TRAVERSED BY CATHODE RAYS. 
51 
anode and applying a coating of wax, an airtight connection could be readily made 
without interfering with that which secured the aluminium foil to the disc. This 
latter connection was effected by placing a thin coating of wax upon the brass disc 
and gently applying heat after the foil was laid upon it. All the space within the 
projecting shoulder was then covered with a thick coating of the wax, excepting the 
central portion of the aluminium. 
In all the experiments with these tubes the anode was well earthed, as was also the 
positive terminal of the induction coil used to produce the discharge. 
As regards the distance between the cathode and the anode, it was found best not 
to make it too small. Otherwise, the discharge would pass in the tube before the 
available maximum potential difference was reached. The velocity of the carriers has 
been shown by J. J. Thomson'* to vary with the potential difference between the 
electrodes, and as a consequence an intense radiation was more readily obtained when 
the distance between the anode and cathode was considerable. 
In the case of tubes constructed with a short distance between the electrodes, the 
device adopted by McClelland! of inserting an air gap in series with the tube very 
largely increased the intensity of the radiation. 
The foil used by Lenard for the aluminium window was '003 millim. in thickness. 
In practice it was exceedingly difficult to obtain such foil free from holes. Aluminium 
about three times as thick was, however, much better in this regard. The induction 
coil used in the experiments was, besides, very powerful, and, as a radiation 
sufficiently intense could be obtained with it, this thickness was used throughout the 
investigation. 
2. Ionization by Cathode Rays. 
It has been shown by Lenard]; that air, when traversed by cathode rays, acquires 
the property of discharging electrified conductors against which it may be blown, and 
that, further, it retains this property for some time after the rays producing it have 
been cut off. 
According to the theory of Professor Thomson, the air, when in this state, is 
ionized, and the discharging action is brought about by a motion of the ions in the 
gas to the charged conductor. Owing to the separation of the positive and negative 
ions, recombination can take place but gradually, and this readily explains why the 
discharging power is retained by the air for some time. In order to show that these 
positive and negative ions are produced in a gas traversed by the rays, the apparatus 
shown in fig. 1 was used. 
The cathode rays issuing from the aluminium window a passed through a narrow 
tube, b , into an earth-connected metal chamber, A. B was a disc of brass supported 
* ‘ Phil. Mag.,’ October, 1897, p. 315. 
f ‘ Proc. Roy. Soc.,’ vol. 61, No. 373, p. 227. 
| ‘Wied. Ann.,’ vol. 63, p. 253 (1897). 
H 2 
