CHARLES D. WINTERS / SCIENCE PHOTO LIBRARY CHARLES D. WINTERS / SCIENCE PHOTO LIBRARY
Vacuum tubes were once commonly used in electronic devices to amplify a signal. Resembling light bulbs with a filament sealed in a glass envelope evacuated of all air, vacuum tubes work via thermionic emission. The filament releases electrons into the vacuum that are then drawn into the metal plate (anode), resulting in a current of electrons flowing from filament to plate.
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