76 



Mr. W. C. Baker on the Hall Effect in 



was placed between the poles of a small ring electromagnet. 

 The cross-section of the laminated core of this magnet was 

 3 x 15 sq.cms. The space between the pole-faces was 3 cms. 

 The winding was in two sections, one of 368 turns and the 

 other of 519 turns. These could be used separately or in 

 series. The exciting currents were drawn from a battery 

 of storage-cells. 



The magnet and the ebonite plate, with its attachments, 

 were immersed in a bath of kerosene, so arranged that a 

 constant circulation was maintained past the gold, by means 

 of a small pump driven from a water-motor. This was found 

 to keep the temperature practically constant, and to prevent 

 thermoelectric changes at the secondary terminals. 



The differential galvanometer I built specially for this 

 investigation. It was of the Thomson reflecting type — four 

 double coils of 50 turns each, No. 20 d.c.c. wire well insu- 

 lated in paraffin — and a light astatic suspended system of 

 parts of the finest cambric needles * on a fine glass staff, sus- 

 pended on a single silk fibre 30 cms. long — magnetic control. 



* These were magnetized in place by making use of the galvanometer 

 itself. Soft iron cores (cc) were made so as to fit into the galvanometer 

 coils (a a a a) and the needle-system was set between, as shown in the 



sketch. All the coils were put in series so as to drive the magnetic flux 

 across the two gaps in opposite directions. A current of eight amperes 

 was put on for a second at a time, twice or three times, and tbis was 

 found to saturate the needles. See also Wadsworth, Phil. Mag. xxxviii. 

 p. 482 (1894). 



