The primary may consist of solid or stranded wire or of copper ribbon. The stranded, tinned-copper wire which is used for aerials gives excellent results; the ribbon is next best and the solid conductor is the least efficient. With the exciting apparatus to be described, the primary should, contain from 12 to 15 turns of conductor. In the writer's coil, the point of resonance is on the eleventh turn from the bottom and one turn either way will greatly reduce the efficiency. The drum may be of heavy paper wound around two rings of dry wood, as shown in Fig. 2, or a cage may be made of dowels fastened between the rings of wood. The same precautions for drying apply here, although the extreme care is not so essential. The primary is wound around the drum or cage with a small space between turns. This space may be 1/8 in. The starting and finishing ends of the coil may be secured with a piece of thread passed through the cardboard, in the case of that construction, or the end of the wire or ribbon may be bent around a dowel and clamped with a brass bolt. The use of iron in the construction of the coil should be avoided.

The end of the secondary wire is soldered to the end of the helix and connections are made to the helix by means of brass clips and heavy flexible wire, or, preferably with thin copper ribbon.

The primary and secondary of the Tesla are wound in exactly the same manner as the Oudin. Both ends of the cylinder, however, are finished off with the flange as described for the top of the Oudin. The primary drum is supported on brackets as shown in the end elevation in Fig. 5. The baseboard may be of any dry, well-seasoned wood and it should be provided with short pieces of glass rod at its four corners to serve as feet. The method of supporting the ends of the secondary is illustrated in Fig. 4, which is an end elevation of the completed instrument.

In the Tesla coil the starting and finishing ends of the winding are soldered to pieces of copper ribbon which are carried through slots in the supporting pieces and up to the brass ball terminals as suggested in Fig. 4. The primary and secondary windings have no electrical connection whatever in this case. The discharging rods are of aluminum or brass and they should be sharply pointed and highly polished. The end on which the handle is fastened should be rounded to lessen the leakage in the wrong direction. The handles may be of hard rubber, fiber, or of small glass test tubes which have been filled with sealing wax with the rod inside.

The dimensions given are so proportioned that the coil may be worked with the full output of a 1/2 kw. transformer without having sparks jump from secondary to primary. For larger transformers, the size should be increased a trifle if longer sparks are desired. If sparks longer than 10 in. are to be taken between points, the discharger pillars should be raised somewhat. This will prevent the discharge jumping to the primary helix.

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