Page:Collected Physical Papers.djvu/219

Rh 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 this may be 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 successive responses are obtained which are extraordinarily regular and consistent amongst themselves.

That the response is 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. The wire is next released from the clamp and vibration imparted as before; there is now no electromotive response.

In fig. 44 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 of rest 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.

With strain cells, there is no permanent change; the stimulated wire returns exactly to its original condi-