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INVENTIONS OF NIKOLA TESLA.

Among the many experiments which may he performed withsuch a machine, of not the least interest are those performedwith a high-tension induction coil. The character of the dis-charge is completely changed. The are is established at muchgreater distances, and it is so easily affected by the slightest cur-rent of air that it often wriggles around in the most singularmaimer. It usually emits the rhythmical sound peculiar to thealternate current arcs, but the curious point is that the soundmay he heard with a number of alternations far above ten thou-sand per second, which by many is considered to be about thelimit of audition. In many respects the coil behaves like a staticmachine. Points impair considerably the sparking interval, elec-tricity escaping from them freely, and from a wire attached toone of the terminals streams of light issue, as though it wereconnected to a pole of a powerful Toepler machine. All thesephenomena are, of course, mostly due to the enormous differ-ences of potential obtained. As a consequence of the self-induc-tion of the coil and the high frequency, the current is minutewhile there is a corresponding rise of pressure. A current im-pulse of some strength started in such a coil should persist to ■flow no less than four ten-thousandths of a second. As this timeis greater than half the period, it occurs that an opposing electro-motive force begins to act while the current is still flowing. Asa consequence, the pressure rises as in a tube filled with liquidand vibrated rapidly around its axis. The current is so smallthat, in the opinion and involuntary experience of the writer, thedischarge of even a very large coil cannot produce seriously in-jurious effects, whereas, if the same coil were operated with acurrent of lower frequency, though the electromotive force wouldbe much smaller, the discharge would he most certainly injuri-ous. This result, however, is due in part to the high frequency.The writer’s experiences tend to show that the higher the fre-quency the greater the amount of electrical energy which maybe passed through the body without serious discomfort; whenceit seems certain that human tissues act as condensers.

One is not quite prepared for the behavior of the coil whenconnected to a Leyden jar. One, of course, anticipates that sincethe frequency is high the capacity of the jar should be small. Hetherefore takes a very small jar, about the size of a small wineglass, but lie finds that even with this jar the coil is practicallyshort-circuited. He then reduces the capacity until he comes to