McKAY. — HALL EFFECT AND CURRENT DENSITY IN GOLD. 361 



1000 ohms 



WW 



iohm 



Potentiometer Wire 



<D 



A/WV 



AA/VWV 



Figure 3. 



S is the galvanometer whose sensitiveness is to be measured. 



C, a Carhart-Clark cell. A, storage cell. G, a sensitive galvanometer. 



cell, both recently put up, the Carhart-Clark cell, which had been used 

 by the writer, was found to have an electromotive force of 1.426 volts 

 at 21 °C, instead of 1.437 volts, which was the value derived from the 

 certificate. The value of the e. m.f. as corrected by the cadmium cell 

 was used in the calculations below. 



Measurement of Primary Current. 



The primary current, when above 0.01 ampere, was measured with 

 a commercial ammeter which had been corrected at various points as 

 described below. At first a Siemens and Halske ammeter was used ; 

 but in most of the measurements in which currents of the above order 

 were used, a Weston ammeter was used in series with the Siemens and 

 Halske ammeter. 



For currents from 0.025 ampere up to 0.72 ampere the ammeters 

 were compared with a tangent galvanometer which is described below 

 under the account of the measurement of H. In this comparison the 

 22 turns of wire for which the constant of the galvanometer had been 



