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INVENTIONS OF NIKOLA TESLA.

An experiment of special interest shown by Mr. Tesla, is therunning, by the use of only one insulated line, of a motor oper-ating on the principle of the rotating magnetic field enunciatedby Mr. Tesla. A simple form of such a motor is obtained In'winding upon a laminated iron core a primary and close to it asecondary coil, closing the ends of the latter and placing a freelymovable metal disc within the influence of the moving field.The secondary coil may, however, be omitted. When one of theends of the primary coil of the motor is connected to one of theterminals of the high frequency coil and the other end to aninsulated metal plate, which, it should be stated, is not absolutelynecessary for the success of the experiment, the disc is set inrotation.

Experiments of this kind seem to bring it within possibility tooperate a motor at any point of the earth’s surface from a cen-tral source, without any connection to the same except throughthe earth. If, by means of powerful machinery, rapid variationsof the earth’s potential were produced, a grounded wire reachingup to some height would be traversed by a current which couldbe increased by connecting the free end of the wire to a body ofsome size. The current might be converted to low tension andused to operate a motor or other device. The experiment, whichwould be one of great scientific interest, would probably bestsucceed on a ship at sea. In this manner, even if it were notpossible to operate machinery, intelligence might be transmittedquite certainly.

In the course of this experimental study special attention wasdevoted to the heating effects produced by these currents, whichare not only striking, but open up the possibility of producing amore efficient illuminant. It is sufficient to attach to the coilterminal a thin wire or filament, to have the temperature of thelatter perceptibly raised. If the wire or filament be enclosed ina bulb, the heating effect is increased by preventing the circula-tion of the air. If the air in the bull) be strongly compressed,the displacements are smaller, the impacts less violent, and theheating effect is diminished. On the contrary, if the air in thebulb be exhausted, an inclosed lamp filament is brought to in-candescence, and any amount of light may thus be produced.

The heating of the inclosed lamp filament depends on somany things of a different nature, that it is difficult to give agenerally applicable rule under which the maximum heating