318 



THE ELECTRIC TELEGRAPH. 



is r, is then put into the circuit, in the place of E, and the 

 resistance altered to B^, until the needle is again deflected < 

 degrees. 



The intensity with the first element is 



E 



B + 9 + r 



and that when the second element is used, 



E' 



These expressions equalled give the value of E 1? the electro- 

 motive force of the second element in terms of that of the 

 first, 



' R + r 



The resistances, E + 9 anc *- RI + 9i ma j be made so 

 great in comparison with r and r lt that the latter may be 

 neglected, and the calculation simplified by the disappear- 

 ance of these magnitudes from the numerator and denomi- 

 nator of the fraction, or 



E' = E. R + ^ 



It sometimes happens that a large electro-motive force has 

 to be measured by a comparatively small one, as in measur- 

 ing the battery used for testing the insulation resistances of 

 telegraph cables by the force of a Daniell's cell. In this 



case it is better to use a shunt for 

 obtaining the common deflection, 

 degrees. If both the batteries are 

 large, a shunt may be used in both 

 measurements with advantage. 



The battery E (Fig. 149) has a re- 

 sistance r units, the galvanometer g 

 units, the shunt s units, and the in- 

 terposed resistance E units ; we get 

 a deflection of the needle through 

 degrees. The battery is substituted by E 1 with r 1 resist- 



