300 



THE ELECTRIC TELEGRAPH. 



copper or zinc pole of the battery to earth at pleasure, and 

 those of U 2 so as to determine to which side of the zero line 

 of the galvanometer the needle shall be deflected. 



The galvanometers used by Messrs. Siemens with these 

 bridges are either Dubois' sine multipliers or Weber's 

 reflecting galvanometers. Professor Thompson's reflecting 

 galvanometer is well adapted for such work ; better, perhaps, 

 than either of the others, as its sensibility is greater than that 

 of the sine instrument, and it is less unwieldy than Weber's. 

 With a very sensitive galvanometer, and a given battery 

 power, it is sometimes found that the needle is deflected 



beyond the range of observation. 

 In such an event the operator 

 inserts a shunt by completing 

 the circuit of the upper branch 

 r r , making it, instead of in- 

 finite, equal to some resistance 

 . which will take as much of the 



T~t current from the galvanometer as 



is necessary. The current goes 

 then (Fig. 141) from the key to 

 c ; here it is split into two parts ; 

 one goes through p, b, L T 2 , and 

 galvanometer to a, and the 

 other through r to a. At a these 

 parts combine again, and the 

 whole current goes over L L 

 into the cable. The resistances 

 r r' consist of four coils of 10 1 , 

 10 2 , 10 3 , and 10 4 units, which can be introduced separately 



or combined, and take off the -- part of the current from 



9+r 

 the galvanometer (g being the galvanometer resistance). 



With very rare exceptions these powers of 10 will be found 

 sufficiently comprehensive for supplying the required shunt, 

 and the function of the deflection of the needle will have 



Fig. 141. 



to be multiplied with in order to arrive at the 



value 



