An electronic vacuum tube with seven electrodes, used in radio and early electronics to amplify or control electrical signals.
From Greek 'hepta' (seven) plus 'hodos' (path or electrode). This term became standard in radio engineering during the 1920s-1940s as tubes became more complex.
Before transistors, engineers kept adding more electrodes to vacuum tubes like adding more strings to a guitar—a heptode had seven different metal pieces working together to perform sophisticated signal manipulation in old radios and amplifiers.
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