394 Mr Wilson, The Hall Effect in Gases 



II. The 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 pre 



Diagram of Apparatus. 



It consisted of a glass tube AB, 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 betAveen two aluminium disks G 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. 

 Flexible 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 15 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 aWeston 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. 



