RESPONSE IN METALS 87 



the exciting disturbance reaches A and B at the same 

 time and with the same intensity ; (b) if the molecular 

 condition is similar at the two points ; and (c) if the 

 rate of rise and subsidence of excitation is the same at 

 the two points. In order that a resultant effect may be 

 exhibited in the galvanometer, matters have to be so 

 arranged that the disturbance may reach one point, say 

 A, and not B, and vice versa. This was accomplished by 

 means of a clamp, in the method of block. Again a 

 resultant differential action may be obtained even when 

 the disturbance reaches both A and B, if the electrical 

 excitability of one point is exalted or depressed by 

 physical or chemical means. We shall in Chap. XVI 

 study in detail the effect of chemical reagents in pro- 

 ducing the enhancement or depression of excitability. 

 There are thus two other means of obtaining a resultant 

 effect (2) by the method of relative depression, (3) by 

 the method of relative exaltation. 



Electric response by method of depression. We may 

 thus by reducing or abolishing the excitability of one 

 end by means of suitable chemical reagents (so-called 

 method of injury) obtain response in metals without a 

 block. The entire length of the wire may then be 

 stimulated and a resultant response will be produced, 

 owing to the difference between the excitability of the 

 two ends. A piece of tin wire is taken, and one normal 

 contact is made at A (strip of cloth moistened with 

 water, or very dilute salt solution). The excitability 

 of B is depressed by a few drops of strong potash or 

 oxalic acid. By the application of the latter there will 

 be a small P.D. between A and B ; this will simply 



