Which vacuum device is defined by having two electrodes: a cathode and an anode plate?

• A. Triode
• B. Pentode
• C. Diode
• D. Tetrode

Answer: (C)

In vacuum tubes, current is controlled by how many electrons the heated cathode emits and how they are directed to the anode. The simplest configuration includes just two electrodes: a heated cathode that emits electrons and an anode plate that attracts them. This two-electrode device is a diode. Because there’s no third electrode to control the flow, a diode acts mainly as a rectifier or detector, passing current when the anode is positive relative to the cathode.

The other options add extra electrodes to influence the flow: a grid between cathode and anode in a triode allows voltage on the grid to modulate current and enable amplification; adding more grids creates a tetrode or pentode, which further improve control and performance. So the two-electrode, cathode-and-anode device is the diode.