54 
me. j. c. Mclennan on electrical conductivity in gases 
The arrangement was that shown in fig. 2. A copper wire, terminated by a disc, A, 
of the same metal, was insulated by ebonite from an earth-connected copper tube B, 
through which it passed to the electrometer. To this tube there was fastened, as 
shown in the figure, a large, finely-meshed copper gauze which completely protected 
the disc from electrostatic induction. The tube B also carried a short concentric 
cylinder a, made of copper, which could be slid out when desired so as to surround 
the projecting end of the wire and the disc. 
On placing this apparatus in front of the aluminium window so that the cathode 
rays fell on the disc, it was found that, although the rays caused a discharging of 
positive and of negative electricity, still in no case observed was a negative charge 
on the disc and wire ever completely dissipated. 
Negative charges fell, however, to limiting values, represented in some cases by 
potentials of the order of ’25 volt, and then remained stationary. In the case of 
initial positive charges the discharging was not only complete but the disc also gained 
this limiting negative charge. A similar charging action was observed Avhen there 
was no initial charge on the disc. 
Here the disc was subjected to two influences, namely, the cathode rays carrying 
a negative charge to it and the ionized gas about it acting as a conductor and tending 
to discharge it. This limiting charge can, then, just as in the case already cited, be 
looked upon as representing a state of equilibrium in which the convection to the 
disc was just equal to the conduction away from it. 
As the electric field produced by a given charge on the disc would vary with 
the distance between it and neighbouring conductors at a different potential, the 
conduction from the wire could consequently be increased or decreased according 
as an earth-connected conductor was brought close to the disc or removed farther 
from it. If then a means were devised of altering in this way the conduction without 
altering the intensity of the rays impinging on the disc, the value of this limiting 
charge could be subjected to definite variations. 
The sliding cylinder a afforded a simple means of accomplishing this result. If 
when the tube was excited a stationary state was reached, with this cylinder shoved 
well back, and it was then brought forward over the wire and disc, the limiting 
negative charge at once dropped and assumed a steady but smaller value. In order 
to restore the charge to its original value it sufficed merely to slide the cylinder back 
to its former position. 
Another simple verification of this view was afforded by the use of a blast of air. 
If when the rays were impinging on the disc a blast of air was directed towards it 
and at right angles to the rays, the limiting charge at once increased to another 
limiting value, and when the blast stopped it again dropped to its original amount. 
As the velocity of the cathode rays has been estimated by J. J. Thomson* to be of 
the order of 10 LU centims. per second, it is clear that any ordinary blast could produce 
* 1 Phil. Mag.,’ October, 1897, p. 315. 
