354 



THE ELECTRIC TELEGRAPH. 



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galvanic battery, although the whole current passes through 

 the instrument, no deflection of the needle is produced ; but 

 when one of the parallel circuits is interrupted, the needle is 

 deflected to the right, and when the other is interrupted 

 instead, $ to the left of zero. 



Such a galvanometer, G (Fig. 163), has the ends of its 

 coils, a and b, on one side, connected together, in the point c, 



to the pole z of the battery E. 

 The opposite end of the coil a 

 is connected to the cable con- 

 ductor, or resistance to be mea- 

 sured ; the opposite end of coil 

 b to an adjustable resistance or 

 set of coils, R ; and the further 

 ends of these, in a point, d, are 

 connected with the pole c. 

 Fig. 163. The current of E is therefore 



divided into two circuits : the one, a and cable ; the other, b 

 and R. 



The intensity in the upper circuit being = i, and that in the 

 lower one = i lf while the resistance of the cable conductor is c, 

 Kirchhoff's equation gives us the general expression, 



i (a + r ) i\ (b + ft) = 

 Whence the resistance of the conductor 



r = -li- (b +ft) a 



When the resistance R is adjusted so that the needle rests 

 upon the zero line, 



t = f , 



and if, at the same time, the resistance of the galvanometer 

 coils are also equal (a = b), the value of r is 



or the resistance of the conductor is observed directly from 

 the adjustable resistance in the opposite circuit. 



Wheatstone's bridge method is more generally used for this 



