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239
EXPERIMENTS ON THE CATHODE DISCHARGE
of the current in a space in which various paths are open to
it? I have tried to answer both questions for a space con-
taining gas and traversed by a current by determining
experimentally the current-lines from the deflections produced
by the discharge in a small magnet in its neighbourhood.
Before attacking this problem it was necessary to solve a
preliminary one. Whether the cathode rays form the path of
the current or not, there is no doubt that they are affected
by a magnet. Conversely, it was not improbable that the
cathode rays would in any case produce a deflection of the
magnet; and this effect might be other than an electro-
magnetic effect. If such an effect existed, the proposed ex-
periment would be useless. The following experiments show
that no such effect occurs.
In a tube 300 mm. long and 28 mm. wide was intro-
duced a cathode consisting of a circular turned brass disc
which just filled the cross-section of the tube. Through a
hole bored in the centre of the disc was passed a thermometer
tube; inside this, and quite centrically with reference to the
disc, was a wire of non-magnetic metal. The ends of the
wire, projecting but little beyond the disc in the gas-space,
formed the anode. The wires used to carry the current in and
out were twisted around one another. Now the current-lines
must at all events be symmetrical with reference to the axis
of the tube; if we suppose the currents replaced by magnetic
surfaces, these would be closed ring-magnets which would have
no external action. But the cathode rays were fully developed
and, according to the density, filled either the whole tube or
a part of it with blue light. If they have any action peculiar
to themselves upon a magnet outside the tube, it would here
exhibit itself apart from any electromagnetic effect. In order
to avoid any electrostatic effects the tube was surrounded with
tinfoil which was connected to earth; without this precaution
the experiments could not have been carried out.
The magnet
upon which the cathode rays were to act was the one which
was used in the subsequent experiments; it was a strongly
magnetised piece of watch-spring, 12 mm. long, and was
attached to a small mirror of very thin glass. This was
hung by a single spider-thread in a very narrow space between
two plates of plate-glass. Thus the arrangement was the