1928 Nobel Prize in Physics
Reason for Award
for his work on the thermionic phenomenon and especially for the discovery of the law named after him (Richardson’s law)
Laureates
United Kingdom of Great Britain and Northern Ireland
Explanation
When a metal is heated very hot, some of its electrons jump out of the surface. This is called “thermionic emission.” Richardson carefully measured how many electrons come out at different temperatures and found a fixed relationship between temperature and electron flow. This rule is now called “Richardson’s law,” and it underpins devices such as vacuum-tube radios and electron microscopes. Even the glowing screen of old televisions relied on this principle.
Related Keywords
thermionic emission
When a metal or semiconductor is heated, internal electrons gain enough thermal energy to overcome the surface potential barrier and escape into vacuum. The resulting current can be steered by an external circuit and is used in cathodes of vacuum tubes and electron guns. The emission strongly depends on temperature and work function and is approximated by J=AT²exp(−φ/kT). Emitted electrons have a broad energy distribution, and surface conditions or oxide layers strongly affect efficiency. Thermionic emission remains indispensable in X-ray tubes, ion thrusters, and other modern devices.
Richardson’s law
Proposed in 1901 and refined in 1912, Richardson’s empirical formula describes thermionic emission. The current density J is proportional to T² and decays exponentially as e^{−φ/kT}. The work function φ is a material-specific parameter that varies with surface cleanliness and adsorbed gases. The constant A, called the Richardson constant, has a theoretical value from the free-electron model but experimental values often span orders of magnitude. Modified versions are used for semiconductors and oxide cathodes, making the law a critical guideline in designing electron sources.
work function
The minimum energy an electron at the Fermi level needs to reach the vacuum level outside a solid. It varies among metals, being low for alkali metals and high for noble metals. Surface adsorbates or oxides modify the potential, so controlling the work function directly impacts electron emission efficiency. It plays a decisive role in the photoelectric and Schottky effects, making it a central concept in surface science. Recently, organic over-layers have been explored to tailor low work functions for displays and solar cells.
vacuum tube
An electronic device in which multiple electrodes are placed inside an evacuated glass envelope. Thermionic emission from a heated cathode supplies electrons whose flow is controlled by applied voltages. Vacuum tubes were indispensable in radios, televisions, radar, and early computers during the first half of the 20th century. Although largely replaced by semiconductor transistors, they remain in use for high-power transmitters and microwave-band tubes. Audiophile vacuum-tube amplifiers have recently enjoyed a revival for their characteristic sound.
electron gun
A device that gathers and accelerates electrons emitted from a thermionic cathode into a narrow beam using electric fields. It is essential in CRT displays, electron microscopes, and as an injector for accelerators. Beam diameter and energy spread depend on cathode material, work function, and lens-electrode design. Low-work-function materials such as barium oxide or LaB6 single crystals have been developed for high brightness and long lifetime. In next-generation free-electron lasers, high-quality electron guns remain a key performance factor.
Richardson constant
The proportionality constant A in Richardson’s law, theoretically given by 4πm e k²/h³, where m is the free-electron mass, e the elementary charge, k the Boltzmann constant, and h Planck’s constant. In practice it is modified by surface roughness, dip layers, and effective mass, spanning tens to thousands of A cm⁻² K⁻² for different metals. Measuring A helps diagnose surface conditions of cathode materials; a low value often indicates contamination or oxidation. Developing stable cathodes aims at simultaneous optimization of A and the work function. The constant also serves as a key parameter in analyzing reverse saturation current of semiconductor Schottky junctions.