McKAY. — HALL EFFECT AND CURRENT DENSITY IN GOLD. 359 



the coil and cone made a cylinder 1.5 cm. in height and nearly 1.9 cm. 

 in radius. Four of these coils were used in two branches of two each. 

 The resistance of this arrangement was 2.22 ohms at 16.3° C. By 

 means of a control magnet placed about half-way between the upper 

 and lower systems and 20 to 30 cm. away, the period was increased 

 to give the required sensitiveness. A second control magnet placed 

 nearly two meters away sufficed to direct the system without producing 

 anv considerable change of sensitiveness. 



With the needle 1.7 m. from the scale, and a period of complete 

 vibration of the needle of 12 seconds, the deflection was 1 mm. for 

 7.7 X 10-^° amperes. 



Considerable difficulty was experienced on account of induced currents 

 from outside electric disturbances, such as those caused by the passage 

 of electric cars in the neisfhborhood. In reading the deflection of the 

 galvanometer needle, an effort was made to get the mean position of 

 the needle at times when it did not seem to be influenced by these in- 

 duced currents. To make less the error caused by the wandering of the 

 needle the difference of position on reversal was made large, in nearly 

 every case greater than 18 cm. on the scale. The vibration of the needle 

 when the readingr was taken was often as much as 2 cm. or 3 cm. on the 

 scale. To lessen tlie error, five or seven or sometimes more reversals 

 were made in a single set of observations with given conditions. At first 

 it was attempted, before and after each measurement of the transverse 

 effect, to measure also the direct effect of the electromagnet, about 45 m. 

 away, and of the lead-wires of the magnetizing current, which came 

 within 3 meters of the galvanometer. This effect was small in compari- 

 son with the transverse current deflection and could not be measured 

 with certainty. 



Resistance of Transverse Circuit. 



The resistance of the galvanometer was measured on the Carey Foster 

 bridge in comparison with coils of raanganin wire of temperature- 

 coefficient 0.00002 per degree centigrade. These coils were made by 

 Mr. Persons and by Mr. Coulson of the Jefferson Physical Laboratory. 

 Some of them were made by direct comparison with standard coils made 

 by Wolff of Berlin and tested at the Physikalisch-Technische Reichsan- 

 stalt. The other coils were made by indirect comparison with these 

 standard coils. All such coils of mancfanin wire made in the above 

 way by Messrs. Persons and Coulson are referred to in this paper as 

 standard coils. 



