102 



words, we thus measure the dirtierence between E, and E^ in terms of the stand- 

 ard cell W. This is expressed by the formnla 



^ ^ Ew C 



E, — E, =n^jg- = nk 



Where E„ = E. M. F. of the cell W; E, = E. M. F. of the cell W^ ; 

 E, -= the E. M. F. of the cell W^ 

 n = number of millimeters between p and 7 

 1 ^ total length of the wire ab in millimeters 

 c = resistance of the wire ab 

 B = total resistance of the B-branch 



F C 



k = or the constant of the wire ah. 



The resistance of IV consists of a few coils of wire varying in resistance from 

 zero to fifty thousand ohms, but their actual resistance need not be known. 

 Neither is it necessary to know the resistance at J5' and R, nor that of the galva- 

 nometer G, since the method of no deflection is used. Care was always taken, 

 however, never to close the circuit through the galvanometer without including a 

 high resistance at i?^ unless it was first known that the system was almost ex- 

 actly at equilibrium. For it is important that as little current as possible shall be 

 allowed to pass through the cells. But when the system has been carefully ad- 

 justed, the resistance of i?' can be gradually cut out, so as to utilize the full sensi- 

 tiveness of the galvanometer. Measurements made by means of this apparatus 

 were limited in accuracy only by the galvanometer's .sensitiveness. For by taking 

 the resistance in B large enough we can make fc as small as we please. Thus we 

 found that we were able to detect diti'erences in E. M. F. as small as three one- 

 millionths doosooo) of ^ ^^o^^ with a considerable degree of certainty. 



One difficulty which we had to overcome was the change in position of the 

 galvanometer's zero, caused l.)y the passing of electric street cars some four hun- 

 dred feet distant. It was found necessary to make the final measurements be- 

 tween 12:30 and 5:30 A. m., when the cars were not running. 



Measurements made at various times upon a number of Clark and Weston 

 cells are given in the followiiis; talile : 



