3. Self-recovery.
It was said that the acted wire, usually speaking, becomes zincoid. This is not universally the case; there are substances which become cuproid under mechanical stimulus. I have previously said that electric radiation produces opposite effects on different substances; silver is often found to show an effect (increase of resistance) opposite to the generality of metals. It is very curious that silver is also often found to exhibit an opposite electromotive effect under twist, that is to say, the acted wire becomes C.
As long as the wire is not overstrained there is always a recovery. Observe the extremely regular recovery in the curve for Zn when the twisting was stopped. It will be noticed that the recovery is very rapid at first, but slow in the later part, and that the recovery is complete.
4. Irreversible Molecular Effect of Twist.
In the case of electric radiation or light, the impulses are of a vibrational nature, unlike the one-directioned mechanical twist used in the above experiments. To make the two sets of phenomena comparable, we should have the mechanical disturbance of a vibrational nature also. I therefore next tried to see what the effect would be of reversing the direction of the twist, and found that the induced electromotive force is independent of the direction of twist.
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 gave 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 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 and with those of radiation stimulus.
