What is Thermionic Emission?

The Edison Effect was an observation made by Thomas Edison while working with his incandescent light bulb.

Thermionic emission, also known as thermal electron emission, is the process by which charge carriers such as electrons or ions move over a surface or some type of energy barrier by the induction of heat. Charge carriers naturally restrict activity; however, in thermionic emission, thermal energy is introduced into the carriers, causing them to overcome these forces. The reason behind the ability of charge carriers to perform this action is because electrons and ions are mobile and not attached to the normal chains of atomic structure that affect other particles. Traditionally, these charge carriers were called “termions”.

A property of the thermionic emission theory is that the emitting region is supported by a charge opposite to the original, but of equal magnitude. This means that locating the charge carrier before emission will generate a positive charge in the case of electrons. However, this can be changed with a battery. The emission is neutralized when the carriers are further from the region, resulting in no change to the original state.

Historically, the prime example of thermionic emission is that used in the Edison effect. Electrons are emitted by a hot metal cathode, which uses a polarized electrical device to cause electric current to flow into a vacuum tube. This allows a device to maintain control over the movement of electrons and amplify or modify the electrical signal.

Anything used to cool or generate energy uses the concept of thermionic emission theory. As the temperature increases, the magnitude of the flux increases. In addition to the traditional use of vacuum tubes for electronics, solid-state devices can also be used to create thermionic motion of electrons, allowing modern technology to work.

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Thermionics was first reported by Frederick Guthrie in 1863. He was able to identify a change in the positive charge of a highly heated iron sphere that did not occur if the object was negatively charged. However, it wasn’t until 1880 that science was readily harnessed by Thomas Edison. As he worked with his incandescent light bulbs, he noticed that certain areas remained dark. This allowed him to identify the flow of electrons due to heat, resulting in the creation of the diode.

Richardson’s law describes why electrons are able to flow this way. Specifically, metals contain two electrons in the atomic structure that are able to move from atom to atom. In 1928, Sir Owen Willans Richardson, a British physicist, discovered that some electrons were able to leave the atom without returning. This process requires a certain amount of energy depending on the metal. The term for this purpose is work function.

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