The Evaporation Theory

Author: J.B. Hoag

As explained earlier, there are free electrons moving between the fixed atoms which make up the metal of the filament. Some of these are moving rapidly and some slowly.

At the surface of the filament some of the electrons escape into the vacuum where they travel along curved paths and come back to the filament, like a ball thrown upward from the surface of the earth. If the wire is heated, some of the electrons acquire sufficient speed so that they can escape completely from it. This is called thermionic emission. In fact, the escape of electrons from a metal obeys the same mathematical laws as does the escape of molecules from water when it evaporates.

The facility with which electrons escape from different filaments, all at the same temperature, depends on the metal of this filament and upon certain sensitizing coatings placed upon it.