How did Lee De Forest’s Audion vacuum tube amplify radio signals?

Lee De Forest’s Audion, developed in 1906, functioned as an amplifier by introducing a third electrode—a control grid—between the cathode filament and the positively biased anode plate.12

In this triode configuration, the anode is kept at a positive potential relative to the cathode, which draws electrons across the vacuum. By applying a small, variable negative bias to the control grid, De Forest could modulate the flow of these electrons. Because the grid is an open-mesh structure, it intercepts very little current itself; however, even minor fluctuations in the grid's voltage significantly alter the number of electrons reaching the plate.213

This mechanism allowed a small voltage change on the grid to produce a much larger voltage change at the plate. By applying a weak radio-frequency signal to the grid, the Audion could amplify that signal, enabling the detection and utilization of much weaker transmissions than were previously possible.42