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XIII
EXPERIMENTS ON THE CATHODE DISCHARGE
ing on the phosphorescent patch at a distance of 12 cm., served
as an accurate indicator of any deflection. A magnetic force
only half as strong as the horizontal intensity of the earth's
magnetism, acting perpendicular to the direction of the ray,
was sufficient to change quite notably the position of this
shadow. The tube was now placed between two strongly
and oppositely electrified plates: no effect could be observed
in the phosphorescent image. But here there was a doubt
whether the large electrostatic force to which the tube was
subjected might not be compensated by an electrical distribution
produced inside it. In order to remove this doubt, two
metallic strips were placed inside the tube at a distance of 2
cm. from one another, and were connected to external con-
ductors by which they could be maintained at different
potentials. After passing the wire which produced the
shadow, the rays had to travel a distance of 12 cm. between
these strips. The latter were first connected with the poles
of a battery of twenty small Daniell cells. Opening and
closing this connection produced not the slightest effect upon
the phosphorescent image. Hence no effect is produced upon the
ray by an electromotive force of one Daniell per millimetre acting
upon it perpendicular to its length. 240 Planté cells of the
large battery were next charged and connected with the two
metallic strips. By themselves these 240 cells were not able to
discharge across the strips; but as soon as the induction coil
was set to work and the cathode rays filled the space between
the strips, the battery also began to discharge between them;
and, as there was no liquid resistance in the circuit, this at
once changed into an arc discharge. The same phenomenon
could be produced with a much smaller number of cells-down
to twenty or thirty. This is in accordance with Hittorf's
discovery that very small electromotive forces can break through
a space already filled with cathode rays. The 240 cells were
next connected up through a large liquid resistance: during
each separate discharge of the induction coil there was now
only a weak battery-discharge lasting for an equally short
time. The phosphorescent image of the Ruhmkorff discharge
appeared somewhat distorted through deflection in the neigh-
bourhood of the negative strip; but the part of the shadow in
the middle between the two strips was not visibly displaced.