168

INVENTIONS OF NIKOLA TESLA.

neighborhood the enormous potential may be made to alternateor fluctuate perhaps many million times a second. The airmolecules are violently attracted and repelled, and by their im-pact produce such a powerful heating effect that a fire is started.It is conceivable that a ship at sea may, in this manner, catch fireat many points at once. When we consider, that even with thecomparatively low frequencies obtained from a dynamo machine,and with potentials of no more than one or two hundred thous-and volts, the heating effects are considerable, we may imaginehow much more powerful they must be with frequencies and po-tentials many times greater; and the above explanation seems, tosay the least, very probable. Similar explanations may have beensuggested, but I am not aware that, up to the present, the heat-ing effects of a brush produced by a rapidly alternating potential

Fig. 111.

have been experimentally demonstrated, at least not to such aremarkable degree.

By preventing completely the exchange of the air molecules jthe local heating effect may be so exalted as to bring a body toincandescence. Thus, for instance, if a small button, or prefer-ably a very thin wire or filament be enclosed in an unexhaustedglobe and connected with the terminal of the coil, it may berendered incandescent. The phenomenon is made much moreinteresting by the rapid spinning round in a circle of the top ofthe filament, thus presenting the appearance of a luminous fun-nel, Fig. Ill, which widens when the potential is increased.When the potential is small the end of the filament may performirregular motions, suddenly changing from one to the other, orit may describe an ellipse; but when the potential is veryhigh it always spins in a circle; and so does generally a thin