Q.In a triode valve, explain how a small change in the control-grid voltage produces a comparatively large change in the plate current.

The control grid is positioned close to the cathode, while the plate is comparatively far away. Since electric field strength depends on both voltage and distance, a given change in grid voltage -- applied over a short distance to the cathode -- creates a much stronger change in field at the cathode surface than the same voltage change applied to the distant plate would. This strong local field change directly controls how many of the thermionically emitted electrons succeed in passing through the grid's open mesh and continuing on to the plate, so a small grid-voltage swing produces a much larger swing in plate current.

Compare the grid's closeness to the cathode against the plate's distance, then link this to field strength and hence emitted-electron flow.

• Saying the grid simply 'blocks or unblocks' electrons like a physical gate -- it acts through its electric FIELD, not by physically obstructing them.
• Missing that this closeness, not the grid's mere presence, is the reason the effect is amplified.