CHAPTER XI. THE CONSTRUCTION OF SPARK COILS._Home-made Electrical Apparatus

A Spark Coil is one of the most interesting pieces of apparatus that an

experimenter can possess. Not only can it be used to transmit wireless

messages, but numerous and various experiments may be performed with its

aid.

The purpose of a spark coil is to generate enormously high voltages

which are able to send sparks across an air space which ordinary battery

currents of low voltage could not possibly pierce.

[Illustration: FIG. 109.—Induction or Spark Coil.]

It consists of an iron core surrounded by a coil of heavy wire known as

the "primary" and a second outside winding of finer wire known as the

"secondary." The primary is connected to a few cells of dry battery in

series with an "interrupter."

Every time that the interrupter shuts off the battery current in the

primary, currents are induced in the secondary which are of sufficiently

high voltage or pressure to leap across a space in the shape of sparks.

A spark coil is not very hard to construct, but it requires careful

workmanship and patience. It is not usually a job which can be finished

in a day, but will take quite a little time, especially in winding the

secondary.

In describing the construction of a spark coil in this chapter,

directions and dimensions have been given for a one-quarter inch coil.

The general method of procedure is the same for all sizes of coils, the

dimensions only, are different. The experimenter may therefore easily

build a larger coil than the one-quarter inch by consulting the tables

of dimensions.

Parts for spark coils are for sale by many electrical houses and it is

possible to purchase the complete parts for less than the separate

materials would cost.

Such parts are made by special machinery and therefore are produced

quite cheaply.

The Core is made of soft iron "core wire" about No. 20 or 22 B. & S.

Gauge. Each piece should be four and three-quarters inches long. Iron

wire may be bought already cut to length from various houses dealing in

experimenter’s supplies. Considering the amount of labor that is

required to cut each piece to length arid then straighten it out, it is

cheaper to purchase the wire already cut.

[Illustration: FIG. 110.—The Primary and Core.]

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Size of Coil. O Length of P Diameter of Primary Wire

Core Core

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1/4 inch 4 3/4 inches 1/2 inch 18 B. & S.

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1/2 inch 5 inches 5/8 inch 18 B. & S.

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1 inch 7 inches 5/8 inch 18 B. & S.

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"Core wire" is a special grade of wire which is very soft and has been

annealed.

If ordinary iron wire is used in making a core, it must first be

annealed. This is accomplished by tying the wire in a compact bundle and

placing it in a wood fire until it is red hot. After the wire is hot,

cover it with ashes and allow the fire to die away.

When the fire has gone out and the wire has cooled the annealing process

is complete. Enough wires each four and three-quarters inches long to

form a compact bundle one-half inch in diameter will be required.

[Illustration: FIG. 111.—The Secondary Winding.]

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L Size L Length of M No. of M Size No. of Turns Wire

of Coil Secondary Sections of Hole per Section

1/4 in. 2 3/8 in. 1 1 1/18 5,000 37

in. en.

1/2 in. 2 3/8 in. 1 1 1/8 10,000 37

in. en.

1 in.¹ 2 2/8 in. 1 1 1/8 10,000 37

in. en.

¹ There are two sections of secondary used on the 1-inch coils.

Cut a piece of tough wrapping paper about six inches long and four and

one-half inches wide. Roll it up in the form of a tube so that it is six

inches long and one-half inch in diameter inside.

Glue the inside and outside edges of the paper so that the tube cannot

unroll and then slip the core wires into it until the tube is packed

tightly and no more can be slipped in.

The core is now ready for the primary, which consists of two layers of

No. 18 B. & S. Gauge cotton covered wire wound over the core for a

distance of three and three-quarters inches. The winding should start

five-eighths of an inch from one end of the core. The wire must be wound

very smoothly and carefully. The inside end of the wire can be fastened

so that it will not become loose by placing a short piece of tape

lengthwise of the core and winding two or three turns over it. Then

double the end back and complete the winding over it. The end of the

wire can be fastened by imbedding a piece of tape under the winding with

a small loop projecting and then passing the wire through the loop.

Figure 110 shows the complete core with primary winding in position.

*The Secondary* is a much more tedious job than the primary and must be

very carefully made. Whenever it is possible for the experimenter to

purchase a secondary already wound he is advised to do so.

The secondary consists of 5,000-6,000 turns of No. 37 B. & S. Gauge,

enameled wire wound in smooth, even layers, with two layers of paper

between every two layers of wire.

The primary winding is wrapped with six layers of "empire cloth or

paper." Empire cloths and papers are a yellow insulating material made

by treating linen or paper with linseed oil.

Roll up a paper tube of five or six layers of paper so that the finished

tube is two and one-half inches long and of proper inside diameter to

just slip over the primary winding when the empire cloth or paper is in

position.

The secondary is wound over this paper tube. It will be necessary to

mount the tube on a round wooden mandrel fitted with a small crank or

handle so that the tube may be revolved. A "winder" may be very easily

made by mounting a round wooden stick of the same diameter as the inside

of the paper tube in a pair of wooden supports. Bore a hole in one end

of the stick and bend a piece of stiff wire in it so as to form a crank.

[Illustration: FIG. 112.—The Fixed Condenser.]

Size Length Width Length Width No. of No. of

of of of of of Paper Tinfoil

Coil Paper Paper Tinfoil Tinfoil Sheets Sheets

1/4 30 in. 4 3/8 30 in. 4 in. 5 4

in. in.

1/2 30 in. 4 3/8 30 in. 4 in. 5 4

in. in.

1 in. 30 in. 4 3/8 30 in. 4 in. 7 6

in.

The paper placed between each two layers of the secondary winding should

be the special waxed paper which is made for that purpose.

Start and end each layer of wire one-half inch from the edges of the

paper. Wind the wire in smooth, even layers, permitting each turn to

touch the other, but none to lap over. Wind on two layers of waxed paper

between each layer of wire and the next. The paper must be put on

smoothly and evenly so as to afford a firm foundation for the next layer

of wire.

The wire should never come nearer to the edges of the paper than

one-half inch or the insulation of the secondary will be weak and the

coil not liable to give a long spark.

The utmost care should be used not to break the wire. If it should

break, the ends must be very carefully connected. The number of turns

that each layer averages should be carefully noted so that by keeping a

record of the number of layers it is possible to tell how many turns

have been wound on. When five to six thousand turns have been wound on

the secondary it is sufficiently large, and the outside end of the wire

should be fastened and prevented from unwinding by securing it with a

drop of sealing wax.

When winding a secondary remember that if at any point in the work, you

allow the winding to become irregular or uneven, the irregularity will

be much exaggerated on the succeeding layers. For this reason do not

allow any irregularities to occur and if the wire tends to go unevenly,

wind on two or three extra layers of the waxed paper to smooth it out.

[Illustration: FIG. 113.—Details of the Wooden Coil Heads.]

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Size of Coil D K J

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1/4 in. 3 in. 2 1/2 in. 1/2 in.

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1/2 in. 3 in. 2 1/2 in. 5/8 in.

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1 in. 3 1/2 in. 3 in. 5/8 in.

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*The Condenser* consists of alternate layers of tinfoil and paraffined

paper piled on top of each other and then rolled up.

Four sheets of tinfoil, thirty-six inches by two and three-quarters

inches wide will be required. Eight strips of waxed condenser paper,

three and one-quarter inches wide and thirty-six inches long are

necessary.

Lay one sheet of paper out flat on a table or board and place a sheet of

tinfoil over it. The tinfoil should be along the centre of the paper

strip so that there is a one-quarter inch paper margin along the sides.

One end of the tinfoil should project over the end of the paper about

two inches. Lay two sheets of waxed paper over the tinfoil making them

line up with the first paper sheet all around. Then place another strip

of tinfoil on the paper. It should be directly above the first sheet of

tinfoil, but the end of the tinfoil must overlap the paper by two inches

at the opposite end from the first sheet. Place two more sheets of paper

over the tinfoil and on top of that a third strip of foil. The third

strip of foil should line up exactly with the first strip. Then lay two

more sheets of waxed paper over the third sheet of foil and place the

fourth sheet of tinfoil in position making it exactly line up with the

second sheet. Place a single sheet of paper on top.

The result should consist of four sheets of tinfoil insulated from each

other with two strips of paper between each two strips of foil.

Connect the first and third sheets together with a wire and the second

and fourth sheets likewise.

Cut out a piece of cardboard, one and one-quarter inches wide and three

and one-quarter inches long. Lay the cardboard on one end of the

condenser and roll the condenser up around it very tightly. Tie it with

a piece of string to keep it from unrolling, and dip it in some melted

paraffine. Then place it between two boards with a weight on top so as

to press it out flat.

*The Coil Heads* are made from two hardwood blocks three inches square

and one-half an inch thick. A circular groove, two and one-half inches

in diameter and one-quarter of an inch deep is turned in each block to

receive a cardboard tube of the same diameter. The two upper corners of

each of the blocks are rounded slightly to improve their appearance. One

of the heads has a round hole one-half inch in diameter bored through

the centre so that one end of the core may be passed through. The core

should project through about one-eighth of an inch. The other head is

bored with a hole of the same diameter but the hole should not pass all

the way through, and serves merely to receive and support the other end

of the core.

[Illustration: FIG. 114.—Details of the Wooden Base.]

Size of Coil A B C

1/4 in. 8 in. 4 in. 1 1/8 in.

1/2 in. 8 1/4 in. 4 1/2 in. 1 1/8 in.

1 in. 10 1/4 in. 4 1/2 in. 1 1/4 in.

*The Base* of the coil is a box eight inches long, four inches wide and

one and one-eighth inches deep. It is made in the form of a hollow box.

Give the base a coat of stain, preferably some dark color, such as

mahogany, and then a coat of shellac or varnish.

The wooden heads should be stained black.

The parts of the interrupter are shown in Figure 115. They cannot be

made by every experimenter on account of the tools required, but can be

purchased very cheaply from concerns dealing in material for

experimenters.

*The Standard* is a piece of hexagonal brass rod one and

three-sixteenths inches long and three-eighths of an inch in diameter.

The lower end of the standard is tapped to receive an 8-32 machine

screw. A hole is bored at right angles to the axis of the standard, one

inch from the lower end and tapped to receive an 8-32 thumbscrew.

[Illustration: FIG. 115.—Details of the Interrupter. The Spring and

Standard for the One inch coil should be made one-quarter of an inch

longer.]

The Thumbscrew should be about seven-eighths of an inch long and be

provided with a lock nut so that it can be fastened in position. The end

of the thumb-screw is drilled with a small hole and a contact point

driven in tightly. The contact point is usually a piece of platinum

wire, but in the case of a small coil may be a piece of the special

contact wire which is sold for just this purpose.

[Illustration: FIG. 116.—The tube.]

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Size of Coil Length of Tube Diameter of Tube

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1/4 inch 4 1/4 inch 2 1/2 inch

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1/2 inch 4 1/2 inch 2 1/2 inch

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1 inch 6 1/2 inch 3 inch

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*The Spring* is a piece of thin steel, three-eighths of an inch wide and

two and one-eighth inches long. Several holes are drilled in it. The

illustration shows the exact location of these holes. The holes A and C

should be just large enough to allow an 8-32 machine screw to slip

through. The hole B is just large enough to receive a piece of contact

wire.

*The Contact* wire should be about three-thirty-seconds of an inch long

and be slipped in the hole and riveted in place.

*The Armature* is a piece of soft iron cut out of a rod one-half an inch

in diameter. It should be about one-quarter of an inch thick, but should

have a small stud left on one face so that the stud may be slipped into

the hole marked C in the steel spring and the armature then riveted

firmly to the spring by flattening out the stud.

[Illustration: FIG. 117.—The Bridge.]

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Size of Coil E Length I Width

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1/4 inch 4 7/8 inch 3/4 inch

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1/2 inch 5 1/8 inch 3/4 inch

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1 inch 7 1/8 inch 3/4 inch

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The spring is bent at right angles about one-half inch from the end as

shown by the dotted line in Figure 115.

*A Cardboard Tube*, two and one-half inches in diameter and four and

one-quarter inches long, is used to inclose the primary and secondary

windings when they are in position. The tube may be given a very fine

appearance by gluing a single layer of black bookbinders’ cloth over it.

*The Bridge* shown in Figure 117 is a strip of wood about one-eighth of

an inch thick, four and seven-eighths inches long and three-quarters of

an inch wide. It is stained to match the base and is then given a coat

of shellac or varnish. Two holes are bored in, the bridge, one inch from

each end.

The coil is now ready for assembling. Figure 118. shows a cross-section

of the complete coil.

Slip the secondary over the insulating cloth or paper wrapped around the

primary. Pass one end of the core through the hole in the coil head

making sure that the groove is on the side towards the secondary. The

secondary should be exactly in the centre of the primary.

[Illustration: FIG. 118.—Section of the Spark Coil showing the

arrangement of the Parts.]

Slide the cardboard tube over the primary and secondary into the groove.

Lead the secondary terminals out through small holes in the cardboard

tube at opposite ends so that the wires are separated as far as

possible. Slip the other coil head over the end of the core and the tube

and then mount the coil on the top of the base. The coil should be in

the centre of the base when considered from front to back. It should be

to the left of centre, however, when viewed from side to side.

The two primary heads should be four inches apart When the coil is

assembled.

The primary wires are led down close to the wooden head and through the

base under the coil. They should be kept as far away from the secondary

as is possible.

The coil is fastened to the base by a wood screw passing through the

base into each of the wooden heads.

Figure 122 will show best how the interrupter is arranged. The brass

spring carrying the armature at its upper end is mounted in front of the

core end projecting through the wooden head. The standard carrying the

thumbscrew and the lock nut are mounted directly behind the spring in

such a position that the contacts touch each other and line up

perfectly.

[Illustration: FIG. 119.—End View of the Complete Coil.]

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Size of Coil B Width of Base D Width of Head

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1/4 inch 4 inch 3 inch

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1/2 inch 4 inch 3 inch

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1 inch 4 1/2 inch 3 1/2 inch

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A small brass washer is placed between the standard and the base so that

it will not "bite" into the wood when the screw is tightened. The

standard and the spring are both mounted by 8-32 brass machine screws

passing through the base.

Two binding posts are mounted on the corners of the base near the

interrupter.

[Illustration: FIG. 120.—Side View of the Completed Coil.]

Size of A C D E F G H Q

Coil

1/4 in. 8 in. 1 3 4 3 1 5/8 4

1/8 in. 7/8 in. 7/16 in. 1/4

in. in. in. in.

1/2 in. 8 1/4 1 3 5 3 1 5/8 4

in. 1/8 in. 1/8 in. 7/16 in. 1/2

in. in. in. in.

1 in. 10 1 3 7 4 1 5/8 6

1/4 1/4 1/2 1/8 in. 7/16 in. 1/2

in. in. in. in. in. in.

The condenser is fastened to the under side of the base by means of a

wooden strip passing across it and secured to the base by means of two

small wood screws.

One of the binding posts is connected to one of the primary wires. The

other primary wire is connected to the spring. The other binding post is

connected to the standard.

The condenser is connected directly "across" the interrupter, that is,

one wire is connected to the spring and the other wire to the standard.

Two binding posts are mounted on the wooden bridge and then the bridge

is fastened across the top of the coil by means of two brass nails. The

secondary terminals are connected to these binding posts.

The Coil is now ready for operation. Connect two pieces of wire to the

secondary binding posts and bring them toward each other until they

almost touch. The space between should be about one-quarter of an inch

long.

[Illustration: FIG. 121.—Diagram of Connections.]

Connect three or four good dry cells to the primary posts and turn the

thumbscrew until the contact point on the end touches the contact on the

spring. The interrupter should immediately commence to "buzz" and if the

coil has been properly constructed and connected, a stream of sparks

will jump between the wires connected to the secondary.

It may be possible that the action of the interrupter and consequently

the strength of the sparks at the secondary can be improved by bending

the spring’ either away or towards the core. A little experimenting will

soon show just how much tension the spring should have. When the proper

position of the thumb-screw is found, tighten the lock nut so that it

cannot change its adjustment.

[Illustration: FIG. 122.—Perspective view of Coil showing names of the

various parts.]

Details and instructions for building and experimenting with larger

spark coils are given in our books "Experimental Wireless Construction"

and "Induction Coils."

EXPERIMENTS WITH SPARK COILS.

Experiment 1—An Imitation Gassiot’s Cascade.

Wrap a piece of copper wire around a small glass jar and connect it to

one of the secondary terminals of the coil. Fill the jar up with water

and dip a wire connected to the other secondary terminal into the water.

If the coil is then set into operation, the entire outside of the jar

will be covered with sparks starting from the wires and rushing upwards

into the neck of the jar. If the coil used for this experiment is a very

small one, the glass jar should also be very small, as for example, a

small medicine vial. This experiment shows off the best when performed

in the dark.

Experiment 2—A Ghostly Light

Grasp the bulb of an old incandescent electric light bulb in one hand

and touch the base of the lamp to one of the secondary terminals of the

coil. Set the coil in operation and the bulb will glow with a weird

greenish light. This experiment will work best in the dark.

Experiment 3—Lighting Geissler Tubes.

The most beautiful and wonderful effects may be obtained from Geissler

tubes operated from a spark coil in a dark room. These tubes come in an

almost endless variety of styles and shapes. They are exhausted of air

and fitted with metallic wires, sealed into the ends, a portion of the

wire projecting outside in the form of small rings, so that the

connections are easily established.

Some of the tubes are of ordinary glass, while others are made of glass

having in its composition certain metallic oxides. Some of the tubes are

filled with various gases in a highly rarified state. Others have a

double wall with the outside filled with a fluorescent liquid, such as

an aqueous solution of sulphate of quinine or an alcoholic solution of

strammonium, and other compounds.

The tubes are not very expensive. They can be obtained in a great

variety of sizes. Most well stocked electrical houses carrying goods for

the electrical experimenter have them on hand.

Such tubes are of necessity very fragile articles and should be handled

with extreme care. It is a good plan to keep them wrapped in cotton and

to put them away in a strong wooden box, so as to reduce the danger of

breakage.

The very smallest coil, even the one-quarter inch size, will serve to

light a large tube. . In fact, if too large a coil is used, there is

danger of spoiling the tube.

The tubes are connected to the spark coil by running a wire from each of

the small rings at the ends of the tubes to the secondary terminals.

Several tubes may be connected in series when a or 1-inch coil is

available.

To show the best effects, the room should be in perfect darkness and the

tubes should be placed against a background of black velvet.

Experiment 4—Flickering Light.

Spread your fingers out and move your hand rapidly back and forth over a

lighted Geissler tube in a darkened room. Your hand will appear to have

ten or twelve fingers instead of five. This is due to the fact that the

light from a Geissler tube is flickering and not at all steady. Each

time the interrupter of the coil opens, the current is sent through the

tube and lights it momentarily. The flickering is almost too rapid to be

perceived by the naked eye until you have something like the fingers or

a pencil in front of it.

Experiment 5—Rotating a Geissler Tube.

An extremely pretty effect can be secured by rotating a Geissler tube at

high speed while it is lighted. This may be accomplished by means of an

electric motor of small size. The tube is tied tightly to a stick of

wood which has a hole drilled exactly in its center into which the motor

shaft fits tightly. Two rings of metal are mounted on the motor shaft,

and a wire is led from each to a terminal of the tube. A copper strip

rests on each ring, and these two copper strips are connected to the

secondary of the spark coil.

When the motor is started and the coil set into operation, the tube will

appear like the spokes of a wheel. The intermittent light will make the

tube visible for a fraction of a second in various positions around the

circle as it rotates, and this gives the effect which is much like that

of a pinwheel.

Experiment 6—Fluorescent Writing.

Dissolve some sulphate of quinine in water and then draw a picture or

design or write a sentence on a piece of white paper with it, and allow

it to dry. The writing will be invisible, but if you hold it up to a

lighted Geissler tube in a dark room, it will appear as if written with

a beautiful blue ink.

Experiment 7—An Electric Bomb.

This is a very interesting experiment from which there is no danger if

it is performed carefully.

The mortar is nothing more or less than a piece of wood which is hard

and well seasoned, and has a small hole bored in its axis about

three-eighths of an inch in diameter and an inch deep.

Two short lengths of stiff copper wire should be inserted through the

sides near the bottom of the hole so as to form a small spark gap

between the ends.

Put a little gunpowder in the mortar and connect two wires from the

secondary terminals of the spark coil to the spark gap. Place a cork

loosely in the muzzle of the mortar and then, standing well away, close

the switch in the primary circuit of the coil, so that a spark passes

through the mortar. The spark will ignite the powder and it will explode

with a loud report.

*Caution*.—Do not use more than a pinch of powder at a time as this is

sufficient to cause a sharp explosion.

Experiment 8—Electrifying the Garbage Can.

If you are ever annoyed by dogs or cats that knock the cover off the

garbage can in their search for canine and feline delicacies, you can

give them the surprise of their lives with the aid of a spark coil upon

the occasion of their next visit.

Set the can upon a thick piece of perfectly dry wood and run a wire from

the can to one secondary terminal of the spark coil. Connect the other

secondary terminal to the ground or to a water or gas pipe.

The next time you see a dog with his nose in the can, close the switch

connected to the primary circuit. What happens will not hurt the dog,

but he will travel for home as fast as four legs can carry him.

Experiment 9—How to Make an Electric Spark Photograph Itself.

This experiment must be conducted in a dark room with the aid of a

photographer’s ruby lamp so that the photographic plates used will not

become "light struck" and spoiled.

Place an ordinary photographic plate on a piece of sheet metal with the

coated or sensitive side up. Connect the piece of sheet metal to one of

the secondary terminals of the spark coil by means of a wire.

Then sift some dry starch powder, flour of sulphur or talcum powder

through a thin piece of gauze on to the plate. Lead a sharp pointed wire

from the other secondary terminal of the coil. Complete the primary

circuit of the coil just long enough to make one single spark.

Wipe the powder off the plate thoroughly and develop it in the usual

manner. The negative will show a peculiar electric discharge looking

very much like sea moss. No two photographs will be alike. The test

results are obtained when only one or two sparks are allowed to pass.

Arts and Science Series No. 9

Home-made

Electrical Apparatus

A Practical Handbook for Amateur

Experimenters

In Three Parts

*Volume III*

_Second Edition_

*BY*

*A. M. Powell*

PUBLISHED BY

COLE & MORGAN, Inc.

Publishers of the Arts and Science Series

P.O. BOX 473 CITY HALL STATION

NEW YORK, N. Y.

Printed in U. S. A.

COLE & MORGAN, Inc.