ON PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 



Ill 



maximum permissible value of i is not so great as with the differential 

 galvanometer and Kelvin bridge. 



At the National Physical Laboratory the Kelvin bridge and the 

 potentiometer were employed up to March of the present year. With 

 the former method a tapping current of 02 ampere was necessary in order 

 to measure a difference of 1 X 10^" ohm with certainty. With the latter 

 method the current used was 03 ampere, but the method was far less 

 convenient. At the present time a modification of the Wheatstone 

 bridge is used, and proves to be the most satisfactory and most sensitive 

 of all the methods discussed. The arrangement is very similar to that 

 suggested by R. H. Housman for the evaluation of small resistances 

 (p. 117). 



In fig. 5, P is the mercury standard, of which r and r' are the current 

 leads. R principally consists of a 1-ohm manganin coil which is shunted 

 with a resistance X, usually of the order 30 to 50 ohms, and a resistance 

 X' of several thousands of ohms. The latter is varied in the final 



adjustment of this arm of the bridge so as to obtain a very accurate 

 balance. Q and S are IjOOO-ohm coils of manganin. R' consists of two 

 unit coils in series ; the value of these in terms of other unit coils is 

 known with great accuracy (see build-up method, p. 104). B is a thick 

 copper conductor in series with S. The current through P is 0*03 ampfere. 

 The operations are as follows : The bridge-piece B is placed in position 

 so that R' is out of circuit, and the shunts X and X' are adjusted until 



R,/P=(S-t-B)/(Q-f-r), 



R, being the shunted value of R. The galvanometer lead at a is 

 removed and connected to b, and the battei'y lead at c is placed at a. In 

 practice this change is effected with a rocking commutator. The position 

 of B is altered so as to include R' as one of the arms of the bridge, 



