The emission of electrons from metals heated above 1000 degree centigrade is called thermionic emission. The name is made of thermo (heat) and ion (charged particle). However, the way thermionic emission was discovered and the way electrons were identified is one of the greatest detective stories in physics. Electrons are invisible, and cannot be seen even in the present age of ultra-high-magnification physics, and that makes the discovery a century and a quarter ago all the more exciting.
History: The atom is made of three particles, electrons, protons, and neutrons. Ancient Greek people were the first to discover negative electrical charges exhibited from a cluster of electrons. From there onwards the physical influence of the electrons was spotted in many experiments, but it was identified as a particle only in 1896 by the British physicist J. J. Thomson. The actual credit of discovery goes to him and also to the hundreds of people who made discoveries that led to Thomson's experiment.
About six years prior to that Thomas Alva Edison was trying to discover the reasons why many of his incandescent bulbs were becoming blackened on some spots. He was also anxious to find a reason why many of the filaments were cracking in an unexpected way. While he had no inkling of what result it would give, he made several electrical bulbs with a metal wire emended in the glass which had no connection with the filament. Since it was vacuum inside the bulbs, and since there was no medium to conduct electricity from the filament to this pieces of metal, no electricity was supposed to flow to this unconnected piece of metal.
Picture: The Glowing Filament Attains 1200 to 1800 Degrees Centigrade!
Edison then connected a galvanometer between the filament and the isolated piece of embedded wire (electrode) and found to everyone's surprise that there was a flow of electricity! This surprised everyone and the effect was named as Edison effect. It took 25 years from there for this effect to find application in physics in the form of Vacuum Tube Diode, and that was the birth of electronics.
The Reason: Eventually it became clear that once metals are heated to temperatures above 1000 degrees, most of them emit electrons randomly into all directions. Since the filament of an electric bulb works at temperatures higher than 1000 degrees, the filament emits electrons with great velocity. Since there is no air inside the bulbs to impede their movement through collision, the electrons travel far and reach the unconnected electrode. This is how electricity flows without a medium in the bulb.
When a material (such as a metal filament) is heated, electrons in the atoms absorb heat energy. This will give some of them enough energy to overcome the attraction that binds them to the nucleus. The moment they overcome the attraction, they are released from the atom. Since heat is what is causing this emission, this is known as thermionic emission.
While it is not exactly identical, it is helpful to think of boiling water. The heat that brings in the boil gives enough energy to water vapors to escape into the atmosphere. Thus thermionic emission can in one way be called the boiling and escaping of electrons out of highly heated metals. Thermionic emission became the foundation of vacuum-tube electronics from 1905 onwards. Today technology would not have been what it is if Edison had not discovered thermionic emission.