7. Effect of Summation of Stimuli.
In the case of effect of rapidly succeeding flashes of electric radiation on Ag′, it was shown (see fig. 14) that the partial effects were fused together and there was produced a limiting effect, kept balanced by the force of restitution. With rapidly succeeding mechanical stimuli, we again obtain an exactly similar result. Fig. 15 (a, b) shows the effect of continuous vibration on tin cell, with different intensities of vibration, the vibration-frequency being two in a second. The curve gradually rises and attains a maximum, at which position it is
Fig. 14.—Effect of continuous vibration. (a) and (b) show effects on a Tin cell. In (c) the effect on the particular Silver cell; the sign of E.M. variation is opposite to that of Tin cell. (d) shows the effect on a Nickel cell.
held almost rigid as long as the disturbance is kept up. But on the stoppage of vibration 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 exactly similar curves, 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 the curve similar to that of Ag′ under electric stimulus mentioned above. Also the effects of different intensities of vibration, as shown in (a) and (b).
In (d) is shown the effect of vibration on Ni. After reaching the maximum there is a tendency towards reversal. Ni also shows greater signs of fatigue.
