178 Prof. J. C. Bose. On the Similarities [Apr. 18, 



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 elec- 

 tric 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 vibra- 

 tional 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 revers- 

 ing 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. 



