- PLATE IV. Motor driven Interru t r p e .

The Electro l tic Rectifier. PLATE V . y

P LATE ” II . Trans ormer for 1 2 A a ratus f pp .

” 1 2 Coil .

T H E T E S L A H I G H F R E Q U E N C Y C O I L

I TS C O NS TR U C TI O N A ND US E S

BY 0 , Q GEOR GE F F HALLER AND ELMER TILING CUNNINGHAM

56 ILLUS TRATIONS

NE W Y O R K A R A D M P D . V N NO S T N C O A NY

2 M Y A D 0 2 ' 3 URRA N 19 1 7 WARREN Sr s . o ri ht 1 1 0 b C py g , 9 , y D AN N S N NY . V O TRA D COMPA

” i c . P 1 om ete 1 2 A aratus . F ront s ie e LATE . C pl pp p INTRODUCTION

IN presenting this b ook on the coil to the public the authors hope that they have filled a long felt vacancy in the

practical library of science . No attempt has been made to

n give a mathematical explanatio of the ,

a s h- and other p rt of the hig 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 m ff rent, but have merely ade an e ort to place in the hands of advanced amateurs in electrical science a practical working

- manual on the construction of coils , now so

useful in scientific investigation . The attention of the authors was first called to the when they were fortunate enough to be given the use of

“ ” 1 the 7 standard coil described in the last chapter of this

on book . A systematic line of experiments was carried with V } efiec ts it, in order to study the of a change in the constants k 3of the various circuits . All the mechanical and electrical ‘ 3 5 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

” 1 T finally arrived at the 2 coil described in this book . his

emcient coil they feel assured is as as can be made . It is

especially designed to give a high - frequency discharge of T k great volume . his latter fact ma es it useful for .

0 . In conclusion they have to thank Mr . G . Mitchell for many suggestions and for the kindly interest he has taken in T this work. hey feel that without his help the writing of this little book would have been impossible . CONTENTS

CHAPTER I GENER SURVEY . AL

II THE T NS F E . RA ORM R

III THE N ENSE . CO D R

IV THE OS N T NS F R E . CILLATIO RA O M R

THE IN E U E V . T RR PT R

VI THE NS U N OR THE XE S . CO TR CTIO BO

VII ASSE N . MBLI G

III THE Y THE Con. V . OR OF

IX USE S THE Con. . OF

X D E NS NS OP ’ S N . IM IO 7 TA DARD COIL

APPE NDIX

LIST OF PLATES FACING PAGE ’ Complete 1 2 Apparatus F rontispiece

’ Transformer for 1 2 Apparatus

’ O sc illation T ransformer and Glass for Condenser of 1 2 Coil Motor-driven Inte rrupter The Elec t rolytic Rec tifier

’ Discharge from the 1 2 Coil

’ The 7 Standard Apparatus

Li st of Figures

High -tension B ushing O sc illators and Standards Wiring D iagram

Waves on Wires

’ P rimary and Core of Transformer of 7 Coil ’ Secondary B ob b in of Transformer of 7 Coil Plate and Frame of Condense r ’ O sc illation T ransformer of 7 Apparatus ’ Box for 7 Apparatus W 1r1ug Diagram ” O sc illators and Standards for 7 Apparatus O sc illation T ransfo rmer of Small Coil Completed Transformer of Small Coil P rimary Spark-gap Wiring Diagram Wiring Diagram TH E TE S LA C O IL

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 electromagnetic

to disturbance in the ether, and thus added that subject the realm of electricity . Amongst the various phenomena of

- electricity, those of the high tension current are the most

all interesting and instructive . With such a current 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 S how 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

n T - C an in k own as the esla High Frequency oil , d consists ,

- h : I . Th T . T e general , of four parts e Step Up ransformer "2

Th . T Interrupter " 3 . e Condenser " 4 The Oscillation rans

E of former . ach these will be fully considered in subsequent chapters . Before entering upon the description of the Tesla high

u frequency apparat s , however, it would be well to make a few general remarks which are of the greatest importance . CHAPTER II

THE TRANSFORMER

THE transformer sometimes called a converter is

ee an m r ly that. is connected directly to the

- mains, without the use , of an interrupter, and is used to raise or lower the . In a transformer the number of in the primary equals approximately

r the number of watts in the seconda y .

- u In the case of any step p transformer, the ratio of the number of 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

m - - - of the com on induction coil type, oil immersed , step up transformer . It takes the alternating current from the mains

1 10 at volts or 55 volts , and steps it up to about volts .

The efficient working of a transformer depends largely

The upon the design of the core . 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 l its maximum value to zero , the magnetic flux fal s from its maximum value, not to zero , but to a value which depends 4 The Transformer 5

The on the residual magnetism . 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

of core than with a closed circuit iron . The coil designer is obliged to determine the length of the iron core from the experience of others , as the mathe

r too matics fo calculating it is complex, although simple and useful in the case of closed circuit . If the core is made too long the primary magnetizing current will be

w too too large , hile if made short the secondary coils would

of too to ffi T have to be made large a diameter be e cient . here 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

of for matters "in fact, all the dimensions the transformer this special use were determined in this way .

Th f 20 2 2 S . e iron core is made up of pieces o No . or B .

” 1 The to gauge iron wire 8 long . wire is first cut nearly

of size with a pair pliers, and , when assembled , the ends of the bundle are sawed off square with a hack saw . An ordi

” of 1 8 nary piece iron pipe , a little less than long, and having an internal diameter slightly less than 2 is tightly filled

s with these wires . When putting the wire in, stand the pipe on end on a smooth surface , and force in each wire until 6 The Tesl a coil

it hits this surface . When the bundle is finished, the upper end is sawed off with a hack saw to exactly

' 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 T insuring slow cooling . his heating and subsequent slow cooling so softens the iron wires that their retentivity is re

ce i out du d to a minimum . When cool , the w res are taken

to T and sandpapered remove any superfluous oxide . hey are

one one then, by , dipped into boiling water, wiped dry , and while still warm are coated with thin varnish . When the shellac is dry they are again packed , as tightly as possible ,

i T in the p pe, to hold them in the desired shape . hen , 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

The from any surgical supply house . bandage should be

one no . between and two inches wide, but more When the entire core is wrapped with this cloth , the cloth should be

he w heavily shellacked . T ends of the core are no filed flat and smooth "after this it is put in a warm place to dry thor

f r the The oughly, when it will be ready o primary winding . 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 ffi lower the transformers e ciently .

The primary is wound in two sections of two layers each ,

. 1 2 . S . one above the other . No B gauge, double cotton

o er 2 o n c v ed copper wire is used . About } p u ds will be The Transformer 7

The r required . prima y may be wound by hand, by erecting

” ” 1 2 two wooden supports 7 apart, and having a hole T bored in each, to receive the iron core . hen, by turning the core by hand the wire may be wound fairly well . But iffi l as it is rather d cu t to wind the wire tightly in this way,

to it would be more satisfactory wind it in a lathe, if the

To - amateur has access to one . mount it , cut a half inch

of piece from the end 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

r - core a few strips of tin o other thin sheet metal . Now clamp

The it firmly in the chuck . other end of the core should also be fitted with a half- inch piece of pipe and supported at this

The n place in the steady rest . o e piece of pipe is used to prevent any of the wires from being forced in unequally at m f the points where the chuck cla ps it, and the other to a ford

o a sm oth bearing surface for the steady rest . If there is

for t a any tendency the core o slip out of the chuck, the t il stock, with the centre removed , may be pressed up against it .

About 1 ft . from the end of the copper wire take a couple of turns of tape around it . At this point bind the wire to

” 1 the iron core , about from its end , by taking several turns

f w o tape around it . Proceed no to wind the wire tightly and

” 1 f closely to within o the other end . Here the winding of the primary is stopped for a short time in order to give the d . wire a good coat of . shellac After the shellac has drie , another coating is given it , and then the second layer is wound on while the wire is still wet . When the winding 8 The Tesla Coil has come to within about Six turns of the starting point , a l piece of tape doubled back on itself is laid on the first ayer , with its ends projecting beyond the unwound portion of the

The the second layer . looped end of the tape must be on

See . 1 . outer S ide of the winding . Fig

D i a ra m s s ho w i n m a ne r o a c ta n m a g n g L s t t u m . g f f '

— ME OF F S EN N P Y TE N S - ETED FIG 1 . THOD A T I G RIMAR RMI AL COMPL P R Y IMAR .

The winding of the second layer is finished over the piece b b of tape , the last turn eing rought through the loop in the

The b on tape . loop is drawn tight y pulling the other pro jecting ends . In this way the last turn is kept from slipping ff o . 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

' The off 2 getting loose . wire is cut about from this ending

10 The Tesla Coil

1 this comes another disc and a ring, followed by a disc

” and a gi ring . Then put on two more discs and the remain T ” ing ring . his is followed by the remaining disc and 1 ring . Be sure that each ring is carefully glued in place .

Before putting on the discs , small holes should be drilled in

r Th them , through which to ca ry the wires . e completed i h 2 . bobbin for the secondary is seen n Fig . T e discs num

2 6 . f r bered , 3 , , 7 have the holes o the connecting wires drilled on their inner edge, while the others have them drilled about from their outer edge . Obtain a wooden rod upon

r which the seconda y bobbin will fit tightly . It should be ” 1 85 long .

If the coil builder i s skilled in winding wire in the lathe, the winding may be done there much more rapidly than by

w ho hand "but for an amateur , has had but little experience

or s with lathe winding, for one who does not po sess 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 f the end o the layer is reached, and while the paper is being

n wrapped o before the next layer is wound .

For rod n the hand winder, the wooden , o which the second ary bobbin fits tightly , is drilled in at both ends for about ” ” f ” 4 with a little less than a 1 hole . Pieces o 1 iron are then

to T driven into these holes , serve as an axle . hey should 6” fit tightly, so as to turn with the cylinder . About should

e 1 7 project at on end, which is bent into a handle . } at the

ffi for other end is su cient a bearing .

” ” The of oak 1 standards are made 2 , fastened 91 apart, The Transformer I I 1 2 The Tesl a Coil

’ t” ” to a baseboard 2 long . A piece of oak i 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 T on the joint . his allows the cylinder to be taken out of T its bearings when necessary . w o 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 T them , are put on the other end . his will give the friction required to enable the amateur to stop the winding at any

a time , and still be sure that the cylinder will not rot te and so

The loosen the turns of wire . 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 ” T is 4 longer than the spool . here is a thread cut on one end of this axle for about It is then mounted in two wooden standards fastened to a baseboard . Iron washers are put between the spool and the standards for thespool 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 I S 1 4 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 T bobbin, unknown to the coil builder . o 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 1 wide

” T - and 3 in diameter . his is fastened by several flat headed brass screws to one side of the spool on which the wire is

- H FIG . . W E S E 4 IR POOL OLD R .

e e bought . If the wire has been bought from a r liabl dealer, the inner end will be found projecting outside of the reel .

r f This wi e is soldered to the ring on the outside o the spool .

of A strip sheet copper or brass , which is of such a length

o of that i t will bear on the ring, is fastened t the upper end

on . the standard, on the side which the ring is From here a wire is led to one pole of a dry cell . The Transformer 1 5

On the winder a strip of sheet metal is fastened to one of

to the standards . It is best 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

rical on t contact . A wire is then led from the brush the

to s on e standard a binding po t the bas board . A

receiver is now connected in series with the binding-post and

- the other pole of the cell . A watch case receiver, with a h . t e head attachment, is the best to use If 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

me er no t . E verything is now ready for the winding of the secondary .

’ To 1 of begin , pass about the wire through the hole in the

2 bobbin heads numbered , 3 , from the side on which bobbin

The head 2 is . insulation should be scraped 05 of the end of the wire for about 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

o The circuit through all the wire n the spool . diaphragm in the telephone receiver is drawn down or if a galvanometer

e . is used , the needl will be deflected If the wire should break, the diaphragm will return to its normal position and

or of a click will be heard , in the case the galvanometer the needle will return to the zero position . When this happens

e the break should b located and the wire soldered . Acid

d on should not be used in sol ering, as a little left 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

2 1 1 , 2, is wound from to , 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 previous

The n one . edge of the paper can be held dow with a little

Paraffine i shellac . wax must not be used to increase the n sulation, as the transformer when finished is immersed in

rafline araffine pa oil , which acts as a partial solvent to p wax, thus spoling its insulating properties . All the layers after ” the first should start about i 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

C 6 1 around it . ontinue winding until 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 secti on for making the various connections . This wire is

1 then brought through the hole in disc , and its end is connected to the axle .

Unwind the other wire from the axle and, after polishing

Of the uninsulated part with a piece emery cloth , twist it around the end of the wire from the spool , which has also been polished, and then solder the connection . Wrap the bare part of the wire with some silk thread, so as to thoroughly insulate it .

The t now section be ween the bobbin heads 3 , 4, is wound .

1 8 The Tesla Coil

FIG F E SE N AR T NS E . 5 RAM FOR CO D Y OF RA FORM R.

hl in for oug y dry , and the meantime construct the frame the Th . . e secondary Working drawings are given in Fig . 5

” ” ” 1 x o The base is made of a piece of 9 pine z i long . corners T and edges are rounded to give it a better appearance . hree

” ” of 1 supports x 7 pine , high , are erected at the points

E N OF E E T AN FIG. 6 . S C S E CTIO OMPL T D R FORM R. The Transformer 1 9

of a - e shown in the figure . At the top each a h lf round hol i in diameter s cut . Into these fits the tube on which

The the secondary is wound . distance between the supports is

’ The two terminals from the middle coils of the secondary are now soldered together, and the connection wrapped with

The silk thread to insulate it . 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

o ately spoils all the adhesive qualities f the type . CHAPTER III

THE CONDENSER

A CONDENSER is an apparatus for accumulating a large

on The quantity of electricity a small surface . form may f vary considerably, but in all cases it consists essentially o

w - or t o conductors separated by a non conductor dielectric , and its action depends entirely upon induction . The thinner the dielectric and the greater its specific

of n inductive capacity , the greater is the capacity the co denser . A thin dielectric , however, cannot withstand a high potential . Besides the thickness, the de pends on the character of the material . The condenser used in this apparatus is especially designed

T e for long continued use on high . h dielectric used is glass and the plates are made of sheet brass . When

r fli n finished the condenser is immersed in pure pa a e oil .

” ” ' 10 x 1 2 For the dielectric 95 sheets of glass, l thick and

' T sh l b . in size, ou d e Obtained hese sheets may be had cut

r n to size for about nine o ten cents apiece . I purchasing them each sheet should be examined carefully to see if there are any air bubbles . If any are found in a sheet of glass it should be rejected .

or - The brass used is number 32 or 34 . F ty six sheets

20 The Condenser 2 1

” ” 0 m 8 x 1 are required . In one of the co ers of the shorter

” ” ” I side is a tongue 2 wide and i long . A 1 lip is bent across 8” the top of this tongue . If the brass can be had in rolls

a wide, a little more should be obt ined and the tongues cut

” ” T The 1 out of it . hey should be cut 3 x extra is for R soldering them to the plates . osin, not acid , should be used T as a flux . hese tongues should always be soldered in the position shown in the figure .

” As a rule 1 2 is the only width that can be obtained in

are most places . When this width is used the tongues cut

S right on the heets . The condenser occupies the part that has been constructed

The for it in the oscillation transformer box . frame in which the condenser is built up is made out of well dried

” ” ” The of 1 1 1 1 1 pine . base is made a piece of 5 x pine, 1

”

T e S x 1 . long . h ides are cut out of 5 2 material long

” ” ” T e x 1 1 h ends are also cut from % 2 wood and are 1 long . The sides and ends should be planed up smooth on both

” The sides , so as to make them a little less than 5 thick . 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 C clean so as to remove any dust or moisture . ommence by putting two glass sheets in the frame so that they reach

o r the b ttom of the f ame . Place a brass sheet on top of these

” glass plates so that there is a 1 margin of glass all around the

f the sheet, except were the tongue comes out. I the lip on 2 2 The Tesl a Coil

tongue has been bent carefully it will just fit up against the

s f sheet o glass . Without displacing the brass lay two sheets

on to . of glass p of it A brass sheet is next put in , but in this

o t T case the tongue comes u on the reverse side . here should

1 be a margin around the brass as in the previous case . After

of T the brass come two more sheets glass . his process is

- lit Pr a m: for Co nde ns e r

‘ ha e B . S p of b r a s s s he a f Dot te d Im e s s h o w 8 12 : of ton ue whe n s o ld ne d n g e o .

2 — FIG N E NS E F E AND S S N ENSE S E E . 7 . CO D R RAM BRA CO D R H T.

6 of kept up until the 4 sheets brass have been put in place .

T to hree sheets of glass are placed on p of the last brass plate .

If the glass and the brass used are the size called for in this

s s l us book the la t heet wi l j t go in tightly . In forcing in the last few sheets it is a good idea to lay a cloth between the last

two, to ta ke up any excess pressure, which would otherwise cr he ss ack t gla . The Condenser 23

Set the condenser upright, when finished , and solder a

’ 1 6 b to piece of NO . bare copper wire a out 3 long each of the lips in turn , down the one side, and the same is done on the other side .

Tw o leather straps should be fastened to the sides of the frame to lift it by w hen lowering into the oscillation trans

The former box . object for building the condenser in a

of separate frame , instead in the division in the oscillation a transformer box , is to f cilitate moving should the condenser ever require rebuilding, due to the rupturing of the glass sheets .

b ox When the condenser is placed in the , the end which has the three glass sheets should be placed against the parti T tion , that is , nearest the oscillation transformer . his is to prevent the spark from the oscillation transformer breaking through into the condenser, instead of following its air path . CHAPTER IV

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

Th s . e with which this part is constructed lea t fault, such

s as two wires touching, or many other small similar mistake ,

o may cause a short circuit and require its reconstructi n .

o It is not wise to hurry the work, as it will be necessary t

reconstruct it if careless .

The end supports are made out of any suitable piece of

” The wood . two supports for the secondary are 8 in diam

” ” 1 E eter, and from i to in thickness . ight equidistant points

” are marked off on the periphery and slots deep and 5

See . . T wide cut at these points . Fig 8 hese slots are for

on o the fibre strips, which the secondary is wound , to fit int .

” ” These strips are 1 7 long and if 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

receive a small brass screw, which is to fasten them to the

” 1 end pieces . A wooden rod about in diameter is now T obtained and a shoulder turned on each end . he diameter

” of the shoulder is is and its length just equals the width of 24

26 The Tesla Coil bent or warped they S hould be straightened before being

to fastened in place . A good way straighten them is to lay

tw o o them between b ards, placing some heavy weight on

h . t e top board , and leave them thus over night

f 2 r o . 8 . About 4 o 5 ounces NO B 81 S . gauge double cotton

is T covered copper wire required for the secondary . here are several equally good ways in which the wire can be wound T on . w o the frame 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

” 1 8 o t on long is first turned u , and it are screwed the eight

so strips Of fibre, that their ends are in line, and that they

ne strike o another . If the amateur has a couple Of clamps

f S i made Of strips o heet iron with a bolt through the ends , t will greatly help matters by clamping them around the strips ,

"

6 . about apart , and moving them as needed A light cut is

a first t ken Off the strips, and then they are polished with a

1 8 now file and sandpaper . A NO . thread is cut, starting

" about 1 from the end of the strips to within an inch of the

S . other end . It hould be cut just as deep as possible In order to make a clean cut the tool must be very sharp and

n several light cuts should be taken instead Of o e . 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 The Oscill ati on Transformer 2 7

’ 8 T 1 O . 2 of one of the strips wrap about of N wire . his is to be used for connections . Starting at this point wind the

n wire tightly o the frame, always keeping the wire in the

’ r 1 groove cut fo it . About extra should be left at the end

n . for making the co nections When finished , a heavy coat of shellac is given to the wires where they rest on the fibre

k . strips . When this is dry all the wire is heavily shellac ed A soft brush should be used so as not to displace any of

The the wires . secondary should now be placed in a warm place to dry . The following method can be used in case a lathe is not f available . A spool o silk thread and some silk are re T t quired . here must be enough silk o make two turns

. on around the secondary frame It is wrapped tightly , and smoothly , and shellacked in place . It might be mentioned here that shellac dissolved in wood alcohol should not

" The 1 be used . wire is then wound on , starting about t from the end up to within an inch of the o her end . A silk thread is wound on at the same time between the turns , to keep them apart . When it is all wound the wire is heavily shellacked and the frame put in a warm place to thoroughly dry .

” ” Get two pieces of 52 x 1 2 pine 1 2 long and find the centre f Of each by the intersection o their diagonals . With these

” ” radII points as centres describe two circles , with 45 and 55

” o D n each board . ivide the circles having the radius 55 T into 36 equal parts . hese points can be located with a

’ one protractor or mariner s compass, there being every 28 The Tesl a Coil

” At each of these points drill a 5 hole . In each comer of these

"

1 1 1 . boards cut out a piece x See Fig . ” 6” ' T At a planing mill buy eighteen 5 dowels , 3 long . hese

f r one E one may be had o cent apiece . ach should be cut

” ” - 18 in two , which will give you thirty six 5 rods , long . ” Fasten one of the 1 2 end pieces to each of the ends of the

A 431.

FIG 1 END S P 1 . U . PPORT FOR RIMARY.

flat- completed secondary frame , by four headed brass screws .

See that the circular end pieces of the secondary just fit on

n The the smaller circles drawn o the boards . 5 rods are

on now driven into place , thus forming a circular cage which the primary is wound .

’ r 8 6 r Fo the primary get an piece of No . 3 copper ibbon

” 5 wide . Wrap the end of the ribbon once around the end The Oscill ati on Transformer 29

. 1 2 of the dowel shown in Fig and solder it in place .

e A piece of copp r wire should also be soldered on here . Starting from here take two and a half turns around

FIG 1 2 P . . RIMARY OF O SCIL N T NS R E LATIO RA FO M R.

T the secondary frame . his 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. 1 E E SEC 3 . COMPL T D OND ARY OP O S CILLATION T NS E RA FORM R.

The of soldered on . turns the ribbon should be equally

” ” ” of 1 x 1 18 spaced . Get four pieces pine 5 long and fasten them as braces from the one end of the frame to

in r s the other, the pieces fitting the co ner that were cut out for them . The Tesl a Coil

FIG 1 U S H N S . 4 . B I G FOR S U PPORT OF O S CILLATOR N R S STA DA D .

— FIG . 1 . H RU E C K ON O S N T NS E 5 ARD BB R BLO CILLATIO RA FORM R . The Oscillati on Transformer 3 1

” Turn two bushings out of a piece of 5 brass or copper rod .

” 1 The n They are S hown in Fig . 4 . le gth over all is 5 and

” " the shoulder is 5 thick . A g hole is drilled down the

S centre , and the shoulder is drilled and lightly countersunk

tw o at two opposite points to receive small brass screws .

” ” Tw o of 1 2 pieces vulcanized fibre or ebonite 5 x long , with

” ” to a 5 hole drilled 5 from the end , are fastened the middle

The of the top S ide of the primary frame . bushings just

The made are fitted into the holes and screwed in place . wires from the secondary are soldered onto these bushings . When this is done the oscillation transformer is finished and

The all that needs to be done is to connect it up properly . completed oscillation transformer is seen in plate III . CHAPTER V

THE INTERRUPTER

LET us now consider that important part of the apparatus,

- The the primary . function of this, as previously

o stated, is t provide a path of high resistance until the con T denser is charged to its full capacity . hen 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 have

- ceased , the ideal spark gap should return to its maximum disruptive strength before the condenser can be charged by

of the next cycle from the secondary the transformer .

The In practice this is far from being the case . air

ff s between the discharging balls becomes heated , and o er

- for T a comparatively low resistance path the current . his

re results in an a being formed , which prevents the condenser

The from performing its function . 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 S how several ways of par

tiall . y 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 res ults 3 2

34 The Tesl a Coil

1 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

. C current , which would probably be fatal onnection can be made t o the brass standard near the bottom by drilling

P l a n S i am

~k s

FIG 1 6 S E P AIR- . . IMPL RIMARY GAP.

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 6 ee . 1 . to the Operator . S Fig TheI nterrupter 35

THE AIR-BLAS T INTERRUPTER

- efli i In order to make the air gap more c ent, 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

” To ffi much higher . e ciently accomplish this a piece of 5 hard glass tubing is drawn out into a nozzle having an 3 ” T Opening 1 3 in diameter . his 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 are that tends

to form , thereby increasing the disruptive length of the bright T . he oscillation transformer discharge operator will find ,

to however, that it is a tedious task 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

- - r a magnetic wipe out or a motor driven inte rupter .

THE MAGNETIC INTERRUPTER

The magnetic blow-out is well suited for those who have a — source of at their disposal "either the I I O VO lt

r circuit or a suitable batte y 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 Tesl a Coil

which will give a current suitable for magnetizing the magnet

of this interrupter . Tw o standards of the same form as those used in the simple 6” ” interrupter, but in length and having spark balls 5 instead

” " of u 1 0 5 in diameter, are mo nted on a hard rubber base x The fibre handl es and shields are also necessary for this in

” rr r TWO te upte . electromagnet bobbins 55 long and having

” " an iron core 5 in diameter with fibre heads 3 in diameter

” ” ” 1 x are procured . An iron yoke made from 5 iron 7 long

” 2 1 has holes drilled in it in the middle 5 from both ends , and

Tw o the bobbins are fastened to it by a screw in the core .

” of —S polar pieces 5 square iron, filed into an egg haped point

to The are screwed the upper ends of the core . bobbins

f 1 - o O . are wound full N . 4 B 81 S . gauge cotton covered magnet

n f wire, if they are to be operated o a battery current o from eight to ten volts such as a plunge battery . If a direct cur

i 0 O 1 1 2 . rent volts is available NO . 4 should be used If the

2 rectifier described at the end of this chapter is used NO . 2 wire should be used as the voltage Of the rectified current - on

1 10 0 The volts alternating is about 9 volts . magnets must be thoroughly saturated in order to give the best results . After the magnets are finished they should be fastened to

o the hard rubber base , at right angles t the rods carrying

The the spark balls, by two screws through the iron yoke . balls of the spark- gap should be between the projections

n n Olar of the mag et . A sheet of mica is bent arou d the p projections of the magnets in order to prevent the spark

fr m . 1 . om ju ping to the cores of the magnets . Fig 7 The I nterrnpter 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 1 M NE IN E U E . 7 . AG TIC T RR PT R .

’ The principle of this piece of apparatus is based on Davy s experiment in which he found that the is extin

uis ed g h upon the approach of a magnet .

THE MOTOR- DRIVEN INTE RR UPTE R This interrupter is the one the writers used in their earlier

” experiments with the 7 standard coil described in the latter

of part Of this book . It consists essentially a fan motor

on 1 10 run the alternating current at volts , driving a brass disc having a number Of projections bolted around its

n face, and a brass oscillator so mou ted that the distance separating it from the projections on the disc can be varied

The at will . motor may be of any suitable design that the builder may possess . A small battery motor running 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

n The has not the facilities for building o e . directions for

building a suitable are given at the end of

this chapter .

M r u b“?

10

FIG 1 8 M IN E U E FAN . . OTOR T RR PT R .

" To make the disc for this interrupter turn out of 5 sheet

” 1 0 m no brass a circular piece in dia eter . If lathe is avail i ’ able t may be procured at a model maker s quite reasonably .

” ” La Off e 8 y on its face two concentric circl s , and 9 5 in diam D eter respectively . ivide the inner Of these into thirty equal divisions and draw radial lines from the centre of the

Off disc through each of the points marked , thus dividing The I nterrnpter 39

the outer circle into the same number of equal divisions .

Drill a hole through each of the points laid Off on both — 6 circles and tap them to fit a standard 4 3 machine screw . 1 ” A number of brass angle pieces made by bending “16 brass

M a ta -m l

FIG S AN E P 1 . S E E . 9 BRA GL I C .

S T into the form hown in the figure are procured . w o holes — are drilled and tapped in each one to fit a standard 4 36

- T flat headed machine screw . hese pieces are screwed to

" a the br ss disc wit 5 screws .

” 3 " A 5 hole is drilled in the centre of the disc and three 1 3 holes are drilled on a circle having a radius of 5 Next 40 The Tesla Coil

turn out a circular block of hard rubber 2 in diameter and

20 The the same shape as in Fig . . brass disc is screwed

” to it with three brass wood screws 5 long and the whole is fastened to the shaft of the motor so as to b e well T insulated from it . o make electrical contact with the brass

F G 20 H RU E I . . K ARD BB R BLOC .

plate a brush to bear on the back near the centre is cut out 1 ” T of a piece of 5 5 sheet spring brass . his piece S hould b e ” ” n 10 long and 5 wide . It is mounted o 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

” ” - Of 6 spark gap is used,but instead being 4 long it is in length

The I nterrupter 43

A S MALL SE LF- S TA RTING S INGLE - PHASE INDUCTION MOTOR

T on - o build a motor, to run the single phase alternating

1 1 0 for current Of volts , suitable running a fan interrupter,

ifli The is perhaps very d c ul t. builder will require more tools and a much greater knowledge of machine shop prac

tice to construct an efli c ient motor than to build all of the

For w ho v parts of Tesla apparatus combined . those ha e

had but little experience in motor construction , the writers

suggest that the amateur purchase an alternating- current fan

- The motor or a suitable direct current battery motor .

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

f r T patterns o the base and yokes . here are two castings

Th hin s f r required . e base supporting the punc g o the stator

on is cast directly the standard which supports the motor .

The 2 drawing , Fig . 3 , 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

n f r the mould was made, therefore shri kage must be allowed o

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

n amou t .

” ” Turn out a circular piece 5 in diameter and 1 in thick 44 The Tesl a Coil

” T 1 6 ness . hen a rod in diameter, and long, swelling into a graceful enlargement at the lower end , is turned out and

to of fastened with glue and nails the centre the circul ar disc .

n c or e r fn t d i e P i .

FIG 2 — P TE N Y KE S . . 3 . AT R OF O

” A piece of wood is cut out of 1 stock to the form shown in

” The 2 the figure . radius of curvature of the arc must be 5 T so that the stator will fit it accurately . his piece is glued

The and nailed to the top of the upright . whole pattern is The I nterrupter

FIG. 2 SE N LE E M . C R 4 CTIO OF OMP T D OTO . 46 The Tesl a Coil

v sufli cient gi en two coats Of best shellac varnish , containing T lampblack to make it jet black . his completes the pattern

to for for the base . In order provide a support the bearing

f r Of the rotor shaft we must make a pattern o a yoke .

TO make this pattern we turn out Of 1 5 stock a circular

” disc 55 in diameter and of the same form as in the drawing .

Tw o n one to castings are made from this patter , fit each end

n v to n of the stator . They ot only ser e fur ish bearings for the rotor, but also to enclose the entire motor and thus keep out moisture and dust .

’ k — zz

FIG 2 R D S . 5. OTOR I C .

After the patterns are shellacked they S hould 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

r jou nal . When the castings are Obtained they should be

. fl chipped and all the roughness filed o .

” The rotor consists of a number of iron discs 2 5 in diameter The I nterrnpter 47 and having twelve 5” holes drilled around the edge and a 9 ” T 5 5 hole in the centre . hey should be made of iron about

” " 1 - of 1 5 53 5 in thickness "about forty them making a pile T high . hese discs can be made by the coil builder with the

of s help a lathe and drill pre s , or they can be Obtained already stamped out from any of the large dealers in electrical sup

sufli cient u of on plies . A n mber them are mounted a shaft,

” turned down from a 5 rod of cold rolled steel to the size

”

2 . shown in the figure, to make a pile 5 in height As the

v motor will not have a very hea y load thrown on it, it will n ot be necessary to key them to the shaft . A good driving

s fli cien T fit is u t to keep them from turning . hey can be

The clamped in position by the nut shown in the figure .

” ” of 2 conductors consist twelve 5 copper rods 5 long . One of these rods should be driven in each of the holes around

” the edge of the discs and S hould project 5 beyond them on

T - both sides . o short circuit them , two heavy rings made ” by bending two pieces of 5 copper rod into a circle having

” an outside diameter of 1 5 are soldered to the ends of the

se sufii cient rods . U solder in making these connections in order to prevent heating at the connections by the induced T current in the rotor . his completes the rotor .

The unc hin s next thing to consider is the stator p g . In

not this case they will be punched out, but will be cut out on or r either a shaper milling machine, o cut out by hand after as much metal has been removed as is possible by

The drilling . diameter Of these discs is given in the draw

” T 4 in on 2 . O . f g Fig 7 make them , cut roughly out o T 357,

50 The Tesla Coil

100 6 of sn s iron about pieces in diameter, with a pair ip .

” A 5 hole is drilled in the centre of them and the whole numb e r are clamped on a mandrel between two nuts and turned

” down in a lathe to Next four 5 holes are d rilled as in ” ” x and the figure, and a fibre tube 5 in e ternal diameter 5

” ” internal diameter is driven in each hole . A 5 stud 35 in .

” n th length,with hexago al nuts and 5 iron washers , binds e

discs together . After tightening up the nuts the bolts can T be slightly riveted to guard against possible loosening . he

whole is clamped in a chuck and the centre is bored out to

” 2 . e 5 in diameter Next the slots are cut as in the figur , either on a milling machine or shaper or by the use of a hack

of saw and file . A large bulk the metal can be removed by drilling .

When the stator is complete it is mounted on the pedestal

e with four cap screws which screw into the bottom edg .

” The two ends are fastened to the stator with four 5 fillister

” The for screws 1 long . holes these screws are drilled mid I way between the nuts binding the stator discs together . n order that the heads fit up against the stator four holes should

to to be drilled allow the nuts 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

e to s are to be permanently . It is w ll mark them o that they m T can always be put back in the sa e place . hen the space betw een the shaft and the jou rnal is filled with the best grade Of Babbit metal obtainable . Cardboard washers slipped over the shaft prevent the babbit from running out. The Interrnpm 51

The The next thing to do is to wind the stator coils . Wire

2 2 The of used is No . . coils are wound in a wooden frame

Fi 2 8 . the size shown in g. 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,

f r - means must be provided o making the motor self starting.

This is accomplished by means of a short-circuited copper

m am stator oi l show i n m e . " c g m “10 ata -Abou 6 5 eet o No 22 B S u a o 523N t f f . g n

C C o er wi re is needed or ea h sta tor oil . D . . . c p p f c c

r o r oi l s r e w o und F ame n w hi ch stato c a .

FIG 8 F E . 2 S CoILs . RAM FOR TATOR .

conductor lying in the grooves marked “ A ” in the drawing

n hi s 2 T u c n . Of the stator p g , Fig . 9 his conductor consists

f f 1 he f o a piece o NO . 4 bare copper wire bent into t form o

n ne f a rectangle, so as to fit arou d the o half o the polar

f The tw o projections o the stator . ends are soldered

together . A glance at the figure will make this clear . The coils are next slipped into place over these short

The of circuited conductors . terminals the stator coils are so connected as to induce opposite poles in adjacent polar 52 The Tesl a Coil

The pieces . six coils are in series, the end terminals being brought out to suitable binding- posts fastened to the end f pieces and suitably insulated from it . A coating O black paint completes the motor .

Although the induction motor is a constant- speed motor

S l at varying loads, we can secure some light speed regu ation ,

S hor t C i r c v l fe d conduc to r ‘ ' o No l l Ba re Co e r Wi re f pp , s ho w i ng G lu t e of w ar e m t i i f i be fo al pp nzn o S lo s

FIG 2 SE -S N DE V E . 9 LF TARTI G IC .

which is a great advantage in operating the coil , by inter T posing a liquid resistance in series with the motor . his

of resistance consists essentially any suitable glass jar, such

D n or as is used in a a iell Gravity cell , having two metal

s plates suspended in an , o that the distance b e C tween them can be varied at will . opper sulphate is gen

r e ally used as the electrolyte . The Interrupier 53

AN ELECTROLYTIC CURRENT RE CTIFIE R 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 .

re Its advantages are that it is easily and cheaply built, it

‘ efli ci enc quires only slight attention , its y 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

of o this description a rectifier, which is entirely foreign t

Th f r s T . e o o the esla apparatus reasons doing, however

to ffi f r appeared , the authors at least, su ciently great, o if the amateur constructs the magnetic wipe - Out he will need

of n 80 or 0 a source direct curre t at about 9 volts pressure , since this current can hardly be obtained from the lighting

r circuits which are generally alternating o from batteries .

Then having this source of direct current he will be able

D C f r to substitute a . . motor o 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 efli ciency

The efli cienc of b is due . high y the apparatus devised y the authors depends on what might be called the alternate path

or is tw o connection method "that , there are paths for the

ne f current to traverse , one of enormous resistance and o o

r ve y low resistance .

The idea of this form of rectifier came to the authors in 54 The Tesla Coil

T on som e the following way . hey were experimenting

” 1 2 i n tubes of peculiar construction, using a induct o

The r Of l coil . current from the seconda y this coil is osc i i lator . the d s y in character, of course It was Observed that

r l h e charge through the tube was ve y unsteady, especia ly w n

n f ex eri the interruptions were ot very rapid . A line o p

' ments was carried out to determine the cause of this unusu al

f b e e fect , with the result that the resistance was found to enormously greater for currents in the one direction throu gh

the the tube than in the opposite direction, due entirely to f dif erence in the forms Of the tw o 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 alte r

n nati g currents . From a previous study Of the effects o f various electrodes on the electrolysis of certain solutions w e arrived at several cells w hich exhibited these properties to a marked extent . It was found that an aluminium electrode was the essen

r s ca ab e tial thing in eve y cell , together with some acid alt p l

Th r Of forming an oxide with aluminium . e other elect ode f might be any conductor unaf ected by the solution.

Some of the conductors suitable for the other elec trode The I nterrupter 55

one of By merely putting these cells in the circuit, the one

of r o half the alte nating current wave may be choked ut. But this method gives an efli c iency of less than Thus

w - the authors ere led to devise the alternate path method .

Before describing this method , however , we will take up f in detail the properties o a single cell . After a current is

for one of passed a few minutes through the cells , a coating

Of oxide is formed on the aluminium electrode which is prac

- n tically a non conductor . While this does ot prevent the diflerence of potential from being maintained across the cell, it does prevent the from giving up their charge and in this way it acts like a polarized copper plate in a single gal

T of vanic cell . his condition enormous resistance exists when the aluminium is the . When on the reversal of the current the aluminium becomes the cathode there is

of merely the resistance the electrolyte encountered . Any

cell possessing this property is called asymmetric .

S As stated before , a ingle cell by being merely put in the

out of circuit would the one half the alternating wave,

e but as this gives an intermitt nt current , the following is T the method devised by the authors . hree cells are needed

T Of one of in all . w o these consist of electrode aluminium

Of and one iron, with a solution Of sodium acid carbonate .

The third has two aluminium plates and one iron plate Th e . between them . same solution is used

0 E On looking at Fig . 3 it will be seen that when is positive the current can flow from either plate 2 or 3 across the elec

trol r The Of yte to plates 1 o 4. path from 3 to 4 is enormous 56 The Tesl a Coil

a r resistance, as the aluminium is the anode, but the p th f om

1 to 2 is Of low resistance and hence the current takes th is

H rom path . When becomes positive the current can flow f

6 to 7 or from 5 to 4 . It takes the path from 6 to 7 as th i s

w a al te is of low resistance . In this y both waves of the m a t ing current are used and the only loss is due to the resistan ce

Of the electrolyte . T m hus the direct current from this set has a sine wave fo r , in which all the negative values in the alternating have b ee n made positive .

The following are plans for a rectifier suitable for use

I I o- - - directly on the alternating current light mains . T he T rectifying cells have glass containing jars . he jars are all

” ” ” T e 7 x 6 x 4 inside measurements . h aluminium plates

” out a e are cut Of 5 sheet and are all the same size and sh p .

” ” T e n They are 5 x 7 and four are required . h alumi i um should be comparatively pure to prevent deterioration of

s the plates due t o local action . If the plate are to any d extent impure the cells may fail to work, and if they o

T e r rectify it will be at a very low efficiency . h i on pla tes

” Tw o of e are cut from 5 sheet . them are the same siz as

” the aluminium plate and the third is 8 x This larger

The s plate is to be used in the middle cell . nece sity for making it larger is that it goes between two aluminium

plates and the extra length is required to fasten the bind,

- The e in h ing post to . plates are h ld three eighths of an c ” apart in the following manner : Out of some 5 sheet v ul

canite or hard rubber (fibre must not be used as it sw ells in

The Tesl a Coil

x

Me lhod o or min c lc c fr f f g odc s

F n i n i i o o u c e l s a n d e lec l f odes i a na s how g Pos t n f fl l

‘ U i c a. o A u . W iflfl “t o P . I r n A, ' m m m

E W D FIG 0 RE E P S AND N . 3 . CTIFI R LAT IRI G IAGRAM The I nterrnpter 59

T tw o aluminium electrodes in the middl e . he connections are shown in the figure . With this electrolyte the aluminium plates will form in a

to few minutes, it being merely necessary short circuit the D . C . taps with a resistance and allowing the rectifier to ta ke 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 .

to ' It will be necessary renew the electrolyte at intervals, depending on the use the set is given CHAPTER VI

THE CONSTRUCTIO N OF THE BOXES

TH E next thing to consider in the building of a Tesla coil is the boxes which contain the transformer and high- tension

b ox for - coil . One transformer, condenser, and high tension

for S coil might be used , but a coil Of this ize the weight would

Tw o be Objectionable . separate boxes give the ideal result .

T of not too to hey have the advantage being bulky handle , and the transformer in this form can be used separately , if so

b x . o of desired A single , however, has the advantage taking up less room and of havi ng all the high - potential connections

d e are e o insi e where th y safe , xcept those which lead t the

is c on to of b x interrupter which pla ed p the o .

Oak the s s an a b ox makes mo t sub t ti l , but it is harder to

w n to a make tight , o i g the fact th t the shellac varnish w hich is used for filling up the pores in the wood does not sink

ak w n s w into o ith the same readi ess a it does in a softer ood .

Pine is the best material to use as the joints w ill require considerable filling up to make them impervious to paraffine

w Oil , hich will soak through almost anything in time . Sugar pine may be readily stained and looks very neat when var i n shcd .

For the sides and ends Of the transformer b ox it requires

60 The Constructi on of the Boxes 6 1

- n 1 a piece of straight grained pine free from k ots , 5 x 10

” The ” 6 long . bottom should be made Of a piece of 1 5 x 14

” ” ” ’ 2 6 to of I I I 2 long, and the p a piece Of x , long . Cut the

to . 1 r pieces the size shown in Fig 3 , and plane the edges t ue .

” The end pieces must be mortised into the sides 1 from the T end . hese tongues and grooves may be cut with a saw

n and chisel if a rabbeting plane is ot at hand . After the sides

S IDE VIEW END VIEW

FIG 1 T NS E Box . . 3 . RA FORM R and ends are finished the tongues and grooves are given a

v of hea y coat shellac , which has been dissolved in grain alco

l Six ho , and while still wet are put together . long brass screws with round heads are to be used in each board to hold

” the sides . A brass washer 5 in diameter should be placed

on the screw to prevent the head from S inking into the wood . Next the edges should be gone over with a plane if neces ' so The sary that the bottom board will fit flush in all places. 6 2 The Tesl a Coil

” to I f bottom board is be wider than the width o the box, so

” on that it laps over 5 each side . When the bottom board

o OH is cut t size , the edges are rounded with a plane to give

to b ox a finish , and then it is fastened the with long flat

headed brass screws , placed every four inches along the sides

of and ends . A coating shellac is given to the edges Of the b ox s on to ju t before putting the bottom , help make it tight . The screws must be forced in until they are flush with the

wood .

or of Next the inside should be given five six coats shellac ,

to n the paying especial care get it i to joints , and allowing

each coat to dry before applying the next .

A small brass cock in the end near the bottom is a conven

b ox of ience in emptying the its Oil , but the labor Of putting

it in so that the b ox will not leak is such that a siphon is

for Oi quick enough an occasional emptying Of the l .

The b ox for the high - frequency coil and condenser must

have the same care taken in its construction as in the case

b x The of the transformer o . dimensions are given in the

2 working drawings in Fig . 3 . A partition is put in betw een

the condenser and oscillation transformer, but several holes

' should be bored in it near the bottom to allow of the free

T b ox circulation of the Oil . his must also have several h coats Of shellac , as the insulating Oil used will leak throug

in S pite of all the precaution taken . After the boxes are finished they should be stained or

f n varnished to suit the taste o the builder . Walnut stai

s s looks well , and a it is dark it covers up a multitude of fault The Constructi on of the Boxes 63

in the wood working . If the boxes are well made a good

oiling followed by several coats of shellac makes a very good

finish .

Vi ew covcn Renow n COVER

— 3 6 £

S IDE VIEW END vuew

FIG 2 H H - E NS N Box . 3 . IG T IO .

E now for of s verything is ready the assembling the part , which will be taken up in the next chapter. CHAPTER VII

ASS EMBLING

T t I is not wise o hurry when assembling the apparatus , for - not if the high tension wires are properly insulated ,

- efiects 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 b ox bring its four primary leads to

- The four heavy binding posts on the end of the b ox. two inner terminals Of the tw o sections are brought to two

- w o adjacent binding posts , and the t outer ones to the other

o tw , in such a manner that connecting electrically the tw o middle binding- posts puts the sections in series and short

rs circuiting the two outer pai throws the sections in parallel .

ee T S the diagram . his is accomplished by means of a piece of flat brass with slots filed in it so that it just fits

tw o - or across binding posts, 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 5 in diameter and 3 long, which extend through the opposite end Of the b ox for 1 5 These rods are insul ated 64

66 The Tesla Coil

' l brass set screw . Another hole is drilled at right ang es to

” the first about 5 from the end of the rod to meet the fi rs t

T ffi n - hole . his makes an e cie t binding post to hold the con ductor .

s r Thi finishes the connections on the transfo mer, w hich

n w t T can o be placed in the position which it is o occupy . he box is filled with enough pure paraffine oil to cover the trans

r T e fo mer. his Oil should be of the best quality obtainabl ,

free from moisture and impurities , such as is used for insu

to th lating purposes . It should be allowed soak into all e

The sections for 24 hours before using . primary terminals

are connected to a source Of alternating current by means of of 0 s a suitable and fuse capable carrying 3 ampere . The next step is putting in the connections and terminal s

- b ox in the high tension .

4

- FIG. H ENS N U S H N 34 IGH T IO B I G.

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 b ox next to one end of the T ” oscillation transformer . hese holes should be about 6 ” T e from the bottom and 3 apart . h brass rods project A ssembli ng 67

i in b ox The il 5 w th the . bushings and rods can be made o

’ tight by giving them a good coating Of Le Page s glue before driving them into place .

” A 5 strip Of wood is glued on in the lower inside end of the

to b ox below these rods, prevent the end Of the oscillation transformer from coming in contact with them .

O S D FIG. . S AN S N S 35 CILLATOR TA DARD .

The condenser is first lowered into place in the box and a

one to r wire is run from terminal the nearest outer brass od,

o The t which it is soldered . wire should be enclosed in a glass tube suitably bent and should follow the lower edge of

h r t e box . A wire is run from the other te minal of the con 68 The Tesla Coil

in to denser a similar manner the other outer brass rod . It

b x follows the other lower edge Of the o . A tap wire is led

from this conductor at a point Opposite to where one terminal

of Of the r to the primary band oscillation t ansformer is be . T A wire is also soldered to the middl e brass rod . he oscilla

n Th tion transformer is ow lowered into place . e 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

T S as directly as possible . hey hould also be kept under the

il he S T . o . 6 connections are hown in Fig . 3

FIG 6 W N D M . 3 . IRI G IAGRA .

After all connections have been securely soldered the box

oil s is filled with , o that the entire apparatus is completely immersed .

The oscillators and standards can now be constructed .

” ” Tw o fibre or hard rubber bushings 2 5 in diameter and 1 5

” ” in length , and having a flange 5 thick and 3 in diameter

on one two turned end, are set in holes cut in the cover directly above the holes in the brass bushings on the oscil

” r lation transformer. A 5 hole is drilled through the cent e Assembli ng 69

” " Tw o 10 re of each . 5 brass rods long a enclosed ” ” T in fibre tubes 5 in outside diameter and 95 long . he

he f tubes should fit the rods tightly . T ends o the brass rods project from the fibre and can be slightly tapered to fit the bushings on the oscillation transformer . The oscillators consist Of tw o brass balls 1 in diameter 3 ” ” 1 2 screwed on the end of two 5 5 brass rods long, which

” are to slide easily in tw o holes drilled 5 from the top of the

r standards through both the fibre and the od . 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 tw o vulcanite handles 5 in diameter

on are screwed the ends of the rods, carrying the oscillators ,

” for about 1 5 Slide the standards through the bushings in the cover until the rods make good contact with the bushings on the T oscillation transformer . his completes the connections in

x S now the second b o . It hould be placed in its final

- Of position at the high tension end the transformer, leav

’ ing a space Of about 1 5 for the interrupter between the two boxes . The particular form of interrupter which has already been built is connected to the bindi ng- posts of the high - tension

' T coil as in the wiring diagram . hese leads and those from

o - b ox S f the transformer t the high tension hould be o No .

1 b 2 B . 81 S . gauge hard copper wire and enclosed in the eavi T est glass tubing obtainable . hey should be as straight and 70 The Tesl a Coil as short as is consistent with safety in operating the pri

- mary spark gap .

es A suitable panel switchboard , with the necessary fus , transformer and interrupter , makes a desirable T o . acquisition t the apparatus , but it is not essential his f matter is left to the taste o the individual worker .

When the connections have all been made and the O il

o t has driven u all the air that it can , open the interrupter

” 1 gap about and cautiously close the transformer switch . If no excessive load is taken by the transformer as manifested by the 30 ampere fuses or an alternating- current ammeter

n - in series, if one is obtai able , the spark gap can be slowly

r a T e closed until the condenser discha ges cross it . h n, if the directions have been carefully followed in building the apparatus, a heavy , bluish white , snapping discharge Of over

” 1 2 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 1 1 0 volts

on i in series . If the sections the primary were connected n

on 1 10 parallel volts, the voltage output Of the secondary

to con would be about double what it ought , and hence the denser may puncture .

If an o by y chance a discharge should fail t 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 n T the transformer are connected in oppositio . O deter mine whether the transformer is working satisfactorily or Assembli ng 7 1

not, disconnect it from the rest of the apparatus and see if an arc di scharge of at least 6 or 8 inches can be drawn out T b etween tw o electrodes . his arc is generally of a yellow f color and easily extinguished by any draught O 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

r no a e 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 See 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 R f transformer . ing out by means O a magneto the primary

no and secondary circuits to see that there are open circuits, and then see if there is any S hort circuit between the primary and secondary . If these parts are all right the fault may be due to poor insulation , in having the turns of the secondary T he . touching . remedy is Obvious Finally look and see that all the electrical connections are as they should be , then the apparatus cannot fail to discharge

” I - over a 5 air gap . CHAPTER VIII

THE THEORY OF THE TESLA COIL

H OUGH ALT , 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

out would not be 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 s . fir t thing to consider is the transformer Its action ,

n to as is well k own almost everybody , depends on electro

Th r magnetic induction . e alte nating current flowing in the primary sets up an al ternating magnetic field , which being linked with the secondary induces an electromotive force

n in it . When the secondary is open there is theoretically o

to current passing through the primary , due its high self induction, except that necessary to magnetize the core .

AS on to r a load is thrown the seconda y , 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

” 1 2 2 2 to 2 the r used in the coil is from 5 amperes , seconda y

7 2

74 The Tesla Coil

T essential part of the esla 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 S everal forms , depending only on the three electrical — constants of the discharging circuit , capacity ,

The s and resistance . discharge may be either o cillatory or entirely unidirectional , consisting only of a gradual equali

i n n zat o of the potentials o the two plates . This may be made clear by the following mechanical illus

r i n S U - t at o . uppose a glass tube to be p artly filled with

so , and the mercury to be displaced that the level T in one side of the tube is higher than in the other . here

to iff of t is then a force due the d erence level, tending o cause the liquid to return to an equal height in both limbs. If

r the mercu y is now allowed to return , but is constrained , so that it is released slowly, it goes back to its original

. he position without oscillations If, however, t constraint is

to suddenly removed , then owing 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

a resist nce 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 the mercury and to be closed up at its ends, the compressibility of the enclosed air would l correspond to the electrica 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

so ric l the position of equilibrium in doing . F t a 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

to having electrical capacity with respect each other, such

of as the plates a condenser, and the circuit itself must

w possess inductance and lo resistance . Under these condi tions, the sudden release Of the electric strain results in the production of an oscillatory 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 are connected by a . heavy copper wire . T L K Professor William homson, titled ord elvin, 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 converted 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

a of ff energy , which leads to a cert in class di erential equation

The having two solutions . solutions in this case depend on the relation between the constants inductance, resistance , and capacity .

L C R s s If inductance , capacity , re i tance , then the s s e i s f olution are determined by the r lat ve value o Rand LC .

2 1 I i r i i f s e ’ s, R is " 2 g ater than that if greater than 4 2 LC RC or if is gre ater than the charge in the jar dies away 4 i gradually as the time increases , in such a manner that the discharge current is always in one direction .

The ratio i is called the time constant ( T) of the dis CR charge circuit , and the product is called the time con T’ stant ( ) of the condenser circuit . Hence the discharge is unidirectional w hen 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 circui t Theory of the Coil 77

CR s d s l If, however, is le s than the i charge current wil 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

O R in f the conductor, then the ordinary resistance ( ) and duc tance (L) must be replaced by the high - frequency resist ance and inductance of the circuit .

When the discharge takes the oscillatory form the fre queney is given by the expression,

1 2

R s , If is very mall then 2 can be neglected in compari 4 2 son w ith and then the frequency is given by the expres

LC

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 f are practically used, the resistance o the oscillatory cir cuit is negligible, and the inductance is small and hence

'

r b sol u . e o a te C . easily measur d in centimeters G S . units , one milli - henry being equal to a million centimeters 78 The Tesla Coil

e Also . the capacity is b st measured in microfarads " one microfarad being the one millionth part of a farad or

—15 C . . S . 10 of an absolute G , 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 X 10

x/CL

The energy storing capacity of a condenser is given by

I 2 ex ressI n C V 18 ca acIt the p O , where C the p y of the con 2

denser and V the charging voltage . The main thing in constructing condensers to be used on T high charging voltages is the solid dielectric . here are in

u for - all only a few dielectrics s itable high tension work, and this number is reduced when cost and internal energy loss f in the dielectric are considered . Glass o certain composi

tions, ebonite , mica, and micanite are practically all that

of are suitable ,and these flint glass is the best, as its dielectric

to 1 0 constant is high , being from 5 , and its dielectric strength

. to is also great Glass is brittle , however, and liable have

flaws which sooner or later give way . The capacity of a condenser depends on the area of the

Of plates , their distance apart and the constant the dielectric a used , and is expressed by the following formul in micro

K S Of farads, where is the dielectric constant, the total area

the D plates expressed in square centimeters, and the distance

apart in centimeters, Theory of the Coil 79

KS

5 4 7rD X 9 X 10

5 The constant 9 x 10 comes from the fact that one micro f farad equals electrostatic units o capacity .

The oscillation transformer is nothing but a modified trans

an The former with air core . only important facts about its construction are that it should be built to withstand great ff voltage di erences 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

h r is gained by aving many close turns in the prima y , because ff the increase of inductive e ect on the secondary , due to an f increase in the number O primary turns , is about exactly annulled by the decreased current through the primary due

its w n to o greater inductance .

The function of the interrupter is to destroy any are that may be formed across the terminals of the primary spark

f r re n t n gap , o if this a is o wiped out there will be o true oscillatory di scharge in the condenser circuit or only a feeble

T e f r one . h reason o this is that as long as the arc discharge

r continues, the seconda y terminals of the transformer are t ff reduced o nearly the same potential, or at most di er only by a few hundred volts .

The function Of the primary spark- gap is to regulate the S voltage to which to charge the condenser . ince the poten tial difference between the spark balls is almost equal to the

a potenti l difference across the condenser, the condenser will discharge at a voltage determined by the length of the air 80 The Tesla Coil

Of - gap . Now there is a certain length spark gap which is best

b e n suited for each coil and it can easily determi ed by trial .

- As a rule it is best to start with a rather short spark gap, gradually lengthening it out until a point is almost reached, when opening it out any further would cause it to cease pass T ing . his 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

a for the ordinary resist nce and inductance , when dealing with circuits which are subject to the action of electric oscil l i e at ons . Th 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

- The f r high frequency measurements . main reason o the difference between the tw o cases is to b e found in the fact that a high - frequency current does not penetrate into the

b u interior of a thick solid conductor of good conductivity , t ff is merely a surface or skin e ect .

five When traversed by an alternating current, there are f qualities O 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 1 00 per second .

The f 2 . inductance o the conductor depends on its geo

Of metrical form , material , and the nature the surrounding

The r . greater the frequency, the smalle the induc tance becomes .

8 2 The Tesla Coil

a . b lls, etc , a discharge takes place which oscillates through the primary of the oscillation transformer and back and

— a The e Of e forth across the primary spark g p . fr quency th

w current depends entirely , as sho n before, on the constants

ne of the circuit . On first thought, o 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 S how 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

r gap . Before any discharge has passed and under no mal

conditions the resistance of the spark - gap is very great : the

voltage required to break down one centimeter Of air being about After the initial discharge has passed and the air becomes heated and ionized the resistance may drop as T low as two or three Ohms . his fact plays an important part in the of the oscillation trains . The discharge from the condenser which oscillates through

r of o the prima y the oscillati n transformer sets . up a rapidly alternating magnetic field , which being linked with the sec

r The for onda y induces an electromotive force in it . law the

not in Of induction in this case is nearly as simple as , the case

r the ordina y 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

r The r prima y and secondary . fo mula for calculating the voltage difference across the secondary in either case are t ex remely complex, involving the damping factor , the capaci f D o s . ties the circuits , and other constant rude and Bjerkn es have treated the subject of the oscillation transformer analyt m ically in an admirable anner .

The frequency of the spark in the large spark - gap is not a

Of simple one but consists several , one being the natural period of vibration of the secondary and one a forced vibra

r tion of the seconda y , due to the fact that the primary and T secondary are never exactly in tune . here is also a certain small current of a high frequency , due entirely to the con

n stants of the spark balls and con ectors, 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

Of telegraphy, so that the solution the one will solve the other . CHAPTER IX

USES OF THE COIL

THE Tesla coil readily lends itself to a great number Of ff experiments, some interesting in their e ect, others useful in scientific research . Waves for wireless messages may

X - Ra be sent out into space , y tubes excited , Geissler tubes ff illuminated , beautiful brush e ects shown , and a great f number O 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 ff e ects , which may be obtained even with the coil described f in the Appendix . All o these experiments on the brush discharge should be performed in the dark, as they then

S how to the best advantage .

ff e f These e ects , besid s a fording a pleasing sight , are of

henome great scientific value . It is a known fact that the p non is due to the agitation of the molecules near the terminal ,

hot and it is thought , since the brush is , that some heat must be developed by the impact Of the molecules against the ter

Of minal or each other . A little consideration the matter 8 4

86 The Tesla Coil

” 1 be suffic ient to the 2 or 7 coil , the light from them will

r distingu ish objects in the t oom . Another way of easily exhibiting the brush efiec t is b y

0 6 l d CO er attaching the end Of I o or 2 feet of NO . 3 insu ate pp wire to the one terminal of the coil and the Opposite end to

n . U o an insulati g support , leaving the wire hanging clear p n touching the remaining terminal with a bit of metal held in

w e e am the hand , the ire will break forth into numb rless str s or threads of light palpi tating in unison with the disc h arge of the condenser .

The luminou s intensity of the streams can b e consider ab ly T i increased by focusing them upon a small surface . h s is

T one of illustrated by the following experiment . o the ter

minals of the coil a wire bent into a circle about one foo t in

diameter is attached and to the other terminal a small b rass

The e sphere . centre of the sphere should be in a lin at r ight

angles to the plane of the circle at its centre . When th e

f rm discharge is set up , a luminous hollow cone is o ed , a nd

in the dark one half of the brass sphere is seen strongly ill u i T m nated . o get the best results possible with this exp eri

of ment, the area the sphere should be equal to th e are a of

w the ire .

Another w ay in w hich the luminous effect Of the disc h arge

w : Of t stou i may be shown is as follo s two circles ra her t w re ,

” 2 ar orm one being about 3 in diameter and the other ef ed ,

and to each of the terminals of the coil one of the Circ l es is T attached . he tw o circles must be concentric and in th e

same ne pla . When the coil is turned on the w hol e space Uses of th,Coil 87

T betw een the wires is uniformly filled with streams . he 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 w ill realize that to pass ordinary luminous discharges through gases, no particular degree of exhaustion is necessary , but a very high frequency T is essential , and of course a fairly high potential . his

S how s us that the attempts to produce light by the agitation of the molecules or atoms of a gas need not be limited to the

r a , but the time is to be looked fo w rd to , and

w that in the near future , when light will be produced ithout the use of any vessel w hatever and w ith air at ordinary pres sure . When light is obtained this way , there will be no

Of chemical process nor consumption material , but merely

of a transfer 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 cflect can be readily show n by using only a

- burnt out incandescent globe , in which the vacuum has not

To w been destroyed . sho it the bulb should be suspended k from an insulating nob , so that the discharge will pass T through the centre of the bulb . he 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 T pulsating, delicate purple hue . his color in a few minutes generally turns to a lovely pale green, and then sometimes, 88 The Jesla C oil

lv a es a a e w te h The b rare t. in ut , this ch ng to delic t hi lig

v f th not verv re a b e i n tensit o e light is g t, ut the d l ca te ess

m n be a of the colors is so ethi g to dmired . The disc harge

must not b e continued too long thro ugh the tub e as it is

ec ar n w liable to pierce the glass . A p ul i thi g al ays happens

if a w t a oo fi ament is used namel tha in f w b ulb i h g d l , y , t a e

m utes the fi ame f to b e com l etelv s a tered in l nt is ound p h t .

’ Wi th the 1 2 coil it is onl v necessar v to hold the gl ob e in th e

w v ns he for i h an tho an c n ec t o to t co to t u . h d . i ut o n i il . it l g p

Bv mov the u aroun in the v cin t of the s ar ing b lb d i i y p k,

va ous c an es in the intensitv of the li ht w l b e see n ri h g g il .

If the amateur is fortunate enough to pos ess some Ge issl e r

t bes thts e mav be lit u b v c nnec tin them in series w u . p o g , ith

n - o he i I a a usta e s a a t t co . n the case of the dj bl p rk g p . l large co il tubes w ill light up w hen merel v broug ht into the vicinity

of the discharge.

The hos t X -rav tube to use w ith this Tesla coil is w h a t

is know n as a o b le-foc s ube al t o d u u t . h ugh auv other tub e

ma v b e sed w t not ui e as ood r u i h q t g es ults. As the terminal s

o f the X -mv tub e ar e al ternatel v ca thode and anode , w h en

an alternating current is used the pul s xting or ra ther va ry

i g i ht if -mvs m v b e call i w . X a ed l ht o ld b e n l g g . u objec tion

ab le but due to the hi h fre uenc v . g q of the cu rrent from th is

oi the uls t tions ar e not no i e b c l p r c a le . “h en the doub l e

e e e foc us tub is used the nus ar r flec ted firs t from one t e

fl ec tor and then fr om the other in ra id suu ‘ e p ss ion , and th us

ble h s e is fill w i h he - do u t e pac ed t t was than if a single focus

tube had been used .

90 The Tesl a Coil

’ was in a large barn that the authors verified some of s

v experiments on electric wa es . In dealing with electric waves there are two disturbances to be taken into account, T the electromagnetic and the electrostatic . here are also two classes of waves , the electromagnetic waves and the sta

ion r A t a y waves on wires . method will be given for obtain

t r es ing bo h the electromagnetic and the stationa y wav , which is all that can consistently be brought within the scope of this work . E D . . Jones has made a translation from the German

f he of the original papers o , dealing with t experiments and results which have made his name famous .

The radiator or oscillator used by Hertz consists of tw o

o metallic plates , having attached to them short r ds ending

T n n in knobs a short distance apart . hese k obs are con ected

. r t o to the secondary of the coil Hence, as the seconda y elec r

ff e motive force accumulates, the plates are brought to a di r ence of potential and lines of electrostatic displacement stretch out from the positive side of the oscillator to corre

i o spond ng points on the negative . We have thus a str ng

ce electrostatic displacement created along the lines of for .

When the potential difierence reaches a certain point de

e of - io pending on the l ngth the spark gap , the air insulat n breaks down and a current flows from the one plate to the

- e other across the spark gap . It is merely the discharg of

on con a condenser , for the two plates the oscillator form a

T a in the s e a . a denser , with air as dielectric his cre tes p c

ev r w here around a magnetic flux , the direction of which is e y Uses of the Coil 9 1

Th normal to the direction of the electric displacement . e electrostatic energy is thus transformed into electrokinetic energy . If the electric oscillation is started sufficiently sudden

w 06 some of the energy is thro n as a displacement wave . As the coil is continually in operation we have groups of intermittent oscillations and therefore trains of electric

w off w out waves thro n hich spread through the dielectric .

w u In this way the electromagnetic aves are set p 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 , n - o ultra violet light must fall on the balls, and the balls must be a certain distance apart, best determined by experience . The form of devised by Hertz consists merely of a nearly closed ring or rectangle of wire the ends of which

e end in metallic balls placed very close together . Th one

of T ball is adjustable by the use a micrometer screw . here

of have been many modifications this resonator, but the one the authors found most satisfactory was where a neon or carbon dioxide of the spectrum variety was

- connected directly across the spark gap . Instead of getting

S a park then the tube becomes illuminated .

By using this resonator and the before - mentioned oscilla

oi tor, the magnitude the electric displacements in the space surrounding the oscillator can be mapped out . Hertz ’s great work was in setting up stationary waves in

ir i a . This s accomplished by having a large metal plate 9 2 The Tesl a Coil

The set up in the room to act as a reflector . coil with its ra r r s ta so diato is set up in f ont of it , at ome dis nce from it, that the plane of the plates on the oscillator is parallel to the re ec B he es ra the l c fl tor . v holding t r onator pa llel to meta li re ra d o flector the nodes and antinodes can be easily t ce ut .

The size of the plates on the radiator used by Hertz in

” e r The reso a t w as this xpe iment w as 1 6 square . n or a ring

"

f a . o No. 6 w re 1 bout copper i . 4 in diameter

W 3 ‘ s

- stationarv w axis on w ins the same radiator is

s w e similar pla tes to tizt xe on the ts c illator are gotten

94 The Tesl a Coil

citing them being that they may furnish entertainment to

The r some friend who should chance into his shop . write s first saw them performed on the vaudeville stage with an

w T Ondin resonator hich is far inferior to the esla coil .

To begin the performance the operator should make a

few general statements as to the voltage required to leap

' —a b diflerent air g ps, such as for a out for

” " T s 6 1 2 etc . hen show the di charge across the or gap and let the spectator imagine what voltage that represents . After he has bec ome somewhat impressed with the intensity

a one of the disch rge , the operator can approach of the oscil

the T lators and allow spark to play on his bare hand . o do this without severe burning of the hand , he should keep his hand in constant motion to prevent the spark from playing

T r r on one point . he e is only a ve y slight sensation felt when this current is traversing your body and no injurio us

f r r a The eflec ts result so a as the autho s are aw re . effect is best show n in the dark .

The next experiment is to grasp one oscilla tor with th e hand w hen the coil is in ope ration and to have an assistant

’ touch the ope rator s bare elbow w ith a cotton cloth dipped

l The a e c ie immediatelv t in n s s . i alcoho . h ndk r h f bur flame

To get the result w ithout any uncertaintv the cloth is wrapped

' ss a An rather tightly around the a ist nt s hand . amusing modification of the same experiment is to touch the cloth to

e erat w n d . th hair of the op or. sho ing that the hair is not ig ite

' f u are d If auv o the a dience incline to smoke . a suggestion as to lighting cigars on a w indy night w i thout the annoyance Uses of the Coil 95

m Th of atches blowing out will be greatly appreciated . e

Operator has merely to bring a piece of metal, such as a nail , held in the hand to within Q” 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

e The e s the metal tip of the burn r. writ r 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

so T gap that no more current passes than is necessary . his

o is t 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

of one terminal 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 .

S Perhaps the most pectacular experiment, one which is unaccountable for by the every - day electrician w ho does house wiring and has never been brought in touch with

high - frequency currents is the lighting of an ordinary incan 96 The Tesl a 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

1 10- to full brightness on the volt circuits should be made , f if those present are ignorant o electrical matters . They can

thus see that the energy required to light an ordinary 1 6

C . . P lamp is equivalent to 55 watts , and that this amount

’ 1 1 0 is therefore taken through the operator s body . At

volts this means approximately ampere , while according

to the best authorities {5 of an ampere is fatal to the aver

m The of age hu an . reason why this amount energy can be

not taken with impunity is 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

T eflec t follows the surface . his is known as the skin .

The to operator, get the best results, should stand on an insulated stool and grasp one terminal of the coil with one

of hand , and approach with a piece metal held tightly in the

hand or mouth one lead of a lamp , the other lead of which

The has been previously grounded . 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 .

CHAPTER X

" DIMENSIONS OF 7 STANDARD COIL

F R r i O those amateurs who , having read the p ev ous

w - chapters , think that an apparatus giving a t elve inch

spark is too large for their limited uses , this chapter has

been added . T his 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

a sent successfully over a dist nce of three miles in wet weather . This was the greatest available distance over which the

so S authors could try the coil , that three miles hould not be

considered the maximum transmitting distance . In clear weather messages could easily be sent a distance of about

a: fifty miles, provided your antenn is well insulated from

grounds .

Because this apparatus is not as powerful as the other , doeS not mean that any less care S hould be taken as

regards insulation and mechanical construction , for it depends

entirely on this whether the coil builder is to get a thin , inter

T i r mittent Spark or a fat crackling one . he only d fle ence

between this apparatus and the larger one, besides that of 9 7 98 The Tesl a C oil

t r ser S ize, is in the cons ruction of the transfo mer and conden and then they are only trivial .

The core of the transformer is 2 3 in diameter and is

”

B . e 1 2 . & S built up of pieces of NO . 3 gauge iron wir 3

The m long after the manner described in Chapter II . sa e care should be taken in annealing and insulating the iron wires as was done before .

The primary is w ound in tw o sec tions adjacent to each

E n s 8 . d other, as seen in Fig . 3 ach section is wou d towar

FIG 8 — P I AND E or T ANS ME or ' C L 3 . R MARY COR R FOR R 7 OI .

” 1 the centre , starting from the ends of the core , for a distance

ix a f o I 6 S o N . . e of There are l yers B . 8:S gaug do u ble

E er i cotton covered copper wire in each section . ach lay s thoroughly shellacked w hen put on and the terminal w ires

sa C r II are held by the me method as described in hapte .

At least tw o feet should be left for bringing out the termin al s

- to the binding posts .

The c r w se n . NO . 2 . se onda y is ound in two ctio s 3 B 8:S .

ere re e gauge double cotton cov d copper wi is us d . The tub e

100 The Tesl a Coil

is wrapped with several turns Of paper w hich is shell ac ked

he h r s m er T t t e t an fo r . in place . his completes construction of When finished it S hould be left for some time in a w arm dry

e dr the sh e l place as b hind the stove, to thoroughly y l ac . The reason for this is that green S hellac is a fairly good con

ductor .

” - S s Of rass 6 x 1 0” For the condenser, twenty eight heet b

O . 2 S S e ss sed . are required . N 3 or 34 oft h et bra is u Eac h

” ” S heet has a lug 1 g long and 1 5 wide either cu t direc tly

e r ether h on it or soldered on in the upp r co ner . Wh t ey are cut directly on the sheet or are soldered on w ill depend ” 1 2 rass on the width of the brass sheet used . With b t hey

” on 8 ass A are cut the sheet or soldered on with br .

o . e . 0 T lip is bent across the t p of each lug Se Fig 4 . h irty 1 ” ” ” 1 r sheets Of glass 1 0 thick and 7 x 2 are equired . Any S heet of glass that has an air bubble in it S hould b e rej ec ted

si as it is liable sooner or later to give way, thus cau ng the

h S ee f reconstruction of the condenser . Wipe eac h t o glass

s clean before putting it in to the conden er.

The l s e ne frame is constructed from wel eason d pi . The

” ” w o 7 1 h t sides are made from } x 2 pine 4 long . T e base

” ” ” of f The a i is made a piece o i x 4 pine 8 long . b se s fas

to flat- scr s tened the sides by some headed brass ew . The

of S h the w o heads the screws hould be sunk flus with od .

” On the one S ide of the frame two strips x 1 are fasten ed " one at the top and the other at the bottom of the frame.

See Th figure . e frame is now laid on a flat surface w ith

t een fas e the side on which the strips have jus b t ned dow n . ” Dimensi ons of 7 S tandard 101

FIG . 0. P E AND F E or N ENSE 4 LAT RAM CO D R. 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 1 margin Of l ” n S The glass at the bottom and a g margin o the ides . lip

of should just fit against the upper edge the glass . Without displacing the brass S heet place a sheet Of glass on top Of i T S f t. o his is followed with a heet brass , but in this case the lug on the brass is brought out on the opposite ~ side

n C 2 to the previous o e . ontinue this process until the 8 f T sheets o glass have been put in place . w o glass sheets

n f are placed o top o the last brass sheet . Also remember to S out on S bring the lugs from the brass heets alternate ides .

Mortices S hould be cut in the top and bottom of the upper ends of the sides a little deeper than the point to which the

T o last glass sheet reaches . hese are t receive two strips f ” ” h T o pine i x 1 similar to those on t e other S ide . hese

S hould be screwed down so that they press firmly against

of or the glass . A piece paper cloth placed between the

of strip and the glass will prevent the breaking the latter . Set the condenser upright and solder a piece Of copper

h to wire , which as already been tinned , each of the lips in turn down the one S ide and another wire to all the lips E . 1 6 . on the other side . About NO bare wire will do nough t extra wire should be left o make all necessary connections .

The oscillation transformer is constructed in the same

” r 1 The manner as the one fo the 2 coil . circular supports

” for the secondary are 6 in diameter and are turned out

” E S ts are cut the er of j material . ight equidistant lo in p

” Dimensi ons of 7 S tandard Coil 105

for primary band making the connections . A hard rubber

” ” ” 1 1 2 w strip x j , long, is scre ed across the top of the completed oscillation transformer . Tw o holes are drilled

. tw o in it apart Into these bushings, Similar to those

” 1 2 C e used in the coil and described in hapter IV, are fitt d . When the secondary terminals are soldered to them the oscillation transformer is complete .

” All b ox Th the parts Of the 7 coil are mounted in one . e f O . 2 The x dimensions this box are given in Fig 4 . bo is

” out of t built i well seasoned oak . All the directions given

C b ox Th in hapter VI apply to the construction of this . e cover is divided into two halves , one carrying the interrupter

s and the other the discharge o cillators . The connections from the interrupter to the condenser are made through the hinges so that the cover may be swung back

nn without disturbing the co ections . A partition is put in b e tween the transformer and the condenser . It has a number

Of holes drilled in it to allow Of the free circulation of the Oil .

S of b ox for r uitable handles are put at each end the car ying it .

The transformer is now set on end in the smaller division

b ox two - of the . It is held in place by yoke shaped , wooden supports fastened to the inside of the b ox and encircling the

The core between the two sections of the secondary . primary terminals are brought to four heavy binding-posts at the upper

O T u end f the b ox. hey sho ld 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 .

1 08 The Tesl a Coil

The oscillators consist of tw o brass balls i in diameter 3 ” ” n of screwed o the end two 3 3 brass rods 7 long, which are

” to slide easily in tw o holes drilled 5 from the top of the

ro . standards , through both the fibre and the d 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 . Tw o fibre or

” hard rubber bushings 2 in diameter and 1 7} in length and ” having a flange i thick and in diameter turned on one end are set in two holes cut in the cover directly above the

on holes in the brass bushings the oscillation transformer .

” of T A i hole is drilled through the centre each bushing . w o

” ” brass rods 8 long are enclosed in fibre tubes 35 in outside

1 ” T e diameter and 775 long . h tubes should fit the rods

The tightly . 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 vulcanite handles 25 in diameter

on are screwed the ends Of the rods , carrying the oscillators , for about 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 .

h not If everyt ing is as it ought to be , the trouble may be found in the manner described in Chapter VII . ” Dimensi ons of 7 S tandard Coil 109

” OS C CLL A T O R a TA NDA R D

" a b rass rod

" F B U S H l NG OR. UGO:L

” O L S AND S N S FOR A US. FIG . S . 44 CIL ATOR TA DARD 7 PPARAT

1 1 2 The Tesla Coil

Th The e above sums include the simple interrupter .

s il i other w l br ng the price up in proportion .

The high - frequency coil is made as follows : Cut out two

” ” end pieces of 1 wood 10 square and describe on each one two concentric circles, having diameters of 9 and 7 inches

” ” 1 respectively . On these circles bore a number of 1 holes apart as in the figure . Next procure from a planing mill ” T about twenty i dowels . hese are made of hard wood and

” ” 1 2 come 36 long . Cut each dowel into lengths and fit one

on of in each of the holes the smaller circle of one the boards , When they are all in place the other board is put on the

The other end Of the dowels . outer circle Of holes is left empty until the secondary is wound .

o 2 The secondary winding consists of ne layer of NO . 3

B . S . gauge double cotton covered copper wire . Begin the

” 7 S winding about 3 from the ends . hellac 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

' ne e ing them through the holes from o nd . If they fit too

out . f tightly the holes may be reamed Next six turns O NO .

1 on r 8 bare wire are wound the outer dowels, each tu n being

on over an inch from the e next to it . h The whole coil is then mounted on a base . T e ends of the primary are connected to tw o b indinga posts mounted on a

Tw o piece Of hard rubber . oscillators with standards are

v f r T pro ided o the terminals of the secondary . his com

- pletes the high tension coil .

1 1 6 The Tesla Coil

The secondary winding Of this transformer consists of

2 two sections of No . 3 B . 81 S . gauge double cotton covered

” Of copper wire . First saw out 1 stock four circular pieces

” ” of z wood, 4 in diameter and having a i hole in the centre .

S r s S lip these on the prima y to the position hown in the figure .

” The two end ones are 5 from the ends of the core and the

” n middle o es are i apart . Wind the wire of the secondary on the two spools just formed until the diameter is Thoroughly S hellac each

of layer and wrap a piece paper on before beginning the next . h e T e whole coil is mounted on a suitable bas , the primary

- terminals being connected to binding posts . — If the transformer is to be operated on the 100 1 10 volt

n current, the two sections of the primary are co nected in

0 — series . If, on the other hand, it is to be used on the 5 55 volt current the sections are joined in parallel . It is well,

in s s however, either ca e to bring the primary terminal out

- T to four separate binding posts. hen the desired connec tions may be readily made for either series or parallel .

e Always be certain , though , that the current will travers 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

- The and the high tension coil . method Of making this primary spark- gap is given as follows : Procure two pieces D of vulcanized fibre rod Y in diameter 4 inches long . rill

” ” ” a 1 hole in each 5 from one end . Next bore two i holes ” 6 apart in the base Of the transformer as shown in Fig . 47 .

1 18 The Tesla Coil

tension coil is connected to the spark - gap instead of the sec

n r The o da y of the induction coil . 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 8 W RIN D . 4 . I G IAGRAM.

FIG W N D . 49 . IRI G IAGRAM .

n Of connecting up the condenser . When making con ections between the various parts of the apparatus it is well to enclose f the wires in glass tubes and to keep them back Out O the way .

The operator will soon find that ordinary insulation is Of Appendi x 1 1 9

- 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 .

E mental and Practi cal ied. ASHE S W. E ctri cit x eri , . le y p ly ly Appl 422 i 12mo c oth 37 5 Net i ustrat ons . ll . , l , pp ,

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