MEASUREMENT OF POTENTIAL DIFFERENCE 287 



The difference of these two values is 0.0011 ohm while the value 

 of a single step in II is 0.11 ohm. 

 The minimum value of R lv is 



so the general value is 



R lv = 0.9890 + 0.0011 X n iv 



where n lv is the number of the contact on dial IV. 

 Similarly the general value of R^, is 



R lv > = 0.9989 - 0.0011 X n iv /. 



In decade V the coil of 8.264 ohms is between contacts 9 and 10; 

 in the group of compensating coils, V, it is between 1 and 0, 

 so, in general, 



R v = 0.9989 - 0.001 lw v 



# V ' = 0.9890 + 0.001 Irv 



The coils 6, c, d, have such values that taken in conjunction with 

 the other resistances, they divide the current flowing in at T l 

 in the ratio 10 to 1. 



The voltage between T n and T ul is the difference of the ohmic 

 drops measured from T l} so for any setting (potentiometer current 

 0.001 amp.) 



P.D. = j| X 0.001 [n, X 1 + 0.9890 + 0.0011n IV 



+ 0.11089 + 0.11(n n + 1)] 

 - -fj- X 0.001 [(10 - n,)l + 0.9989 - 0.001 ln v 



+ 0.11(10 -n ni )] 

 n nm f _i_ ni l_i_ ? ' m Urv \ n * 1 



" 10 + 100 + i,ooo + io,oooJ- 



Therefore when the dials are properly graduated the unknown 

 P.D. is measured by the sum of the dial readings, as in the usual 

 instruments. 



A study of the network shows that if the battery circuit is 

 open, the resistance between the galvanometer terminals 



