ox TKACTICAL STANDARDS FOR ELECTRICAL MEASUKEJIENTS. 113 



Comparison of Unit Coils. — Manganin coils with potential points are 

 alone considered. Platinum-silver coils without such points are com- 

 pared with manganin ones by substitution in one of the arms of the 

 bridge. 



The method adopted is analogous to that of Carey Foster. The coils 

 are exchanged in position, but the difference of values is given by the 

 shunts applied to the two ratio coils. Thermal e.m.f.'s are small, and 

 produce no disturbing effect as the galvanometer circuit is continually 

 closed. The self-induction of the coils is very small indeed. 



For coils having potential leads the Kelvin double bridge is used. 

 P=Q are the coils to be compared, P^, P,,, Q,., and Q,, being the resis- 

 tances of the current leads of these coils. E, and S are 1-ohm standards. 



Fig. 6. 



n—ft—l ohm and Q,j-f Pj,=o?. The gahanometer is permanently con- 

 nected as shown in fig. 6, but the battery leads are successively joined to 

 tlie junctions of P-P,, and QQ^, PP,, and SQ„ R-P,, and Q-Q,. The 

 cojls K. and S are shunted to effect a balance. Representing the shunted 

 values of R and S by Ri R^ R^, S, S.^ 83, &c., we have 



(1) P= Q(^i+Ph) , dfi / R,+P, _ u\ 



s,+Q. "+/3+.^Vs,+q; ft)- 



(2) P= (Q + Q'-X^-s + Pb) , d^i _ /R2 + Pb_ «> 



(.3) p= 3^y^ - P + ^^^ f^^ _ «^ 



In practice, the value of d,S / (a + /3 + d) does not exceed 0-00006 ohm, and 

 the expression accompanying this is normally of the order 0-00005 ohm, so 

 that the last term in the above equations is negligible. From (1) and '(''') 

 Q,=(R,/S,-R2/S2)/2, and from (1) and (3) P,=(R3/S3-R,/S,)/2. 



P and Q are now exchanged in position, when Q=P(Rj + Q^)/(S^ + P„), 

 the values of Q^ and P„ being determined as before. If the coils are not 

 very different from their nominal values we may now write 



lyub. 2 



