|In the earliest tubes, the electrons from
the cathode bombarded the glass end wall of the tube. This quickly
heated up, and unless you were careful, could overheat, and crack,
destroying the tube. In this tube, the electron beam is intercepted by a
metal anode, in this case aluminium, that was better able to withstand
the electron bombardment. The cathode is on the right, the anti-cathode,
on the left, and the anode enters from the bottom left.
When under tension, the tube is filled with a cloud of ionized gas. The ions tend to react with the glass walls, and so become fixed. As this process continues, more and more gas is removed, and the pressure falls. As a result the voltage required to initiate ionization rises, so that the operating voltage increases. A point is eventually reached where it is no longer possible to get the tube to strike.
If the tube is left for a period of time, gas is slowly released, and the operating voltage falls.