52 
Mi?. J. c. Mclennan on electrical conductivity in gases 
by an ebonite plug, and surrounded by a guard ring. A wire led from this electrode 
to one pair of quadrants of an electrometer, and the other pair was put to earth. 
Care was taken to screen off electrostatic induction by surrounding the wire and 
electrometer with earth-connected conductors. The second electrode, C, also 
supported by an ebonite plug, was connected by a commutator, D, to one of the 
terminals of a battery of small storage cells, the other terminal being connected to 
earth. 
The tube, b, was made narrow, and penetrated a short distance into the chamber in 
order to confine the rays to a slender pencil, and to prevent their impinging upon the 
electrodes. By means of the key, K, the electrode, B, could be put to earth when 
necessary. 
With such an apparatus, and no field initially between the electrodes, it was found 
on exciting the discharge tube and breaking the earth connection, K, that the 
electrometer gained a small negative charge, which did not go on increasing, but soon 
attained a limiting value. 
On the assumption that the cathode rays produce positive and negative ions 
throughout the gas, the explanation of this is obvious. The cathode rays carried a 
negative charge into the gas, and set up a field which caused the negative ions to 
move to the walls of the chamber and to the electrode, B. The charge which the 
latter soon gained, however, set up a field of its own, and a state of equilibrium was 
reached when the conduction to the electrode was just equal to that proceeding from 
it. If, instead of there being no field initially between the electrodes, C was joined 
to the positive terminal of the battery, then the electrode, B, gained a jiositive charge 
Avhen the tube was excited, and the rate at which its potential rose depended upon 
the capacity joined to B and the electrometer. 
With C joined to the negative terminal of the battery, a similar charging took 
place, except that in this case the charge accumulated was a negative one. 
This reversal in the sign of the charge collected may be shown with a field of a 
few volts a centimetre, and clearly points to the existence of positive and negative 
ions in the gas. Since the cathode rays themselves carry a negative charge, the 
presence of these carriers alone in the chamber would account for the negative charge 
obtained with a negative field. With a positive field, however, these carriers would 
be attracted to the electrode C, and it seems impossible to explain how the electrode B, 
under these circumstances, could receive a positive charge unless ions were produced 
by the rays. 
3. Discharging Action of Cathode Rags. 
In connection with the experiments of Lenard,* already referred to, cathode rays 
were allowed to fall upon a charged conductor surrounded with air at atmospheric 
pressure. This conductor consisted of a wire attached to a gold-leaf electroscope, 
* ‘ Wied. Aim.,’ vol. 63, p. 253. 
