ O 
 
 o 
 o 
 
 
 
 -1^ 
 
 
 ^ o 
 
 o 
 
 
 
 1 
 
 
 o 
 
 o 
 
 o 
 
 
 i 
 
 
 o o 
 
 o 
 
 
 o 
 
 o 
 
 o 
 
 
 o 
 
 
 
 
 o 
 
 
 
 o 
 
 o 
 
 o 
 
 
 o 
 U 
 
 
 o 
 
114 '^^'^ Tesla Coil 
 
 The condenser consists of fifteen sheets of window glass 
 lo" X 12", with a piece of tin foil 8" x 10" between each sheet 
 of glass. The method of arranging this condenser is as 
 follows: Lay a glass plate on a smooth table and gi\e it a 
 coat of shellac. While still wet place a sheet of tin foil on 
 top of it, leaving an inch margin of glass all around. On one 
 corner lay a strip of tin foil projecting an inch beyond the 
 glass. On top of this lay a second sheet of glass and another 
 sheet of tin foil, only the strip in this case is brought out 
 on the opposite side. Continue this until the fifteen sheets 
 of glass are used up. This will give seven sheets of tin foil, 
 with the strips coming out on the one side and seven pro- 
 jecting on the other side. The strips may be fused together 
 with a hot iron and a copper wire soldered on. The whole 
 condenser is bound together with insulating tape and is 
 best mounted in a box. This completes the condenser. 
 
 The transformer for use with the 1 00-110 or 50-55 volt 
 alternating-current circuits is the next piece of apparatus 
 to construjt. It is essentially the same as the two trans- 
 formers already described. The core consists of a bundle 
 of No. 20-22 iron wires, well annealed. The diameter is 
 lY and when formed after the method described in Chapter 
 II is wrapped with insulating tape. The primary is wound 
 in two sections one above the other. See Fig. i for the method 
 of fastening the layers. Each section consists of one layer of 
 No. 16 B. & S. gauge double cotton covered copper wire. 
 After the primary is wound wrap on several layers of paper 
 well shellacked until the diameter is built up to 2}". 
 
Appendix 
 
 "5 
 
ii6 The Tesla Coil 
 
 The secondary winding of this transformer consists of 
 two sections of No. 32 B. & S. gauge double cotton covered 
 copper wire. First saw out of \" stock four circular pieces 
 of wood, 4" in diameter and having a 2 J" hole in the centre. 
 Slip these on the primary to the positions shown in the figure. 
 The two end ones are |" from the ends of the core and the 
 middle ones are \" apart. 
 
 Wind the wire of the secondary on the two spools just 
 formed until the diameter is 3J". Thoroughly shellac each 
 layer and wrap a piece of paper on before beginning the next. 
 The whole coil is mounted on a suitable base, the primary 
 terminals being connected to binding-posts. 
 
 If the transformer is to be operated on the loo-iio volt 
 current, the two sections of the primary are connected in 
 series. If, on the other hand, it is to be u^ed on the 50-55 
 volt current the sections are joined in parallel. It is well, 
 however, in either case to bring the primary terminals out 
 to four separate binding-posts. Then the desired connec- 
 tions may be readily made for either series or parallel. 
 Always be certain, though, that the current will traverse 
 the windings in the same direction. 
 
 In order to set up the high oscillations we must introduce 
 a spark-gap in series with the secondary of the transformer 
 and the high-tension coil. The method of making this 
 primary spark-gap is given as follows: Procure two pieces 
 of vulcanized fibre rod |" in diameter 4 inches long. Drill' 
 a j" hole in each V' from one end. Next bore two -|" holes 
 6" apart in the base of the transformer as shown in Fig. 47. 
 
Appendix 117 
 
 Drive the fibre supports into these holes with the holes in the 
 fibre in line. 
 
 The spark-gap is made of two ■}" brass rods 6" long with 
 fibre tube 2'' long slipped over the end to act as an insu- 
 lating handle. One lead of the secondary of the transformer 
 goes directiy to one rod, the other goes to the primary of the 
 high-tension coil. The return wire from the primary of the 
 high-tension coil is soldered to the other side of the spark-gap. 
 The diagram shows how the condenser is connected and also 
 the connections just described. 
 
 — 6 
 
 i 
 
 
 ^H 
 
 ^-- 1- --^ 
 
 Fig. 47. — Prim-^ry Spakk-g.\p. 
 
 There is no interrupter used with this apparatus so that 
 care must be taken that the spark is long enough to prevent 
 arcing. 
 
 Those possessing a suitable induction coil and who wish 
 to substitute this for the transformer and primarj' spark-gap 
 may do so by changing one connection. Disconnect one 
 terminal of the secondary from the discharger and connect 
 the secondary terminal to a binding-post suitably insulated 
 by hard rubber. One terminal of the primary of the high- 
 
ii8 
 
 The Tesla Coil 
 
 tension coil is connected to the spark-gap instead of the sec- 
 ondary of the induction coil. The other terminal of the high- 
 tension coils primary is connected to the new binding-post. 
 A glance at the figure will make this plain and also the method 
 
 Fig. 48.*-^ WriUNG Diagram. 
 
 Fig. 49. — Wiring Diagram. 
 
 of connecting up the condenser. When making connections 
 between the various parts of the apparatus it is well to enclose 
 the wires in glass tubes and to keep them back out of the way. 
 The operator will soon find that ordinary insulation is of 
 
Appendix 119 
 
 no value whatever in dealing with these high-tension currents, 
 so that all terminals must be kept apart a distance greater 
 than that of the high-tenson discharge gap. If this pre- 
 caution is not observed you will have some very beautiful 
 brush discharges all along the conductors that are in too 
 close proximity. 
 
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