How does a control grid regulate electron flow in a vacuum tube?

In a vacuum tube, the control grid acts as a gatekeeper for the flow of electrons between the cathode and the anode. Typically constructed as a wire mesh placed between these two electrodes, the grid regulates current through electrostatic influence.12

The mechanism functions based on the electrical potential applied to the grid:

• Negative Bias: The grid is generally maintained at a negative potential relative to the cathode. Because like charges repel, this negative bias creates an electrostatic barrier that opposes the movement of electrons emitted from the cathode.3
• Modulation: A large negative bias can completely block electrons from reaching the anode. However, because the grid is composed of open space, reducing the negative bias allows a portion of the electron stream to pass through the mesh toward the anode.3
• Amplification: Because the grid is positioned close to the cathode, small variations in the grid's voltage can produce significant changes in the total current reaching the anode, allowing the tube to function as an amplifier.23