CHAPTER XVII. HOW TO BUILD A TESLA HIGH FREQUENCY COIL.
IF the discharge from a Leyden jar or a condenser is passed through a coil of wire acting as a primary and the primary is provided with a secondary coil of a larger number of turns, a peculiar current known as high frequency electricity is generated in the secondary. Such a device is known as a Tesla Coil or Transformer.
[Illustration: FIG. 170.—Tesla Coil Circuits.]
When a Leyden jar or a condenser discharges through a coil of wire, the spark which takes place does not consist simply of a single spark passing in one direction only, but is really made of a number of separate sparks passing alternately in opposite directions at the tremendous rate of from one hundred thousand to one million times per second.
A Tesla high frequency coil opens a field of wonderful possibilities to the amateur experimenter and brings innumerable weird and fascinating experiments within his reach.
The Tesla coil described in the following pages will give sparks of high frequency electricity two or three inches long from the secondary, when used in connection with a two inch spark induction coil. If used in connection with a small high potential wireless transformer, the high frequency discharge can be increased to six or eight inches.
A Tesla coil, suitable for use with a smaller spark coil, say one capable of giving sparks from one-half to one inch in length can be made by following the same plans but cutting all of the dimensions in half. Make the secondary six inches long and one and one-half inches in diameter instead of twelve inches long and three inches in diameter, etc.
[Illustration: FIG. 171.—Secondary Tube.]
*The Secondary* winding consists of a single layer of No. 28 B. & S. Gauge double cotton covered wire wound over a cardboard tube, twelve inches long and and three inches in diameter. The tube must be thoroughly dried before using it, by baking in an oven. A coat of shellac, both inside and out, will avoid the possibility of having to rewind the tube because of the wire becoming loose, due to drying out of the tube later. The wire should be wound on in a smooth, even layer to within about one-quarter of an inch from the ends and given a coat of hot paraffine when finished. The ends of the tube are fitted with circular wooden heads having a half inch flange.
*The Base* is a rectangular piece of wood, fifteen inches long and six inches wide.
The secondary is supported in position by two hard rubber uprights, four inches high, seven-eighths of an inch wide and one-half an inch thick. A round-headed brass wood screw is passed through the top part of each of the supports into the centre of each one of the wooden secondary heads. High frequency currents are very hard to insulate and wood does not possess sufficient insulating value to fit it for use as supports. Hard rubber or glass are the most satisfactory materials for the supports.
[Illustration: FIG. 172.—Details of the Secondary Heads.]
The secondary terminals are connected to two brass rods, five inches long and having a small brass ball at the upper end, mounted on the top of each of the hard rubber supports.
The lower ends of the hard rubber supports are fastened to the base by means of screws passing upwards through the base into a threaded hole in the bottom of each support.
[Illustration: FIG. 173.—Details of the Primary Head.]
The secondary passes through the centre of the primary. The primary consists of eight turns of heavy copper wire wound around a wooden drum or frame. The wire should be No. 8 or No. 10 B. & S. Gauge. Flat copper or brass ribbon one-quarter of an inch wide can be used in place of the wire. The stranded copper wire, consisting of seven No. 22 B. & S. Gauge wires twisted together and commonly employed for wireless aerials can be used to good advantage in winding the primary, the idea of using a large wire, stranded wire, or ribbon being to obtain a conductor having as much surface as possible. High frequency currents travel only on the surface of wires and conductors. A hollow tube is just as good a conductor for high frequency currents as a solid rod of the same diameter.
[Illustration: FIG. 174.—Primary Cross Bar.]
[Illustration: FIG. 175.—Front View of the completed Tesla Coil.]
[Illustration: FIG. 176—Side View of the completed Tesla Coil.]
The heads of the primary drum are wooden rings, seven inches in diameter outside, four and one-half inches inside and one-half an inch thick. Six cross bars, two and one-half inches long, three-quarters of an inch thick and one-half an inch wide are required to support the wire. They are spaced equidistantly around the rings and held in position by means of brass screws passing through the rings. Do not use iron screws, because iron is magnetic and should be entirely avoided in the construction of a Tesla coil. Small notches should be cut in the outside edge of the cross bars to accommodate the wires. The wires should pass around the drum in the form of a spiral with one-quarter to five-sixteenths of an inch space between the turns. The completed drum will somewhat resemble a squirrel cage. The ends of the primary winding should terminate in two large binding posts mounted on the primary heads. The heads are fastened to the centre of the base by a couple of large wood screws passing upwards though the bottom.
[Illustration: FIG. 177.—Diagram of connections for operating the Coil.]
The illustration in Figure 177 shows how to connect the Tesla coil. The primary should be in series with a condenser and a spark gap. The condenser should consist of two or three Leyden jars or several glass plates coated with tinfoil. It is impossible to determine just how much capacity the condenser should have in advance, because the length of the conducting wires, adjustment of the spark gap, etc., will have considerable effect. The condenser is connected directly across the terminals of the spark coil. The spark gap may consist of two one-eighth inch brass rods supported by two double binding posts mounted on a small wooden block.
The induction coil should be as large as possible. When the coil is set in operation it will charge the condenser and a white, snappy spark should pass across the gap. If the fingers are brought near to one of the secondary terminals of the Tesla coil, a small, reddish purple spark will jump out to meet them. It will be necessary to adjust the apparatus very carefully before a spark of any considerable length can be obtained. Changing the length of the spark gap and the size of the condenser will undoubtedly produce results. It may also be possible to lengthen the high frequency spark by disconnecting one of the wires from the primary binding posts on the Tesla coil and connecting the wire directly to some one of the primary turns. A very small change in some one of the connections may produce considerable result. The purpose of the adjustments is to tune the circuit in the same manner that a wireless outfit is tuned by altering the capacity of the condenser or varying the number of turns in the helix.
[Illustration: FIG. 178.—Plate Glass Condenser.]
There are many interesting experiments which may be performed with the aid of a Tesla coil which space does not permit of describing here. The weird and strange beauty of the Tesla discharge is most evident when it takes place in the dark.
High frequency currents do not produce a shock. If you hold a piece of metal in your hand and bring it near one of the secondary terminals, you can take the shock of a high-frequency coil, throwing a spark several inches long without feeling any sensation except that of a slight warmth.