394 
Mr Wilson , The Hall Effect in Gases 
II. Tice Variation of the Hall Effect along the Discharge 
in Air. 
The apparatus used for measuring the Hall Effect in different 
parts of the discharge is shown in the accompanying figure. 
* — To PUMP PTC G 
A B 
Diagram of Apparatus. 
It consisted of a glass tube A B, 58 cms. long and 4'5 cms. in 
diameter, provided with a glass stopper at each end. At the 
centre of the tube a side tube G was sealed on and also provided 
with a glass stopper which carried two electrodes E and F for 
measuring the Hall Effect. 
The discharge was passed between two aluminium disks C and 
D, each 3 cms. in diameter, which were supported by a framework 
of thin glass rods which kept them at a constant distance apart. 
I lexible spirals of copper wire connected these electrodes with 
platinum wires sealed through the ends of the stoppers at A and 
B. A coil of thin sheet iron H was fixed to the frame carrying 
the electrodes and enabled it to be moved along the tube by 
means of a small electro-magnet. In this way the electrodes E 
and F could be brought into any portion of the discharge between 
G and D. The electrodes E and F were 1*40 cms. apart, and were 
connected through a commutator to an insulated quadrant elec- 
trometer. The discharge between C and D was produced by 
means of a battery of small secondary cells and an adjustable 
resistance and telephone were included in the circuit. The 
resistance and number of cells used were adjusted until a sensibly 
steady discharge was obtained with the kathode nearly but not 
quite covered by the negative glow. Under these conditions the 
Hall Effect and distribution of potential in the tube are nearly 
independent of the current. 
A pair of circular coils each of about 1 5 cms. radius were 
arranged symmetrically one above and one below the tube and 
separated by a distance equal to the radius of either. A current, 
measured by means of a Weston Ammeter, could be passed through 
these coils, and this produced a very uniform magnetic field 
near E and F in a direction parallel to the axis of the stopper G. 
J 
.^SMEUUsL 
