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 two electrodes.

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 devices.