RESPONSE IN METALS 87 
the exciting disturbance reaches A and B at the same 
time and with the same intensity ; () 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 
