ON PRACTICAL STANDARDS FOR KLECTRICAT. MEASUREMENTS. 117 



readjusted in order to eliminate thermal e.m.f.'s. The resistance of the 

 galvanometer branch is not constant unless the potential leads of P are 

 equal in resistance to those of b. In order to neglect the resistance of 

 these leads, and to make bc/G comparatively small, G must be great. This 

 diminishes the sensitiveness. If Q is made greater than 1, the maximum 

 permissible current in the main circuit is reduced, and the sensitiveness is 

 again diminished. Suppose that P=0'0], Q=l, b=l, c = 97, and 

 G = 100 + .'i'wherea;issmall. Then P=l/ (99 + 097— 0-01,t). Hence, if 

 the value of P is desii'ed to be correct to 0-001 per cent., the value of the 

 galvanometer resistance must be known to 1 part in 1000. Although not 

 so sensitive as other methods described hereafter, the pr-ocess is interest- 

 ing. The combination of resistances Q, b, and c was used by Lord 

 Rayleigh in the determination of the ohm by Lorenz' method. 



(c) Housman's Method ' (fig. 9). — The first stage in the process is to 

 measure the ratio of P to Q by shunting E, or (S + Q'). The second is to 

 shift one galvanometer lead and one battery lead and measure the ratio 

 of (P + Q) to Q'. Q' is a 1-ohm coil. For precision work the leads 

 connecting Q' to S and P to R must be known. The greatest dis- 

 advantage of this method is that the current through P in the second 

 measurement must be compai-atively small. Thus,'if P=0-0001, Q=0-01, 



Fig. 9. 



and Q'=l, the maximum permissible current through P (if P is the 

 usual type and size of standard resistance) is 100 amperes ; through P and 

 Q in series, 10 amperes; and through P-fQ-f-Q' in series, 1 ampere. 

 The necessary ratio of the arms S and R is also unsuited for accurate 

 work. 



(d) Two-stej) Method- (A. Campbell) (fig. 10). — A suitable small 

 resistance, whose value need not be accurately known, is inserted at U, 

 and is adjusted by shunting until the galvanometer balances in position a. 

 The galvanometer is then brought into position b and balance obtained by 

 another shunt at R or S. By repeating this process a few times the 

 balance is good in both positions. The method is about 50 per cent. 

 more sensitive than the Kelvin double bridge if equally favourable arrange- 

 ments are made, but it is much less convenient in practice. The leads 



' Electrician, 1897, xl., p. 300. 



» Phil. Mag., July 1903. 



