86 RESPONSE IN THE LIVING AND NON-LIVING 
showed an increase of E.M.F. On stopping the vibration, the 
spot of ight came back to zero. On now vibrating the copper 
wire, a deflection of 23 dns. was obtained in an opposite 
direction, showing a diminution of E.M.F. This transi- 
tory responsive variation disappeared on the cessation of 
disturbance. 
By disturbing the balance of the potentiometer, the 
galvanometer deflection due to a known increase of E.M.F. 
was found from which the absolute E.M. variation caused by 
disturbance of copper or zinc was determined. 
It was thus found that stimulation of zinc had increased 
the P.D. by fifteen parts in 1,000, whereas stimulation of 
copper had decreased it by eleven parts in 1,000. According 
to the old terminology, the response due to stimulation of 
zinc would have been regarded as positive variation, that of 
copper negative. The responses however are not essentially 
opposite in character, the action current in the bar being in 
both cases towards the more excited. For this reason it 
would be preferable, as already said, to employ the terms 
positive and negative in the sense I have suggested, Le. 
positive, when the current in the acted substance is towards 
the more excited, and negative, when towards the less excited. 
The method of block is, as I have already shown, the most 
perfect for the study of these responses. 
In the experiment fig. 50, if the block is abolished 
and the wire is struck in the middle, a wave of mole- 
cular disturbance will reach A and B. The mechanical 
and the attendant electrical disturbance will at these 
points reach a maximum and then gradually subside. 
The resultant effect in the galvanometer will be due to 
£,-E, when E, and £, are the electrical variations pro- 
duced at AandB by the stimulus. The electric changes 
at A and B will continuously balance each other, and the 
resultant effect on the galvanometer will be zero: (a) if 
