This is basically a two-electrode valve [an anode and
a heated cathode] the space between the anode and cathode being evacuated.
Experiments show that there is:
(a) No emission when the cathode is cold - virtually an open circuit
(b) No emission
when the polarity of the anode and cathode are reversed
(c) No current when the tube is
filled with a gas
(d) The anode current increased as the heater is made hotter. The
current reaches a peak no matter what voltage was applied to the anode for a given
temperature of the cathode. This happens when all the electrons emitted by the cathode are
being attracted to, and collected by, the anode (see Figure
2).
