Which vacuum tube uses three active elements: a heated cathode, a grid, and a plate (anode)?

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

Answer: (B)

In vacuum tubes, the name of the type comes from how many active elements are inside. A triode has three: a heated cathode that emits electrons, a control grid that sits between the cathode and the plate, and the plate (anode) that collects the electrons. The grid lets small changes in its voltage produce large changes in plate current, which is how the tube can amplify signals.

The other options include more or fewer elements: a diode has only two elements (cathode and plate) with no grid, so it isn’t a triode; a tetrode adds a second grid (the screen grid) between the control grid and plate, giving four elements; a pentode adds another grid as well, totaling five. So the configuration that uses exactly a heated cathode, a grid, and a plate is the triode.