252 Mr Wilson, On the Hall Effect in Gases 



gauge besides a bulb containing P 2 5 and a three-way tap which 

 enabled fresh air to be admitted when required. The air was 

 kept in a CaCl 2 tube for a long time before admitting it. The 

 whole apparatus was perfectly air-tight, no leak being detectable 

 by the gauge even in a whole week, although a change of pressure 

 of 001 mm. could be detected easily. 



The discharge between the electrodes A and B was produced 

 by means of a battery of small secondary cells, of which one 

 thousand were available when required. The circuit included an 

 Ayrton- Mather galvanometer and an adjustable liquid resistance, 

 besides a telephone to detect oscillations in the current. The 

 P. D. being used was measured by means of a Braun's electro- 

 static voltmeter reading from 100 to 1500 volts. 



The magnetic field for producing the Hall effect was produced 

 by means of a circular coil of 143 turns and 37"4 cms. outside 

 diameter, which was placed so that the axis of AB coincided with 

 a diameter of the coil, the plane of the coil being horizontal. 

 The axis of the tube K was then perpendicular to the plane of 

 the coil, and the centre of the coil midway between the small 

 electrodes E and E'. The current sent through the coil was 

 supplied by six large secondary cells, and measured by a Weston 

 ammeter. Its value could be changed by means of an adjustable 

 resistance. The field due to this coil was vertical and nearly 

 uniform near the small electrodes EE'. One ampere sent through 

 the coil gave a field at EE' of 5'20 c.G.s. 



When measuring the Hall effect the electrodes EE' were first 

 turned round until the electrometer indicated that they were at 

 the same potential. A magnetic field was then applied, when a 

 deflection of the electrometer was obtained which was reversed 

 on reversing the direction of the field ; its direction showed that 

 the negative ions have the greater velocity. 



The present paper only deals with results obtained when E 

 and E' were immersed in an apparently uniform positive column 

 produced by a sensibly steady current. 



If the positive column is striated the Hall effect depends on 

 the position of the electrodes with reference to the striae, and if 

 the current is not steady (as indicated by the telephone), the 

 results obtained are irregular in character. 



Before making measurements therefore at any pressure the 

 current and the number of cells used were adjusted until a 

 uniform column and steady current were obtained, or at least 

 until as near an approximation to these desired conditions as 

 possible was obtained. At pressures of several millimetres there 

 is usually a considerable range of current for which the discharge 

 is steady and uniform, but as the pressure is diminished this 

 range of current rapidly diminishes, until at about 0*2 mm. a 



