356 



COMPARISON OF RESISTANCES. 



resistance.* The galvanometer is put in a lateral branch b of the 

 parallelogram, while the bridge which usually contains the galvano- 

 meter is closed by a wire with a key K (Fig. 187). The needle 

 of the galvanometer is deflected by the passage of the current /3 ; 

 the resistance b' is so adjusted that the deflection is the same 

 whether the bridge is opened or closed. If the resistances of the 

 four limbs satisfy the relation of ordinary equilibrium, the two 

 ends of the bridge are at the same potential, there is no current 

 in the wire joining them, and the suppression or introduction of 



Fig. 187. 



this wire does not at all modify the currents in the lateral 

 branches. 



We see, in fact, in equations (22), that the currents in the 

 lateral branches are independent of the resistance r of the bridge. 



In order to estimate the sensitiveness of this method, we may 

 suppose the bridge formed by a very short wire, and by raising 

 or lowering the key the resistance r is made to vary from zero to 

 infinity. The balance- being nearly established, we may replace the 

 denominator A by the approximate value 



MN N 

 = - 



Equations (21) give then, for r=0, 



Sir W. THOMSON. Proceedings of the Roy. Soc., Vol. xix., p. 253. Jan., 1871. 



