McKAY, — HALL EFFECT AND CURRENT DENSITY IN GOLD. 365 



in series with the ballistic galvanometer and test-coil, it was possible to 

 get nearly the same deflection by turning the earth-inductor through 

 180° as by jerking the test-coil from the field, and this was done. 

 The earth-inductor was mounted with its plane vertical and its axis 

 north and south. 



Determination of H. 



The horizontal intensity of the earth's magnetism at the position of 

 the earth-iuductor was determined by two methods. In the first method 

 tlie tangent galvanometer, already referred to, belonging to Professor 

 Hall, was used. This was of the Helmholtz form. There were 11 

 turns of wire wrapped on each circle. The terms G^, G-^, in Maxwell's* 

 formula were found to be negligible and the constant was given by the 

 expression 



2 n ^^w 



where A is the radius of the coils, B one-half the mean distance be- 

 tween the coils, and n the number of turns in the two circles. The 

 constant so obtained was 6.294. 



A current was passed through two copper voltameters and the tan- 

 gent galvanometer in series. The galvanometer deflection was 41°. 

 The value given by Ostwald and Luther, 0.000329 gm., was used as 

 the amount of copper deposited by the passage of one coulomb of 

 electricity. This method gave for 77" 0.1628 c. g. s. units. H was 

 measured also by Gauss's method,! which gave 0.1632. 



Preparation and Use of Gold Strips. 



In the following account of the writer's observations, most of the 

 preliminary work is given, as well as that to which more weight is 

 attached. The various improvements which increased the reliability of 

 the measurements are briefly mentioned. The dates on which the work 

 was performed (in the case of the first and fourth sets only approximate) 

 are given. 



At first an attempt was made to measure the Hall effect in thicker gold 

 than had been previously used for such measurements. A strip of gold 



* Maxwell's " Electricity and Magnetism," Vol. 2, § 700. 



t The writer is indebted to Professor B. 0. Pierce for the apparatus for the 

 measurement by Gauss's method and for the means of comparison of the resist- 

 ances used with standard resistances. 



