270 ELECTRICAL MEASUREMENTS 



This may be varied by altering either R i or R z . The battery 

 EZ is so inserted that, when the key is depressed, its e.m.f. 

 opposes the potential difference due to the passage of the cur- 

 rent through Ri. If by varying RI or Rz this potential differ- 

 ence is made equal to E 2 , no current will flow through the gal- 

 vanometer and the battery EZ when the key is closed. 

 Consequently the absence of a deflection of the galvanometer 

 when the circuit is closed shows that 



or 



PT^ " O _l_ 7? 

 .JJ. K\ -f- /t2 



If #1 + R-2 be so high that very little current flows through 

 the battery E\, the fall of potential in E\, which is given by 

 IB, where B is the battery resistance, will be so small that the 

 P.D., which is equal to EI IB, may be taken as equal to EI, 

 and 



Wi = Ri + R ' very nearlv ' 

 or 



+ 7} 

 Kz 



ft 



R 



The larger Ri + Rz, the better the approximation. 



It will be noticed that current can flow through EZ only when 

 the key is depressed, and that when the adjustment is perfect, 

 there can be no current through the battery EZ. This is of 

 importance, for if care be exercised it allows cells to be used at 

 Ez without danger of altering their e.m.fs. by polarization. 



Much labor has been expended in the development and study 

 of galvanic cells, suitable for use at EZ, which shall have per- 

 fectly definite e.m.fs., and consequently can be used as standard 

 cells with which P.D. or E l may be compared. On account of 

 their high degree of reproducibility the Clark and the Weston cells 

 are now universally used. These cells are of the open-circuit 

 type and no appreciable current can be drawn from them without 

 temporary alteration of their e.m.f. 



As it is important to avoid short-circuiting EI or connecting 



