The Tesla high frequency coil, its construction and uses

Front matter

UNIVERSITY

• UBRARY

Hii

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 current, 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

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 Transformer. 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.

Throughout the whole work of construction the most exacting 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 insulated 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 obtainable. 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 insulation 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 secondary 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 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 magnetic 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 mathematics 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 ordinary 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 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 reduced 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 thoroughly, 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 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 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.

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 projecting 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 remaining 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 numbered 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 secondary 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,

II r

-Idf ^

1-^

<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 o "n

F\ c -a

I

M O

C5 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.

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 electrical 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 galvanometer.

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 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 insulation, 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 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-

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

!lbi'Anm(Trn-F-n-3T^"'T:::r;

m.

Fig. 6. — Section of Completed Transformer.

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 immediately 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 condenser. A thin dielectric, however, cannot withstand a high potential. Besides the thickness, the dielectric strength depends 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 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