..... Click the link for more information. or ions ion, atom or group of atoms having a net electric charge .
Positive and Negative Electric Charges
A neutral atom or group of atoms becomes an ion by gaining or losing one or more electrons or protons.
..... Click the link for more information. by substances that are highly heated, the charged particles being called thermions. The number of thermions emitted increases rapidly as the temperature of the substance rises. The heated material may be in the form of a metal filament or of some compound that coats and is heated by the filament. If the heated body carries a positive or negative charge, the thermions will be of the same charge. At temperatures below red heat (see black body black body, in physics, an ideal black substance that absorbs all and reflects none of the radiant energy falling on it. Lampblack, or powdered carbon, which reflects less than 2% of the radiation falling on it, approximates an ideal black body.
..... Click the link for more information. ), thermionic emission from uncharged bodies is chiefly positive; at higher temperatures it is negative. The effect was discovered by Thomas A. Edison in 1883 when he was working on filaments for the electric light. Thermionic emission's most important practical application in electronics is in the electron tube electron tube, device consisting of a sealed enclosure in which electrons flow between electrodes separated either by a vacuum (in a vacuum tube) or by an ionized gas at low pressure (in a gas tube).
..... Click the link for more information. , since it is the mechanism by which electrons are emitted from the cathode.
thermionic emission [‚thər·mē′än·ik i′mish·ən]
Thermionic emission
The emission of electrons into vacuum by a heated electronic conductor. In its broadest meaning, thermionic emission includes the emission of ions, but this process is quite different from that normally understood by the term. Thermionic emitters are used as cathodes in electron tubes and hence are of great technical and scientific importance. Although in principle all conductors are thermionic emitters, only a few materials satisfy the requirements set by practical applications. Of the metals, tungsten is an important practical thermionic emitter; in most electron tubes, however, the oxide-coated cathode is used to great advantage.
The thermionic emission of a material may be measured by using the material as the cathode in a vacuum tube and collecting the emitted electrons on a positive anode. If the anode is sufficiently positive relative to the cathode, space charge (a concentration of electrons near the cathode) can be avoided and all electrons emitted can be collected; the saturation thermionic current is then measured. See Schottky effect
The emission current density J increases rapidly with increasing temperature; this is illustrated by the following approximate values for tungsten: 
The temperature dependence of J is given by the Richards (or Dushman-Richardson) equation below.
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