110 



REPORTS ON THE STATE OF SCIENCE. 



however, that the current in the potentiometer is continuous and the 

 heating effects more marked than in the bridge in which a tapping current 

 only is employed. A great practical advantage of the bridge method is 

 the rapidity of measurement. 



Differential Galvanometer Method (fig. 

 resistances of the galvanometer coils, the 



4).— If G and g 

 difference of the 



are the 

 currents 



Fig. 4. 



Gfg 



a 



through them is i(P5'— QG)/G(Q4 g). If P=Q and G!=g, the difference 

 of the currents is i(P— Q)/(Q + G)=tc)P/(Q + G), and the best galvano- 

 meter resistance is G=P=Q. The sensitiveness is then proportional to 

 iA \/ P/2 n/ 2. If the currents through the galvanometer are comparatively 

 large, convection currents are produced in the space containing the sus- 

 pended magnets ; also, the resistance of the coils is subject to small but 

 rapid changes. There is, therefore, a maximum permissible value for the 

 currents through the galvanometer coils, and in general some ballast 

 resistance must be added to the galvanometer arms. This reduces the 

 sensitiveness. 



Mercury Standards of Resistance. — The Kohlrausch differential 

 galvanometer (see p. 120), the Kelvin bridge, and the potentiometer 

 have been employed ' for the measurement of resistance of mercury 

 standards witli current and potential leads of comparatively high resist- 

 ance. These methods are recommended in the Report of the Conference 

 on Electric Units at Charlottenburg (1905). The current used in the 

 measurement of such resistances is limited by the condition that the 

 mercury shall not be sufEciently warmed to produce appreciable error. 



In the Standards Department of the National Physical Laboratory 

 no favourable opportunity has arisen for an exhaustive test of the 

 Kohlrausch method. As used at the Physikalisch-Technische Reichsan- 

 stalt it is very satisfactoi-y ; but, strictly speaking, it is not a null 

 method, as observations of deflections have to be made. From particulars 

 published,- a favourable arrangement for the measurement of mercury 

 standards is when G=5'=6 ohms, P=;Q = 1 ohm, and the ballast resist- 

 ance in each galvanometer arm is 10 ohms. In this case the sensitive- 

 ness is proportional to iAs/12/34=0"098 iA. 



With the Kelvin double bridge, if R=S = 1000, P=Q=1, a=^=100, 

 and G=1000, the sensitiveness is proportional to 0011 iA. If R=S=100, 

 P=Q = 1, a=^^100, the sensitiveness is more than doubled, being 

 equal to 025 iA. This latter case is convenient in practice. 



In the potentiometer, if P=Q, and G=P + Q, the sensitiveness is 

 proportional to 0'35i-!s. The current is continuous, and hence the 



' Wissoischaft. Ahhand. d. Phys.-Tech. Eeichsanstalt, 414, Band II. ; see also 

 Phil. Trans., 1904, A 373, 57. 



» Wissenschaft. Abhand. d. Phys.-Tech. Reichsanstalt, Band III. 



