242 



THE ELECTRIC TELEGRAPH. 



But 



ri c a ri sin. a = M sin. a; 

 therefore also 



S = M. sin. a. 



Any other intensity of current, S', moving in the same 

 ring, will require the instrument to be turned through 

 another angle, a, in order to bring the needle to the zero 

 point of the scale ; and we get another equation, by the 

 same reasoning 



S' = M sin. '. 



The relation of the two currents is 



S : S = sin. a : sin. a'. 



That is to say, the currents are proportional to the sines of 



the angles through which the galvanometer is turned to 



make the coil and needle parallel. 



Instruments constructed on this principle are called Sine 



Galvanometers . 



Multiplier Sine Galvanometer. The galvanometer which 



Professor Du Bois Rey- 

 mond uses in his beautiful 

 experimental researches on 

 animal electricity consists 

 of a multiplier of from 

 twenty to thirty thousand 

 turns of fine insulated cop- 

 per wire, which act upon 

 a Nobili's astatic pair of 

 magnets. The two mag- 

 netic needles are placed 

 upon a common centre 

 with reversed poles, and 

 are of nearly equal direc- 

 tive forces, so that the 

 difference between them, 

 which determines the di- 

 rective force of the system, 

 is very small, whilst, being 



Fig. 125. 



placed one in the centre and the other over the coil, they 

 are deflected in the same sense. 



