
by Cathode and Canal Rays to Exciting Current. 191 
which had been washed out with pure oxygen and hydrogen. 
In these experiments it was above all things necessary to 
avoid greased and cemented joints. or this reason, all the 
joints with the air-pump &c. were blown. A difficulty which 
arose in connexion with several tubes and which could no 
longer be overcome was the following one. When the tubes 
are used a good deal, a metallic deposit gradually appears on 
the glass wall of the tube between the two electrodes. This 
deposit then, on reaching a certain thickness, forms a con- 
ducting coating, which at first is of importance only with 
high, but subsequently also with quite low potentials. This 
action even went so far that the tube no longer showed any 
luminous discharge, although the galvanometer, which was 
inserted beyond the tube, showed that the full current of the 
influence-machine passed through it. And yet the deposit | 
was only faintly visible. I succeeded in getting rid of this 
deposit in the entirely new tube (fig. 2). As electrodes only 
nets of the second kind were used. 
It was very inconvenient to measure the current, as was 
done formerly, before it passed through the tube, as this 
necessitated a considerable length of high-potential conductor 
going to the distant galvanometer. The leakage was in this 
case so great that after a time the influence-machine was no 
longer capable of sending a discharge through the tube. 
I therefore altered the arrangement, measuring the current 
after it had passed through the tube, and only using a short 
well insulated voltmeter-lead on the other side of the tube. 
According to the foregoing experiments, the current leaving 
the tube is diminished by the observed amount corresponding 
totherays. The total discharge current is therefore obtained 
by adding the ray current to the measured current leaving 
the tube. If, therefore, E denotes the ray current and J the 
current leaving the tube, the ratio of the rays to the current 
is given by 
E 
J+E 
The moving coil galvanometer by Hartmann and Braun 
was used for measuring the current, that by Siemens and 
Halske for measuring the rays. 
A favourable condition for this method of observation was 
the possibility of carrying out in direct succession measure- 
ments at various pressures. Since, namely, for the purpose 
of evolving the gas &c. a fairly large system of tubes had been 
fused on to the air-pump, the effect of a single lift became 
only slowly perceptible. Thus the values of the current and 
