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XIII
EXPERIMENTS ON THE CATHODE DISCHARGE
through which the cathode rays can pass; secondly, of wire-
gauze, about 1 sq. mm. in mesh, through which the cathode
rays have to pass; thirdly, of a protecting metallic case, which
completely surrounds the greater part of the tube and screens
that part of the gas-space which lies beyond the wire-gauze
from any electrostatic forces which might be produced by
induction from without, e.g. from the cathode. If the results
which we have already obtained have any meaning, the cathode
rays are to be regarded as pure after they have passed through
the opening in the metal cylinder and the wire-gauze beyond
it. They are none the less vivid; at low densities they cause
the glass at B to shine with a brilliant green phosphorescence,
upon which the shadow of the wire-gauze is plainly marked.
The part of the glass tube which lay within the protecting
case was now enclosed in a metallic mantle y, which was
connected with one pair of quadrants of a delicate electrometer;
the protecting case and the other quadrants were connected to
earth. When even a small quantity of electricity was brought
inside this mantle, it attracted by induction electricity of the
opposite sign from the electrometer, so that a deflection was
produced. The electricity could, e.g., be introduced by re-
placing the tube AB inside the protected space and the mantle
y by a metal rod which had about the same size and position
as the cathode rays. This was placed in metallic connection
with the cathode, while the current from the induction coil
passed, as it did in the actual experiments, through the tube.
The deflection then produced in the electrometer was too great
to be measured, but could be estimated at two to three thou-
sand scale-divisions. When the current was stopped the
electrometer needle went back to about its old position; and
this could be repeated at will. Now if the cathode rays
consisted of a stream of particles charged to the potential of
the cathode, they would produce effects quantitatively similar
to the above, or qualitatively similar if they produced any
electrostatic forces whatever in their neighbourhood. On
trying the experiment the following results were obtained.
When the quadrants of the electrometer were connected
together and the induction coil started, the needle naturally
remained at rest. When the connection between the quadrants
was broken, the needle, in consequence of irregularities in the