The Vacuum Diode

Construction. A vacuum diode consists of two electrodes sealed inside an evacuated glass (or metal) envelope: a heated CATHODE, which supplies electrons by thermionic emission (Section 9.2), and a cold PLATE (or anode), a metal surface positioned to collect those electrons. The envelope is evacuated to a high vacuum specifically so that the emitted electrons can travel from cathode to plate without being scattered or absorbed by gas molecules along the way.

Working: one-way conduction. When the plate is made POSITIVE with respect to the cathode (by an external high-tension battery), the resulting electric field between the electrodes attracts the cloud of thermionically emitted electrons across the gap to the plate, completing the circuit -- the tube conducts. When the plate is instead made NEGATIVE with respect to the cathode, the field repels the emitted electrons back toward the cathode, and essentially no current can flow (the plate itself, being cold, emits no electrons of its own to be pulled the other way). A vacuum diode therefore conducts appreciable current in only ONE direction -- plate positive relative to cathode -- exactly the one-way behaviour later reproduced, far more compactly and efficiently, by the semiconductor p-n junction diode of Sections 9.9-9.12. …