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 





FIG. 52. KESPONSE BY METHOD or DEPRESSION (WITHOUT 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 (6) 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 B in (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 B' but still 

 from the relatively less to the relatively more excitable 

 (fig. 52, b\ 



