1901.] 



betvjeen Radiation and Mechanical Strains. 



179 



5. Effect of a Single Stimulus. 



For studying the effect of mechanical stimulus, a voltaic element 

 made of " tin " wire* is very suitable. Normal responses are easily 

 obtained after annealing. 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 compara- 

 tively strong ; under favourable conditions the electromotive variation 

 obtained is 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 a succession of responses is obtained which 

 are extraordinarily regular and consistent, amongst themselves. 



That the responses are 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 pro- 

 duced a strong molecular disturbance with the attendant electromotive 

 variation. If the wire is now released from the clamp and vibration 

 imparted as before, there would be no electromotive effect. 



In fig. 13 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 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. 



If sufficient time be allowed the recovery is complete. (In the 

 curves given only 30 seconds were allowed, hence the recovery was 

 not complete.) On the cessation of disturbance the electromotive 

 variation gradually disappears, the wires returning to their original 

 condition, after which similar cycles of operation may be repeated for 

 any length of time. 



In the case of molecular distortion due to electric radiation we met 

 with similar instances of complete recovery. It was then shown that 

 the effect was not primarily due to any chemical action, but was due 

 to physical strain, the recovery taking place as it were by the release 

 of strained molecular springs. 



With strain cells, there is no permanent change ; the stimulated wire 

 returns exactly to its original condition on the cessation of disturbance. 



In the borderland between physics and chemistry no sharp line of 



* By tin wire is meant what is sold as such, and used as electric fuse. It is a 

 pliable alloy of tin and lead. 



