Page:On the Similarities between Radiation and Mechanical Strains.djvu/6

1901.] 5. Effect of a Single 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. As has been said before, any other metal may be used; I have, in fact, obtained as good results with platinum. But the advantage of tin is that the electromotive variation is comparatively strong; under favourable conditions the electromotive variation obtained is as high as 0·4 volt; another advantage is that it shows very little fatigue. On freshly making the cell, signs may be exhibited of abnormal irritability; this is due to the fact that a stable molecular condition has not yet been reached; but a more settled state soon supervenes, and after that a succession of responses is obtained which are extraordinarily regular and consistent amongst themselves.

That the responses are due to molecular disturbance in the acted wire may be shown by the following experiment. The wire is clamped below; when the wire is subjected to torsional vibration, there is produced a strong molecular disturbance with the attendant electromotive variation. If the wire is now released from the clamp and vibration imparted as before, there would be no electromotive effect.

In fig. 13 is given a series of curves for different "intensities" of vibration. For want of space I have given a few only of each series. As a matter of fact, the succeeding series would have been mere repetitions of those which preceded. I have taken as many as 500 successive records, and each record is a mere duplicate of the rest. The substance does not exhibit any appreciable fatigue, especially if a period be allowed for complete recovery. It will be seen that the rise is quick, whereas the fall is comparatively slow, specially in the later part.

If sufficient time be allowed the recovery is complete. (In the curves given only 30 seconds were allowed, hence the recovery was not complete.) On the cessation of disturbance the electromotive variation gradually disappears, the wires returning to their original condition, after which similar cycles of operation may be repeated for any length of time.

In the case of molecular distortion due to electric radiation we met with similar instances of complete recovery. It was then shown that the effect was not primarily due to any chemical action, but was due to physical strain, the recovery taking place as it were by the release of strained molecular springs.

With strain cells, there is no permanent change; the stimulated wire returns exactly to its original condition on the cessation of disturbance.

In the borderland between physics and chemistry no sharp line of