MUSIC FROM ELECTRIC OSCILLATIONS. The mathematical theory of electric oscillations was given by Lord Kelvin so long ago as 1853, in a famous Paper in The Philosoph ical Magazine called "Transient Currents." Helmholtz had al ready shown that the conservation of energy required an electric discharge under certain conditions, even a flash of lightning, to be oscillatory. And the fact that the discharge from a leyden-jar or other condenser consisted of an alternating current was subse quently verified by Feddersen, who photographed the spark in a revolving mirror, and saw that the band to which it was spread out was beaded. The merit of Lord Kelvin's paper was that it recognized the two chief causes of the oscillation, the electrostatic capacity of the condenser, on the one hand, which may be likened to the storage of energy in a spring with elastic recoil, and what he called the electrodynamic capacity of the discharger, on the other, which may be likened to the momentum of a stream of water or other material. These two properties are essential to oscillations. They may be called elasticity and inertia or momentum.
Whatever may be the reason, electricity exhibits these two properties. An electric charge displays the one, an electric current the other. Every detail of the oscillations was worked out by Kelvin, considered as the free and dying out oscillations resulting from the running down of stored energy, on the analogy of a tun ing-fork. But whereas the oscillations of matter are to be esti mated at the rate of a hundred, or at most a few thousand, per second, the rate of electric oscillation, which is concerned not with matter but with the ether, is to be reckoned in millions a second. Consequently electric oscillations are far too rapid to affect the ear, unless they are rectified and made tractable by some such device as Fleming's vacuum valve.
Of late it has become possible, by means of valves, to maintain the oscillations continuously, from any convenient source of power, and such oscillations are in common use for wireless telegraphy. When two such circuits are operating in the same neighbourhood, though independently, they produce interference phenomena or beats, corresponding to the difference in their rate of vibration ; and then, if the circuits are not too different or far apart in their rate of vibration, the beats may be of audible fre quency when they are rectified, and communicated to a telephone or loud-speaker. For instance if one oscillated at i,000,000 a
second, and the other at 999,000, the beats would be i,000 a second, which corresponds to a note, say Cb, above the treble clef. Any variation in the capacity of one of the circuits, modifying its rate of vibration so as to bring the two nearer together, would lower the pitch of the beat-note, and might abolish it altogether if the two circuits were exactly in tune. The pitch of the beats can in fact be raised or lowered at will. The process of extracting beat-notes is often called "heterodyning," and is well known to wireless operators.
A young Russian, M. Leo Theremin, by constructing the circuits so that the condenser of one of them has an exposed part whose capacity can be varied by bringing the hand near it, found that he could control the pitch of the beat-note in a satisfactory man ner by alternately approaching and withdrawing his hand, and that by another device he could vary the loudness by similarly moving the other hand. He thus had what might be called a musical instrument, in which electric oscillations were controlled both in pitch and in loudness by the movement of the two hands, and thus, after a good deal of practice, by skilfully waving his hands, he was able to perform serious music. The surprising thing about his achievement was, not the control of electric oscillations for sound production over an extensive range, but the accuracy with which many of the tones of instruments such as a violoncello or a cornet could be imitated, and the skill with which an essen tially continuous series of vibrations could be so managed—by sliding along the scale quietly and then swelling out—as to give the impression of the discontinuous notes required for melody. Eminent musicians did not disdain to express admiration of the performance, and even to consider the possibilities of the instrument for orchestral purposes. (0. J. L.)