53 
TRAVERSED BY CATHODE RAYS. 
and was placed within a zinc box in which was a small opening covered with a film of 
aluminium, thin enough to allow the rays to pass through. The end of this wire was 
placed in front of the window and close to it, with the electroscope clear of the direct 
path of the rays. The box itself was connected to earth and set in position, with its 
window opposite that of the discharge tube. 
Using this apparatus, Lenard found that positive and negative charges alike were 
completely dissipated by a single discharge through the tube when the aluminium 
windows were at any distance up to 4 centims. apart. At greater distances than 
this a similar hut only partial discharging of both kinds of electricity occurred when 
the same amount of rays was used. 
This loss of charge was no doubt brought about by means of the ionization in the 
air surrounding the conductor. The known behaviour of an ionized gas, however, 
would have led one to expect a somewhat different result, especially in regard to the 
effect obtained w r ith short distances between the windows. When an insulated metal 
conductor is placed in air ionized by Rontgen rays, Zeleny* has shown that, owing 
to the greater velocity with which the negative ions diffuse, this conductor takes up a 
small negative charge, while the gas itself is left with a positive one. If then the 
ionizations in the two cases are of the same nature, one would have expected that in 
Lenard’s experiments the wire and electroscope would not, under any circumstances, 
have been finally discharged completely, but would have been left with at least a small 
negative charge. When, further, it is remembered that the impinging cathode rays 
themselves carried a negative charge to the wire, this fact affords an additional reason 
for expecting such a result. 
H. 
Now the gold-leaf electroscope, as used by Lenard (Exner’s type), was not 
sensitive to small differences of potential, and it was consequently not a suitable 
instrument for the detection and measurement of effects of this kind. As the 
explanation of his results seemed, then, to be connected with this lack of sensibility 
in the measuring instrument, his experiments were repeated, and a quadrant elec¬ 
trometer was used in place of the electroscope. 
* ‘Phil. Mag.,’ July, 1898, p. 134. 
