300 



ELECTRICAL MEASUREMENTS 



coefficient; exceedingly small lag error; long life, due to freedom 

 from cracking at the negative electrode; continuity of action, 

 due to the fact that gas is not formed at the negative electrode. 



The Weston Secondary Standard Cell. The Weston Instru- 

 ment Co. has placed on the market the convenient, portable 

 form of cadmium cell shown in Fig. 174. The solution is satu- 

 rated at 4C., and therefore does not contain an excess of cad- 

 mium sulphate crystals. This type of cell is to be used between 

 the temperatures of 4 and 40. The temperature coefficient 

 is much smaller than that of the normal cell and is negligible for 

 any ordinary measurements; each cell is accompanied by a 

 certificate stating its e.m.f. The extreme variation among 145 



Mercurous _ 

 Sulphate Paste 



Mercury Cement 



FIG. 174. Weston secondary standard cell. A working standard of elec- 

 tromotive force. 



cells of this type was found to be 0.0009 volt. This is the best 

 form of secondary standard cell, being remarkably permanent. 

 The e.m.f. is very closely 1.0186 international volts; the resistance 

 about 200 ohms. 



Precaution in Using Standard Cells. No appreciable current 

 can be taken from a standard cell without alteration of its e.m.f., 

 due to polarization. It is found that the change is not perma- 

 nent, for the cell gradually recovers its original e.m.f.; the cell is 

 unreliable until the recovery is complete. This being so, stand- 

 ard cells are used only in compensation methods and must always 

 be protected by a key, and the manipulation must be such that 

 the key is closed for but an instant. 





PROPERTY OF ELECTRICAL LABORATOI 



FACULTY OF APPLIED SdJLNCE. 



