350

INVENTIONS OF NIKOLA TESLA.

portant element than the frequency. When both of these aresufficiently high, the heating may be almost entirely due to thepresence of the rarefied gas. The experiments to follow willshow the importance of the rarefied gas, or, generally, of gas at or-dinary or other pressure as regards the incandescence or otherluminous effects produced by currents of this kind.

I take two ordinary 50-volt 16 o. p. lamps which are in everyrespect alike, with the exception, that one has been opened at thetop and the air has filled the bulb, while the other is at the ordi-nary degree of exhaustion of commercial lamps. When I attachthe lamp which is exhausted to the terminal of the secondary ofthe coil, which I have already used, as in experiments illustratedin Fig. 179® for instance, and turn on the current, the filament, asyou have before seen, comes to high incandescence. When Iattach the second lamp, which is filled with air, instead of theformer, the filament still glows, but much less brightly. Thisexperiment illustrates only in part the truth of the statementsbefore made. The importance of the filament’s being immersediu rarefied gas is plainly noticeable but not to such a degree asmight be desirable. The reason is that the secondary of this coil iswound for low tension, having only 150 turns, and the potentialdifference at the terminals of the lamp is therefore small. WereI to take another coil with many more turns in the secondary,the effect would be increased, since it depends partially on thepotential difference, as before remarked. But since the effectlikewise depends on the frequency, it may be properly stated thatit depends on the time rate of the variation of the potential dif-ference. The greater this variation, the more important becomesthe gas as an element of heating. I can produce a much greaterrate of variation in another way, which, besides, has the advan-tage of doing away with the objections, which might be made inthe experiment just shown, even if both the lamps were con-nected in series or multiple are to the coil, namely, that in con-sequence of the reactions existing between the primary andsecondary coil the conclusions are rendered uncertain. This re-sult I secure by charging, from an ordinary transformer which isfed from the alternating current supply station, a battery of con-densers, and discharging the latter directly through a circuit ofsmall self-induction, as before illustrated in Figs. 183®, 1835,and 183c.

fn Figs. 186®, 1865 and 186c, the heavy copper bars bb„ are