192 ELECTRICAL MEASUREMENTS 



Assume that the galvanometer stands at zero. Between the 

 terminals of a there must be a point at the same potential as c; 

 let d be this point; suppose it to be joined to c by a connection of 

 zero resistance, thus bringing the points c and d together; this is 

 allowable from the manner of locating the point d. The ar- 

 rangement has then become a Wheatstone bridge with arms. 



n 

 From the manner of locating d, 



Consequently 



1 n + 2 

 and 



MP / n \ / M\ MP 



/ na2 \ Ijn M\ t 



\n + a/ \n "" ^V/ 



Obviously, if the resistances are adjusted so that = -^ the 



second member becomes -^rr > an ^ X = -^-. The measurement 



N N 



is then independent of a. 



For general laboratory purposes P may be a variable standard, 

 and is frequently a slide wire or, better, a resistance divided into 

 tenths, the last tenth being a slide wire of perhaps 0.001 ohm. 

 This standard, shown diagrammatically in Fig. 109, should have 

 ample carrying capacity, 50 amp. at least. 



A convenient form of this bridge, employing such a low- 

 resistance standard, and adapted to commercial work, is shown 

 diagrammatically in Fig. 110. In this arrangement it is assumed 



