360 



THE ELECTRIC TELEGRAPH. 





m' ' E' + / 



Sometimes the proportion between m and m 1 is so great that 

 it would be inconvenient to obtain the resistance W large 

 enough to establish the balance in measuring the constant of 

 the apparatus. 



In this case the operator employs a shunt across the ends of 

 the larger coil of the galvanometer, and introduces its value 



into the calculation by the con- 

 stant K. This shunt, r,,, Fig. 

 165, does not exert any de- 

 flecting force upon the needle, 

 its duty being simply to take off 

 part of the current from r, that 

 the needle may not be acted upon 

 Fig. 165. by the full intensity, i. The 



currents in r and r being i and (,, respectively, by Kirchhoff's 



laws, 



I __ i ._ { ti - o 



and 



i r i ti r t = 

 whence 



And since 



m 



I m' r 

 By Ohm's law, however, 



W-f 



