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INVENTION'S OF NIKOLA TESLA.

it affects mostly the size of the apparatus, for, when excessivelyhigh potentials would be used, the light-giving devices would belocated not far from the apparatus, and often they would he quiteclose to it. As the air-bombardment of the insulated wire is de-pendent on condenser action, the loss may be reduced to a trifleby using excessively thin wires heavily insulated.

Another difficulty will be encountered in the capacity and self-induction necessarily possessed by the coil. If the coil be large,that is, if it contain a great length of wire, it will be generallyunsuited for excessively high frequencies; if it be small, it maybe well adapted for such frequencies, but the potential mightthen not be as high as desired. A good insulator, and prefera-bly one possessing a small specific inductive capacity, wouldafford a two-fold advantage. First, it would enable us to con-struct a very small coil capable of withstanding enormous differ-ences of potential; and secondly, such a small coil, by reason ofits smaller capacity and self-induction, would be capable of aquicker and more vigorous vibration. The problem then of con-structing a coil or induction apparatus of any kind possessingthe requisite qualities I regard as one of no small importance,and it has occupied me for a considerable time.

The investigator who desires to repeat the experiments whichI will describe, with an alternate current machine, capable ofsupplying currents of the desired frequency, and an inductioncoil, will do well to take the primary coil out and mount the sec-ondary in such a manner as to be able to look through the tubeupon which the secondary is wound. He will then be able toobserve the streams which pass from the primary to the insulat-ing tube, and from their intensity he will know how far he canstrain the coil. Without this precaution he is sure to injurethe insulation. This arrangment permits, however, an easyexchange of the primaries, which is desirable in these experi-ments.

The selection of the type of machine best suited for the pur-pose must be left to the judgment of the experimenter. Thereare here illustrated three distinct types of machines, which,besides others, I have used in my experiments.

Fig. 91 represents the machine used in my experiments beforethis Institute. The field magnet consists of a ring of wroughtiron with 384 pole projections. The armature comprises a steeldisc to which is fastened a thin, carefully welded rim of wrought