86 RESPONSE IN THE LIVING AND NON-LIVING 



showed an increase of E.M.F. On stopping the vibration, the 

 spot of light 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, i.e. 

 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 A -E B when E A and E B are the electrical variations pro- 

 duced at A and B 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 



