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INVENTIONS OF NIKOLA TESLA.

sufficiently large. The passage of the streams heats, and, aftera while, softens, the rubber so much that two plates may bemade to stick together in this manner. If the spheres are so farapart that no spark occurs, even if they are far beyond the strik-ing distance, by inserting a thick plate of glass the discharge isinstantly induced to pass from the spheres to the glass in theform of luminous streams. It appears almost as though thesestreams pass through the dielectric. In reality this is not thecase, as the streams are due to the molecules of the air whichare violently agitated in the space between the oppositely chargedsurfaces of the spheres. When no dielectric other than air ispresent, the bombardment goes on, but is too weak to be visible;by inserting a dielectric the inductive effect is much increased,and besides, the projected air molecules find an obstacle and thebombardment becomes so intense that the streams become lumi-nous. If by any mechanical means we could effect such a vio-lent agitation of the molecules we could produce the same phe-nomenon. A jet of air escaping through a small hole underenormous pressure and striking against an insulating substance,such as glass, may be luminous in the dark, and it might be pos-sible to produce a phosphorescence of the glass or other insulatorsin this manner.

The greater the specific inductive capacity of the interposeddielectric, the more powerful the effect produced. Owing tothis, the streams show themselves with excessively high poten-tials even if the glass be as much as one and one-half to twoinches thick. But besides the heating due to bombardment,some heating goes on undoubtedly in the dielectric, being ap-parently greater in glass than in ebonite. I attribute this to thegreater specific inductive capacity of the glass, in consequence ofwhich, with the same potential difference, a greater amount ofenergy is taken up in it than in rubber. It is like connecting toa battery a copper and a brass wire of the same dimensions. Thecopper wire, though a more perfect conductor, would heat moreby reason of its taking more current. Thus what is otherwiseconsidered a virtue of the glass is here a defect. Glass usuallygives way much quicker than ebonite; when it is heated to a cer-tain degree, the discharge suddenly breaks through at one point,assuming then the ordinary form of an arc.

The heating effect produced by molecular bombardment ofthe dielectric would, of course, diminish as the pressure of the