260 
Mr Wilson , On the Hall Effect in Gases 
in the positive direction along x tends to move, then Z is positive 
when the velocity of the negative ions is greater than that of the 
positive ions. That is, Z helps on the positive ions in the positive 
direction of z but retards the negative ions. 
When a steady state is reached evidently 
nX ( Z + hXH) -kJ?£ = 0, 
where K x is a coefficient of diffusion of the positive ions. Simi- 
larly 
njc 2 (-Z + h XH) = 0. 
But n 1 = n i = n (say) so that 
Ko du 
k ■j z = n{ - z+h ‘ XH) - 
But ~ ~ according to the kinetic theory of gases, so that 
A?2 
Z+k x XH=-Z+k 2 XH\ 
Z^HXih-k). 
In this expression for Z the change in the transverse distribution 
of the discharge produced by the magnetic field is taken into 
account so that no error is introduced into this investigation by 
this effect. 
In the above calculation the influence of the walls of the 
discharge tube is left out of account and it is possible that this 
influence may not be unimportant in some cases, especially at low 
pressures. It is well known that when a gaseous ion strikes a 
solid body such as glass it remains stuck to it (or at least its 
charge remains stuck), thus for example it is easy to remove all 
the ions from a gas by passing it through a glass wool plug. 
Consequently when a discharge is passing through a glass tube 
the walls of the tube ought to be regarded as perfect absorbers 
of the ions so that close to the glass n x = 0 and n 2 = 0. 
The glass must therefore get charged up sufficiently to make 
the number of negative ions striking it in any time equal to the 
number of positive ions. Since the negative ions diffuse quicker 
than the positive ions the glass will get a negative charge. 
In the same way the electrodes E and E' ought to be regarded 
