64 ELECTRICAL MEASUREMENTS 



it gave readings 10 per cent, high, and at 750,000 cycles, 4f> per 

 cent, high (see page 67). 



The explanation of the distribution errors will be seen by 

 examining the theory of the three-wire instrument shown in 

 Fig. 29, where the arrangement is such that these errors are 

 pronounced. 



SYMBOLS USED AND THE DATA FOR A PARTICULAR CASE 



I = length of wires, 10.00 cm. 

 5 = diameter of wires, 0.008 cm. 

 d = distance between wires, 0.40 cm. 

 R a = mean resistance of a and a', 0.347 ohm 

 Rb resistance of b, 0.352 ohm. 



L =! coefficient of self-induction of one wire, 155.00 cm. 

 Ma6, M '6, Moo' = coefficients of mutual induction. 

 Mob = M a 'b by symmetry. 



v = instantaneous potential difference between the ends of 



the wires. 



i = total instantaneous current. 

 i*, i a ', ib = instantaneous currents in the wires. 



It will be necessary to calculate the self-inductance of the 

 straight wires by the approximate formula 



L = 2ZJlo & y - 0.75}. 

 The mutual inductance will be given by 



40 



The potential difference between the ends of the wires will be 





