The Edison effect. Thermionic emission
In 1883, the American scientist and inventor, Thomas Edison, was experimenting with an incandescent electric filament lamp which also contained a small metal plate supported by a wire sealed through the glass. When a battery and galvanometer were connected between the plate and filament he found that a small current flowed through the galvanometer if the plate was positive with respect to the filament, but not if it was negative. This was called the Edison effect, and remained nothing more than a scientific curiosity for many years until an explanation was forthcoming from O. W. Richardson.
Early in the twentieth century Richardson, who was for’ a time Professor of Physics at Princeton University, carried out investigations of the emission of electrons from hot bodies. He explained the Edison effect by assuming that electrons evaporate from the hot filament in much the same way as vapour molecules leave a hot liquid. Thus, in a very short time the region surrounding the hot filament becomes occupied by a space charge of electrons. Eventually, a state of equilibrium is reached in which the rate at which the electrons leave the filament is balanced by the rate at which they return to it. This was happening inside the bulb of Edison’s lamp. When the plate was positive it attracted electrons and a current flowed in the plate circuit, but if the battery was reversed so as to make the plate negative the electrons were repelled and current ceased to flow. The process by which electrons leave the hot filament is called thermionic emission.