0^ PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 119 



ment is restored and the bridge balanced again. Thus, by succes.sive 

 approximations wc have 



P _ P+« _ « ^ 



Q Q + /3 /8 



R' 



where E' and S' are tlie shunted values of E and S. 



E' 

 Thus — is equal to 



If 



then 



a//3 within the limits of the errors of measurement. It does not follow, 

 however, that d/S / (<t + ft + d) x (R'/H'-a/ft) is negligibly small. It is 

 only so if the value of dft/{a + /3 + d) does not exceed the value of P 

 the value be NPand the probable error of an observation is 1 X 10 ", 

 the error of the final result is not less than N x 10"". It will be seen 

 from this that the current leads of standard resistances intended for 

 measurement on the Kelvin double bridge should have a resistance not 

 than the standard itself. In some commercial standards the 



greater 



Fig. 11. 



resistance of the current leads plus the connectors necessary for their 

 measurement is greater than that of the standard strip. In such cases 

 the potentiometer or Kohlrausch differential galvanometer should be 

 employed. In the department of Electrotechnics at the National Phy- 

 sical Laboratory the potentiometer is used. The sensitiveness of the 

 Kelvin bridge is less than that of the potentiometer, but it is more 

 convenient m practice. In the bridge, if P=00001, Q=0001, E=l, 

 g_10, a = l, /3=10, the sensitiveness is proportional to 0-0034 iAVP. 

 The galvanometer resistance is supposed to be 2 ohms. An example 

 follows. For simplicity P=01 ohm. 



P = No. 2484 = 01 ohm with potential leads. 



Q = No. 23.51 = 10 „ 



R = No. 2483 = I'O „ 



S = No. 1693 = 100 ohms. No potential leads. 



a = 1 /3 = 10 t = 170° 0. 



Value desired. 

 Value = 1-00000, at 17-0° C. 

 Value = 100002, „ 

 Value - lOOOOls „ „ 



