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

The cathode is heated so it emits electrons by thermionic emission (Section 9.2); the plate is a cold metal electrode some distance away, the whole assembly sealed in an evacuated envelope so emitted electrons are not scattered by gas. When the plate is made positive relative to the cathode, the resulting field pulls the emitted electron cloud across to the plate and current flows. When the plate is made negative, the field repels the electrons back toward the cathode, and since the (cold) plate itself emits nothing, essentially no current flows the other way.

Describe the two electrodes and the vacuum, then explain conduction only occurs for a positive plate since only the cathode emits electrons.

• Thinking the plate also emits electrons -- it is cold and emits essentially none.
• Forgetting the envelope must be evacuated, or scattering by gas would prevent electrons reaching the plate at all.