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XIII
EXPERIMENTS ON THE CATHODE DISCHARGE
deduced partly from the behaviour of single cells and partly
from experiments in which the current from the whole
battery was sent through very high resistances, was about
600 Siemens units. This potential difference kept up, when
the battery was on open circuit or only very slightly used, for
twelve to fourteen hours; after which there was loss of charge
(mainly through chemical action, but partly also through
short-circuiting), and the potential difference sank rapidly to
lower values. If the circuit was only closed from time to
time through rather large resistances, as was usually the case
in these experiments, the battery remained in good working
order for about six hours. It could supply for two or three
hours the current required for continuously lighting a Geissler
tube: but if it was closed through a small resistance or short-
circuited, it became exhausted in a few minutes or even in a
fraction of a minute. It then exhibited the well-known
partial recovery of charge. On closing the circuit in free air
the battery gave a spark nearly half a millimetre long. It lit
up Geissler tubes of the usual form (without capillary)
through an interval of pressure from 1 mm. to a few
hundredths of a millimetre; at the former limit the blue
glow-light surrounded the cathode as a thin layer; at the
latter the cathode rays attained a length of 120-150 mm.
In general the connecting wires should not be attached to the
electrodes of a Geissler tube without introducing a resistance
of several thousand Siemens units; otherwise the cathode
discharge passes into an arc discharge, and generally the tube
breaks and the battery becomes exhausted in a few moments.
This battery came to grief in the following way. The
sulphuric acid crept up in the capillary space between the
lead strips and the layer of varnish, and went on spreading
farther and farther in this space. If the apparently uninjured
varnish was scraped away at any point, it was easy to detect
the presence of the acid by its taste. Thus the acid worked
its way to the copper wires of the end strips and produced
upon these growths of copper sulphate which spread along the
wires. After the battery had been in use for three or four
weeks, these growths on the wires of the front row of end
strips reached to the mercury in the commutator-cups. The
mercury then amalgamated the wire along its whole length,