1902.] accompanying Mechanical Disturbance in Metals. 217 



at A and B), one contact, say A, is touched with a few drops of 

 Na 2 C0 3 , which is an exciting agent. The electric excitability of A 

 will now be found to be greater than that of B ; on simultaneous 

 vibration of A and B there will be a disturbance of balance, giving 

 rise to an upward deflection (current of response towards the more 

 excitable A). 





Response of A. 



Response of B. 



Resultant 

 response. 



Both contacts of normal 

 Contact A touched with 



+ 12 



+ 32 



-12 

 -12 







+ 20 





Similarly, when A is depressed by a trace of oxalic acid the electric 

 excitability of A is less than that of B, the resultant deflection being 

 now downwards (current of response towards the relatively more 

 responsive B). It is to be remembered that in all cases the resultant 

 current of response in the ivire is towards the more excitable point. 



An interesting line of investigation rendered possible by a modifica- 

 tion of method of balance described above is to compare the relative 

 excitability induced by various chemical reagents, the influence of the 

 same reagent of different strengths, and the modification of the effect 

 caused by the duration of application. We may thus compare the 

 effect of the reagent in relation to the normal effect of water or dilute 

 NaCl solution. There is again an extremely delicate method of com- 

 parison of the relative effects of a series of compounds like NaoCOs, 

 KoC0 3 , &c. Balance having been previously obtained between the 

 normal sensitiveness of A and B, the two different solutions are now 

 applied at the two points ; the slightest difference in their relative 

 action is at once exhibited by. the upsetting of the balance during 

 stimulation, the direction of the resultant deflection indicating the 

 more exciting reagent. 



Resultant Response by Method of Relative Depression or Exaltation. 



From what has been said, it will be seen that by rendering A and B 

 unequally excitable, a resultant response may be obtained. The block 

 may be abolished, and the wire may be vibrated as a whole ; the response 

 will now be due to the differential effect at A and B. To produce dif- 

 ference in excitability we may subject one point, say A, to a prelimi- 

 nary vibration, or apply at the point a suitable chemical reagent. By 

 the application of the latter there will be a small P.D. between A and 

 B : this will simply produce a displacement of the zero. (By means of 



