360 COMPARISON OF RESISTANCES. 



The difference of potential between the summits A and B 

 that is, between the armatures of the condenser is 



for r it becomes 



and for r=<x> 



The change of potential is then 



8V 2 - SVj ab' - ba' ac 



E 6 (b + b')(a + a' + b + b') (b + b 1 ) (a + a' + b + b 1 ) ' 

 If we assume b = b' and a = a' = mb, we get 



m 



E & 





In the actual case the sensitiveness is greater, the greater is 

 the ratio m, and the relative variation of potential to be observed 

 tends to become the quarter of the error in respect of the 

 resistance. 



964. COIL BRIDGE. The two forms of the Wheatsone electrical 

 balance now most used are the coil bridge and the wire bridge. 



The resistance boxes are ordinarily arranged so as to give at 

 the same time the elements of a Wheatstone's Bridge. Fig. 181 

 represents Elliott's pattern. MN is the resistance box described 

 above ; a series of coils CAD form the two arms a and a' of 

 the balance. The resistance b to be measured is connected to 

 the binding screws D and B by the wires R and R', and the 

 series of dials represents the standard resistances ; the binding 

 screws form thus the four summits of a parallelogram. The 

 battery is interposed between the two summits A and B by the 

 wires P and P', and the galvanometer between the summits 

 C and D by the wires G and G'. The two circuits, those of the 



