642 • Mr. Nikola Tesla [Feb. 4, 



between the knobs. But when the frequency is very bigb, and the 

 arc of the discharge produces a sound which is loud and smooth 

 (which indicates both that oscillation takes place and that the 

 sparks succeed each other with great rapidity), then the luminous 

 streams formed are perfectly uniform. They are generally of a 

 purplish hue, but when the molecular vibration is increased by 

 raising the potential they assume a white colour. 



The luminous intensity of the streams increases rapidly when the 

 potential is increased ; and with frequencies of only a few hundred 

 thousand, could the coil be made to withstand a sufficiently high 

 potential difference, there is no doubt that the space around a wire 

 could be made to emit a strong light, merely by the agitation of the 

 molecules of the air at ordinary pressure. 



Such discharges of very high frequency which render luminous 

 the air at ordinary pressure we have very likely occasion to witness 

 in the Aurora borealis. From many of these experiments it seems 

 reasonable to infer that sudden cosmic disturbances, such as erup- 

 tions on the sun, set the electrostatic charge of the earth in an 

 extremely rapid vibration, and produce the glow by the violent agita- 

 tion of the air in the upjjer and even in the lower strata. It is thought 

 that if the frequency were low, or even more so if the charge were 

 not at all vibrating, the lower dense strata would break down as in a 

 lightning discharge. Indications of such breaking down have been 

 repeatedly observed, but they can be attributed to the fundamental 

 disturbances, which are few in number, for the superimposed vibration 

 would be so rapid as to not allow a disruptive break. 



The study of these discharge phenomena has led to the recognition 

 of some important facts. It was found that gaseous matter must be 

 most carefully excluded from any dielectric which is subjected to 

 great, rapidly-changing electrostatic stresses. Since it is difficult to 

 exclude the gas perfectly when solid insulators are used, it is 

 necessary to resort to liquid dielectrics. When a solid dielectric is 

 used, it matters little how thick and how good it is ; if air be present 

 streamers form, which gradually heat the dielectric and impair its 

 insulating power, and the discharge finally breaks through. Under 

 ordinary conditions the best insulators are those which possess the 

 highest specific inductive capacity, but such insulators are not the 

 best to employ when working with these high frequency currents, for 

 in most cases the higher specific inductive capacity is rather a 

 disadvantage. The prime quality of the insulating medium for these 

 currents is continuity. For this reason principally it is necessary to 

 employ liquid insulators, such as oils. If two metal plates, connected 

 to the terminals of the coil, are immersed in oil and set a distance 

 apart, the coil may be kept working for any length of time without a 

 break occurring, or without the oil being warmed, but if air bubbles 

 are introduced, they become luminous ; the air molecules, by their 

 impact against the oil, heat it, and after some time cause the insula- 

 tion to give way. If, instead of the oil, a solid plate of the best 



