Electron Tube Thermionic Valve

Vacuum tubes with two active elements ("diodes") are used for rectification. Ones with 3 or more elements ("triodes", "tetrodes", etc.) are used for amplification, functions which rely on amplification such as oscillators,

Modern vacuum tubes, mostly miniature style

In electronics, a vacuum and switching.
Standard tubes with cathode, grids, and anode are classified in various ways according to requirements. They

Early RCA triode vacuum tube, type 808

may be classified by frequency range (audio, radio, vhf, uhf, microwave), power rating (small-signal, audio power, high-power radio transmitting), design purpose (sharp versus remote cutoff; amplifying and switching, control and signal amplification, rectification, mixing, general-purpose versus very low microphonic and low noise audio amplification, and so on). Distinctions are not necessarily sharp; for example similar dual triodes can be used for audio preamplification and as flip-flops in computers, although linearity is important in the

Triodes as they evolved over 40 years of tube manufacture, from the RE16 in 1918former case and long life in the latter.
Other tubes which could be called vacuum tubes have different construction and different functions, such as cathode ray tubes which create a beam of electrons for display purposes (such as the television picture tube)

Vacuum tube with plate (anode) cut open revealing grid.

in addition to more specialized functions such as electron microscopy and electron beam lithography. X-ray tubes are also vacuum tubes. Phototubes and photomultipliers also rely on electron flow through a vacuum, though in this case the emission of electrons from the cathode depends on energy from photons rather than thermionic emission. Since these sorts of "vacuum tubes" have functions other than electronic amplification and rectification they are described in their own articles.