McKAY. — HALL EFFECT AND CURRENT DENSITY IN GOLD. 369 



layer of shellac dissolved in alcohol. The alcohol was allowed to evap- 

 orate. Then the plate was heated and pressed on the thin gold leaf. 

 A strip about 7 cm. long and 1.5 cm. wide was cut from this, with two 

 short arms for the secondary electrodes as described at the beginning of 

 this paper. Contact with the electrodes was made by pressing polished 

 brass pieces on tin-foil in contact with the gold. The brass pieces were 

 kept in place by a cement of resin and beeswax, which also prevented the 

 water from reaching the electrodes. The gold was then covered with a 

 thin layer of paraffin. 



The first two sets of observations gave the following results : — 



Mat 9. Plate 4. 



Mat 11. Plate. 4. 



Hitherto in almost every case the magnetic field had been measured 

 before and after each measurement of the transverse effect for a given 

 strength of primary current. This required the removal of the plate 

 from the field at every change from weak to strong or strong to weak in 

 the primary current. Hereafter direct determinations of the strength 

 of the magnetic field by means of the test coil were made only at the 

 beginning or end, generally at both the beginning and end, of each 

 period of work, which included one or more changes from strong to 

 weak or weak to strong of primary current. 

 vol. xxxix. — 24 



