Nikola Tesla - Free-Energy Devices

356 INVENTIONS OF NIKOLA TESLA.would be of advantage, so far as the lamp is considered, to employhigh frequencies for lighting, as they allow the use of shortand thick filaments and smaller currents.If a wire or filament be immersed in a homogeneous medium, allthe heating is due to true conduction current, but if it be enclosedin an exhausted vessel the conditions are entirely different. Herethe gas begins to act and the heating effect of the conduction current,as is shown in many experiments, may be very small comparedwith that of the bombardment. This is especially the case ifthe circuit is not closed and the potentials are of course very high.Suppose that a fine filament enclosed in an exhausted vessel beconnected with one of its ends to the terminal of a high tensioncoil and with its other end to a large insulated plate. Thoughthe circuit is not closed, the filament, as I have before shown, isbrought to incandescence. If the frequency and potential becomparatively low, the filament is heated by the current passingthrough it. If the frequency and potential, and principally thelatter, be increased, the insulated plate need 'be but very small, ormay be done away with still entirely the filament will become;incandescent, practically all the heating being then due to the bombardment.A practical way of combining both the effects ofconduction currents and bombardment is illustrated in Fig. 188,in which an ordinary lampis shown provided with a very thinfilament which has one of the ends of the latter connected to ashade serving the purpose of the insulated plate, and the otherend to the terminal of a high tension source. It should not bethought that only rarefied gas is an important factor in the heatingof a conductor by varying currents, but gas at ordinary pressuremay become important, if the potential difference and frequencyof the currents is excessive. On this subject I have alreadystated, that when a conductor is fused by a stroke oflightning, the current through it may be exceedingly small, noteven sufficient to heat the conductor perceptibly, were the latterimmersed in a homogeneous medium.From the precedingit is clear that when a conductor of highresistance is connected to the terminals of a source of high frequencycurrents of high potential, there may occurconsiderabledissipation of energy, principally at the ends of the conductor, inconsequence of the action of the gas surrounding the conductor.Owing to this, the current through a section of the conductor ata point midway between its ends may be much smaller than