THE MEASUREMENT OF RESISTANCE 171 



.Definiteness is the only requirement in the balancing re- 

 sistances. They must not change through heating or be erratic 

 through ill-fitting plugs or defective sliding contacts. 



The box to be calibrated is placed in series with the standard 

 as shown. If the resistances of the equal extension coils, m, 

 are properly adjusted to that of the slide wire, a given dis- 

 placement of the slider may be made to correspond to an assigned 

 percentage difference of X and S. 



Suppose the slide wire has a length of 1 meter and a resistance 

 of 10 ohms. It is desired that a difference of Jfo per cent, 

 between X and S shall correspond to a displacement of the 

 balance point of 10 cm. When X = S, the balance point is to 

 be at the middle of the slide wire. 



Fi<;. <M't. Connections for comparing resistance boxes. 



A displacement of 10 cm. to the left takes 1 ohm from the left- 

 hand side of the bridge and adds it to the right-hand side. Then 



-A and m = 1,996 ohms. 

 l.UUl m -f- b 



Assume a balance to be obtained with the 1-ohm coil in X 

 by unplugging the corresponding coil in the balancing resistance, 

 the standard S having been cut out. If needful, R may be used 

 to bring the balance point to the middle of the slide wire. The 

 reading of the slider is taken and then the standard S is sub- 

 stituted for X. If X = S there will be no change in the balance 

 point; if X differs from *S, balance is restored by moving the slider 

 and another reading taken. The difference of the two readings 

 and the known displacement of the slider corresponding to 



