Today, I made a simple video demonstrating how the control grid of a vacuum tube triode may work. Essentially, the purpose of a control grid, is to control the flow of electrons from the cathode to the anode of a vacuum tube. So, im my experiment, I put my ammeter in series with the B+ and the tube plate and applied a positive charge from a 9v battery and then I inverted the battery polarity, applying a negative charge. In the first case, It is possible to see the current flow increasing, while in the second case, the current practically doesn't flow anymore. You can see this simple experiment in the YouTube video below: For this experiment I used an early type 30 tube, a 1.5v battery and a 90v stack of 10 * 9v batteries and a couple of wires. Hope you like!