Page:Collected Physical Papers.djvu/221

Rh active (zinc-like), and that the wire recovers its original condition on the cessation of stimulus.

In fig. 44 are given the curves of response for single vibration, of amplitudes of 90°, 180° and 360°, the period lasting for 0.5 second. It will be noticed that the intensity of response increases with the energy of vibration.

In the case of effect of rapidly succeeding flashes of electric radiation on Ag′, it was shown (see fig. 36d) that the partial effects became fused together and that there was a limiting effect, kept balanced by the force of restitution. With rapidly succeeding mechanical stimuli, we again obtain precisely similar results. Fig. 45 (a, b) shows the effect of continuous vibration on tin cell, with different intensities of vibration, the vibration-frequency being twice in a second. The curve gradually rises and attains a maximum, at which position it is held almost rigid as long as the vibration is kept up. But on the stoppage of stimulation there is an immediate recovery, and if sufficient time be allowed the recovery is complete, as seen in the last curve of the series. The disturbance was kept up for 1 minute, and the period of recovery allowed was also 1 minute. In this way I obtained a long-continued series of similar responses, there being little fatigue; this is the case when a period of repose intervenes. But if the vibration is kept up without intermission, signs of fatigue begin to appear and the curve tends to fall. in some metals there may even be a reversal. Observe the flat top of