An Air Insulated Tesla Coil

Experimenters who have wireless sending stations have all the apparatus necessary for the operation of an air insulated Tesla coil.

The coil here described is suitable for use with spark coils giving from a two to six inch spark or with a ¼ k. w. transformer. It is not advisable to make a larger coil unless oil is used for insulating. The coil shown in the photograph will give a seven inch spark when used in connection with a ¼ k. w. closed core transformer (magnetic leakage type), glass plate condenser and a rotary spark gap. When a stationary gap is used the sparking distance is about five inches.

The drawing gives the necessary dimensions. Part of the primary and secondary tubes are cut away in the drawing in order to make the construction clearer. The post in the end view is also cut away for the same reason. The primary consists of 20 turns of No. 14 B. & S. gauge rubber insulated wire wound on a paper or fiber tube six inches outside diameter. The length of the tube is not shown in the drawing because there is a great variation in the thickness of the insulation of different kinds of wire. There should be just space enough between the end pieces to accommodate 20 turns of the wire used. If stranded wire can be obtained it is preferable.

The secondary coil consists of one layer of No. 32 B. & S. gauge enameled wire wound on a tube three inches outside diameter. The winding should be eleven inches long. The other dimensions may be changed somewhat to suit the builder. The posts, however, must not be lower or the spark will jump into the primary winding.

The framework may be made of any hard wood. It should be put together entirely with dowels, for if nails or screws are used the spark will jump into them. The tubes may be fastened to the end pieces with glue. The wood-work may be finished with any substance which is a good insulator. A good imitation of hard rubber can be made by using maple and finishing with black shellac.

Very beautiful experiments may be performed with this coil in a dark room. The spark may be allowed to jump to a piece of metal held in the hand but no sensation of pain will be experienced. If the spark jumps directly into the body only a slight burning sensation will be felt. Lubricating oil may be placed on the end of the finger and lighted by means of the spark without burning the finger.