XIII
231
EXPERIMENTS ON THE CATHODE DISCHARGE
Hence if an alternating part had been present, an oscillating
current would have flowed through the dynamometer, and would
have been strong enough to produce a perceptible deflection.
The preceding experiments prove that the current flows
continuously through the greater part of the metallic circuit
even when an air-gap is introduced. They only enable us to
form a conclusion as to the current in the tube itself if we
assume that the current is uniform in all its parts. But if
the number of the partial discharges amounts to 100,000 or
more per second, the assumption is unsafe; indeed there is no
doubt that the current-variations can only penetrate a small
distance into the coil, on account of its large self-induction,
and that inside it they must be effaced. Thus only a fraction,
and probably a very small fraction, of the effect under con-
sideration would actually occur. On this account coils are
avoided in the following experiments.
FIG. 28.
4. The current was sent through
a Wheatstone bridge (Fig. 28) of
which the four arms consisted of equal
liquid resistances of 700,000 S.U.each.
These were made by filling thin glass
tubes 30 cm.long with a dilute solution
of zinc sulphate. The mean potential difference at the points
a and b-more accurately the mean square of this difference-
could be observed by means of a gold-leaf electroscope: this was
enclosed in a metal case connected to the point a, while the
leaves themselves were connected to the point b. In this and
the following experiments the gold leaves were observed under
a microscope. The difference of potential which could just be
observed was about one-tenth of that which existed between a
and c when the current was flowing. By means of short metallic
conductors the points a and c could be connected with the two
coatings of a condenser of very large capacity. The resistances
were so adjusted that the gold leaves showed no divergence
when the current was allowed to pass in the absence of the
condenser. On introducing the condenser it was again found
that not the slightest divergence could be perceived.
result again tells against discontinuity. For let us suppose
that a very rapid intermittent current flows through the
apparatus, and let us, as before, conceive of this as being
This