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.

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.

"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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Details and instructions for building and experimenting with larger spark coils are given in our books "Experimental Wireless Construction" and "Induction Coils."