SCIENCE AND PRACTICE. 



239 



C and C' being constants of sensibility of the two arrangements. 

 They are functions of the diameters of the rings, and of the 

 distances of the magnetic poles. If n s (Fig. 123) is the 



direction of the magnetic meridian .p 



and the plane of the deflecting ring, 

 and n s the direction taken up by the 

 deflected needle, by plane trigono- 

 metry 



a b 



tan. a = 

 an 



Following Weber's equation, however, 



and, since a n is equal to p M the Fig- l 23 - 



attraction between the magnetic pole and the earth we get 



27TV 2 



tan. a = 



M 



in which g (or S), the strength of the current, is 



2 tan. a. 



Therefore 



and by the same reasoning 

 C' =M 



Pouillet's tangent galvanometer is constructed on these 

 principles. It consists of a copper ring of large diameter, 

 erected in the plane of the magnetic meridian, and of a short 

 magnetic needle in its centre. 



A circular copper band, or ring, is bent outwards at the 

 ends to form parallel connections, which are properly insulated 

 from each other and attached to the terminal screws for re- 

 ceiving the wires of the galvanic circuit. In the lower half of 



