The Ferranti valve is a type of vacuum tube developed by Messers. Ferranti Ltd., in measuring the charge-to-mass ratio of an electron. It consists of an anode and two co-axial guard rings. A cathode, as a thin wire of tungsten filament, runs through this arrangement. When heated to a particular temperature using an HT electricity supply, electrons are emitted.
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Ferranti valve Wikipedia(Text) CC BY-SA