194 ELECTRICAL MEASUREMENTS 



With massive conductors it is absolutely necessary that the 

 relative positions of the potential and current terminals be such 

 that the stream lines between c and d are in their normal posi- 

 tion; for instance, in measuring a low-resistance shunt, such as is 

 used for large currents on switchboards, a serious error may be 

 introduced, even if c and d are at the proper points, if the current 

 is not led into the short, heavy terminals exactly as it is to be in 

 the subsequent use of the instrument. 



Expression for Galvanometer Current. Let the arrangement 

 of the conductors and the mesh currents be as shown in Fig. 111. 

 The mesh equations are 



(x + y)(M + Ro + m + X) - yR G - zm - I B X = 



(y)(N + P + n + Ro) - (x + y)R - zn - I B P = 



(z)(m -f- n + a) (x -f y)m yn I B a = 0. 



1 



FIG. 111. Mesh diagram for Thomson bridge. 

 Solving for x, which is the galvanometer current, gives: 

 NX-MP+ 



y _ r = T _ m + n + a __ , . 

 B R (M + N + X + P) + (M (1 



where [ ] = 



m + n + a 



(14) 

 If the galvanometer stands at zero, I G is zero, and 



im M\ 

 x = MP _ a(mN - nM) _ MP (n ~ W/ 



N AT ( _l_ _l \ ~ AT" an ~~ 1^1. (*-**) 



