High Frequency Apparatus Page 46

46 HIGH FREQUENCY APPARATUS Fig. 1.-Wiring diagram for Oudin coil windings may be insulated by air space, oil, wax, or petrolatum. The primary may be an open helix of bare copper bar, strip, ribbon, or tubing, or it may consist of a coil of heavily insulated cable. The secondary may be wound in a single layer upon a cardboard or wooden cylinder, or upon a wooden cage, or it may be wound in a series of narrow layers on insulating paper or cloth. The secondary may also be in the shape of a cone or a flat spiral resembling a spider web. Oudin and Tesla Types.-Oscillation transformers may be broadly classed under these two headings. The Oudin coil is in reality an auto transformer, i. e., its primary and secondary are connected together at a neutral point which, in the case of high frequency apparatus, should be grounded. In the Tesla coil, the only connection between the primary and the secondary is an inductive one; the two windings are separate and distinct. (See Fig. 2.) The third type that should be mentioned, as it is perhaps the most practical of all, is a combination of the Tesla and Oudin circuits. This consists of two Oudin resonators placed side by side and having their ground connections