280 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



through the galvanometer the electromotive force between c and 

 d must be equal to zero. Therefore the electromotive force ai f 

 between a and c is equal to the electromotive force 72" be- 

 tween a and d, that is, 



and similarly we find 



ai' = yi" 

 &' = &'" 



CO 



Dividing equation (i) by equation (ii), member by member, we 

 have equation (82). 



155. Measurement of resistance by the slide wire bridge. A 

 stretched wire ab t Fig. 207, an unknown resistance a, a 



cc 



known resistance y8, and a sensitive galvanometer G are con- 

 nected as shown to a battery B. The lettering in Fig. 207 cor- 

 responds to that in Fig. 206. The sliding contact d is adjusted 

 until the galvanometer gives no deflection and then equation (82) 

 is satisfied, but 7/8 is equal to the ratio of the lengths of the 

 corresponding portions of the wire ab, and it is easily deter- 

 mined by measuring the lengths ad and db. Therefore, /3 

 being known, a may be calculated. 



156. Measurement of resistance by the box bridge. The box 

 bridge is a resistance box containing three sets of resistances, 

 yS, 7 and 8 connected as shown in Fig. 208. The dotted lines 

 represent connections outside the box. The portions 7 and 8 

 usually have each a lo-ohm, a loo-ohm, and a i,ooo-ohm coil, 



