There are two basic types of electron tube: the vacuum tube and the gas-filled tube. As their names imply, vacuum tubes have virtually all the gases removed from their envelopes. Gas-filled tubes contain elemental vapor at low pressure.
Vacuum tubes accelerate electrons to high speeds, resulting in large currents. This current can be focused into a beam and guided in a particular direction. The intensity and/or beam direction can be changed with extreme rapidity, producing effects such as rectification, detection, oscillation, amplification, signal mixing, waveform displays, spectral displays, and video imaging.
A neon bulb oscillator, also known as a relaxation oscillator.
Gas-filled tubes have a constant voltage drop, no matter what the current. This makes them useful as voltage regulators for high-voltage, high-current power supplies. Gas-filled tubes can withstand conditions that would destroy semiconductor regulating devices. Gas-filled tubes emit infrared (IR), visible light, and/or ultraviolet (UV). This property can be put to use for decorative lighting. A small neon bulb can be employed to construct an AF relaxation oscillator (above figure).
Even before the year 1900, scientists knew that electrons could carry electric current through a vacuum. They also knew that hot electrodes emit electrons more easily than cool ones. These phenom ena were put to use in the first electron tubes, known as diode tubes, for the purpose of rectification. Diode tubes are rarely used nowadays, although they can still be found in some power supplies that are required to deliver several thousand volts for long periods at a 100 percent duty cycle (that is, continuous operation).