HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 141

exciting phosphorescence is extraordinarily great. It is capableof exciting phosphorescence at comparatively low degrees ofexhaustion, and also projects shadows at pressures far greaterthan those at which the mean free path is comparable to thedimensions of the vessel. The latter observation is of some im-portance, inasmuch as it may modify the generally accepted viewsin regard to the “radiant state” phenomena.

A thought which early and naturally suggested itself to M r.Tesla, was to utilize the great inductive effects of high frequencycurrents to produce light in a sealed glass vessel without the useof leading in wires. Accordingly, many bulbs were constructedin which the energy necessary to maintain a button or filamentat high incandescence, was supplied through the glass by eitherelectrostatic or electrodynamic induction. It was easy to regu-late the intensity of the light emitted by means of an externallyapplied condenser coating connected to an insulated plate, orsimply by means of a plate attached to the bulb which at thesame time performed the function of a shade.

A subject of experiment, which has been exhaustively treatedin England by Prof. J. J. Thomson, has been followed up inde-pendently by Mr. Tesla from the beginning of this study, namely,to excite by electrodynamic induction a luminous band in a closedtube or bulb. In observing the behavior of gases, and theluminous phenomena obtained, the importance of the electro-static effects was noted and it appeared desirable to produceenormous potential differences, alternating with extreme rapidity.Experiments in this direction led to some of the most interest-ing results arrived at in the course of these investigations. Itwas found that by rapid alternations of a high electrostatic po-tential, exhausted tubes could be lighted at considerable distancesfrom a conductor connected to a properly constructed coil, andthat it was practicable to establish with the coil an alternatingelectrostatic field, acting through the whole room and lighting atube wherever it was placed within the four walls. Phosphores-cent bulbs may be excited in such a field, and it is easy to regu-late the effect by connecting to the bulb a small insulated metalplate. It was likewise possible to maintain a filament or buttonmounted in a tube at bright incandescence, and, in one experi-ment, a mica vane was spun bv the incandescence of a platinumwire.

Coining now to the lecture delivered in Philadelphia and St.