XIII
241
EXPERIMENTS ON THE CATHODE DISCHARGE
sure of the air, and could be heated while this pressure was on;
but they bent under it so strongly that the curvature could
easily be observed on looking at them sideways. Through the
brass frame were inserted a tube with stopcock for pumping
out, and also several aluminium electrodes; the latter were
cemented in glass tubes so as to be insulated from the frame.
It was only after several fruitless attempts that the case was
made air-tight. One difficulty arose from the bending of the
glass; this made any accurate grinding impossible, and every
solid cement cracked on pumping out. Another difficulty
arose from the fact that no trace of any decomposable organic
substance could be allowed
inside the case, so that a
free use of any fatty sub-
stance would have been
fatal. Fig. 31 explains
how a tight joint was
at last secured. On the
FIG. 31.
The case
ground projecting inner rim a of the frame was laid a thin
strip of rolled gutta-percha, which was kept about a millimetre
away from its inner edge. The glass plates were then heated
and applied, and the case was exhausted as far as the leakage
would admit; at the same time a mixture of four parts of
rosin and one part of olive oil was poured into the hollow
space B. This mixture proved, after cooling, sufficiently fluid
to follow the movements of the glass plates, and was yet so
tough that only after several months did it begin to flow out
of the hollow space on account of its own weight.
could be kept exhausted for days together. If the current
was sent through it for a considerable time, the pressure of
the gas increased slightly, but not so much as to interfere
with the experiments. The case was next enclosed in a tin-
foil covering, connected to earth but insulated from the brass
frame and the electrodes. After exhausting it to a pressure
of a few hundredths of a millimetre, it was placed upon a
board covered with co-ordinate paper (squared paper) and
provided with levelling screws. Exactly over the zero point
of this co-ordinate system hung the magnetic needle which
has been already described, at such a height that the exhausted
case could be moved about underneath without touching it.
M. P.
R