Page:Collected Physical Papers.djvu/218

198 variation is the same, whether the torsion is right-handed or left-handed.

I next tried the effect of a complete torsional vibration. I twisted the wire suddenly through +90°, then back to zero, then to −90°, and again back to zero, the complete vibration being executed in half a second. It will be seen that under these conditions we have a mere vibration and no resultant twist. This gives rise to an electromotive variation, the magnitude of which simply depended, as will be shown later, on the amplitude of vibration. It did not matter in the least whether the vibration commenced with a right- or left-handed twist.

It may be stated here that similar electromotive variation is obtained by molecular disturbance produced by a mechanical tap.

I shall now describe the effect of mechanical stimulus of varying intensities and durations. The intensity may be varied by varying the amplitude of vibration. We shall also study the effect of a single stimulus, or the summated effect of rapidly succeeding stimuli.

A set of experiments on the effect of mechanical stimulus may thus be carried out parallel to those on the effect of radiation stimulus. It would then be instructive to compare the response-curves of mechanical with those of radiation stimulus.

For studying the effect of mechanical stimulus, a voltaic element made of "tin" wire is very suitable. Normal responses are easily obtained after annealing.