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INVENTIONS OF NIKOLA TESLA.

There are a good many other points of interest which may beobserved in connection with such a machine. Experiments withthe telephone, a conductor in a strong field or with a condenseror arc, seem to afford certain proof that sounds far above theusual accepted limit of hearing would be perceived. A telephonewill emit notes of twelve to thirteen thousand vibrations persecond; then the inability of the core to follow such rapid alter-nations begins to tell. If, however, the magnet and core bereplaced by a condenser and the terminals connected to the high-tension secondary of a transformer, higher notes may still beheard. If the current be sent around a finely laminated coreand a small piece of thin sheet iron be held gently against thecore, a sound may be still heard with thirteen to fourteen thou-sand alternations per second, provided the current is sufficientlystrong. A small coil, however, tightly packed between the polesof a powerful magnet, will emit a sound with the above numberof alternations, and arcs may be audible with a still higher fre-quency. The limit of audition is variously estimated. In SirWilliam Thomson’s writings it is stated somewhere that tenthousand per second, or nearly so, is the limit. Other, but lessreliable, sources give it as high as twenty-four thousand persecond. The above experiments have convinced the writer thatnotes of an incomparably higher number of vibrations per secondwould be perceived provided they could be produced with suffi-cient power. There is no reason why it should not be so. Thecondensations and rarefactions of the air would necessarily setthe diaphragm in a corresponding vibration and some sensationwould be produced, whatever—within certain limits—the velocityof transmission to their nerve centres, though it is probable thatfor want of exercise the ear would not be able to distinguish anysuch high note. With the eye it is different; if the sense ofvision is based upon some resonance effect, as many believe, noamount of increase in the intensity of the ethereal vibrationcould extend our range of vision on either side of the visiblespectrum.

The limit of audition of an arc depends on its size. Thegreater the surface by a given heating effect in the arc, the higherthe limit of audition. The highest notes are emitted by thehigh-tension discharges of an induction coil in which the arc is,so to speak, all surface. If A be the resistance of an arc, and Gthe current, and the linear dimensions be n times increased, then