88 RESPONSE IN THE LIVING AND NON-LIVING 
produce a displacement of zero. By means of a 
potentiometer the galvanometer spot may be brought 
back to the original position. The shifting of the 
zero will not affect the general result. The effect of 
mechanical stimulus is to produce a transient electro- 
motive response, which will be superposed algebraically 
on the existing P.D. The deflection will take place 
from the modified zero to which the spot returns during 
recovery. On now stimulating the wire as a whole by, 
say, torsional vibration, the current of response will be 


A =o 
Fic. 52.—Responsre By Mretuop or Depression (WitTHoUT BLock) 
When the wire is stimulated as a whole the current of response is towards the 
more excitable. 
In (a) A is a normal contact, B has been depressed by oxalic acid; current of 
response is towards the more excitable A. 
In (b) the same wire is used, only A is depressed by oxalic acid and a normal 
contact is made at a fresh point B’, a little to the left of Bin (a). Current 
of response is now from A towards the more excitable B’. 
found towards the more excitable, i.e.. from B- to A 
(fig. 52, a). 
A corroborative reversal experiment may next be 
made on the same piece of wire. The normal contact, 
through water or salt solution, is now made at B, 
a little to the left of B. The excitability of A is now 
depressed by oxalic acid. On stimulation of the whole 
wire, the current of response will now be found to flow 
in an opposite direction—i.e. from A to bB/—but still 
from the relatively less to the relatively more excitable 
(fig. 52, 0). 
