Q.Describe the basic construction of a vacuum diode and explain, in outline, how it conducts current in only one direction.

A two-electrode vacuum-tube device: a heated cathode (thermionic emitter) and a cold plate, sealed in an evacuated envelope. It conducts appreciable current in only ONE direction -- when the plate is positive relative to the cathode, the field pulls the emitted electrons across; when the plate is negative, they are repelled back and the cold plate emits nothing of its own, so essentially no current flows. This one-way behaviour, used historically as a rectifier, is the vacuum-tube precursor to the semiconductor p-n junction diode.