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When a metal wire becomes hot and starts to glow, the air around the wire becomes electrically charged. This phenomenon was used earlier to amplify electrical signals in telephone lines and other applications. In 1901 Owen Richardson explained the phenomenon, declaring that when metal becomes hot, the electrons in the metal become so agitated that they are liberated from their atoms and cast into the air where they act as free charged particles. Richardson also formulated a law that describes the relationship between emissions of electrons and temperature.