A device consisting of a sealed enclosure in which
the electrons flow between. Electrons separated either
by vacuum (in a vacuum tube) or by an ionized gas
at low pressure.
The two principal electrodes of an
electron tube are called the anode and cathode.
The simplest vacuum tube, the Diode, contains only these
When the cathode is heated, it emits
a cloud of electrons, which are attracted to the positive
polarity of the anode and constitute the current through
the tube. Because the anode is not capable of emitting
electrons, no current can flow in the reverse direction,
and diode acts as a Rectifier.
In the vacuum triode, small signals applied to a
third electrode, called a grid, placed between the
cathode and anode cause large fluctuations in the
current between the cathode and anode. A triode can
thus act as a Signal Amplifier. Although formerly
the key elements of electric circuits, electron tubes
have been almost entirely displaced by Semiconductor