to the Wheatstone Bridge. 417 



calculated. Calling the resistance B l5 choose x x in the box 

 so that 



x 1 B x 



Tooo -<>or 



Eemove the ping P, and, including p r in the battery cir- 

 cuit, obtain balance by shifting the slider I, the galvanometer 

 being also shunted if necessary. The arrangement is now an 

 ordinary Wheatstone bridge, 



a 4- B x __ x x _ III 

 b-\-r y r ' 



or a K, B • . i 



— = — x = — approximately. 



Next insert the plug P, short-circuit p r so that now a 

 strong current passes when the battery circuit is closed, and 

 obtain balance by altering x. Let the new value be x 2 . Then. 



n T? 



since approximately r = — , we have, by the formula quoted 

 above, 



E _ x 2 



r ~~ y' 



'■' R=-xr very approximately. 



j 



In any case this new value of E is very much nearer the 

 true value than the approximate value E x . If the value of x 

 has to be but little altered, it may be taken as the true value 

 of E, but if, on the other hand, the first estimate was con- 

 siderably in error, and, in consequence, a large alteration 

 has to be made in x in order to obtain balance, the much 

 closer value thus obtained must be taken as the first approxi- 

 mation, and both experiments repeated. 



Now, since the wires are so close to one another, their tem- 

 perature can be called the same, and, if the tested wire be of 

 copper, their temperature-corrections are the same. There- 

 fore, if E be a certain fraction of r at one temperature, it will 

 be the same fraction at any other. But in the actual bridge 

 on which the experiments described later on were first 

 made, this temperature was 17°' 7 C. Therefore, substituting 

 numerical values in the last equation, 



^I^o^^^lopoo 0111118 ^ 170-70 - 



Of course, if E be not made of copper, the temperature 

 must be noted and allowed for. 



The accuracy and sensibility of this method depend on 

 three conditions : — 



Phil. Mag. S. 5. Vol. 41. No. 252. May 1896. 2 G 



