A three element vacuum tube is called a "triode". The A battery heats the filament so that the filament emits electrons. The plate collects any electrons emitted by the filament that can get through the grid to it. The plate is kept at a positive potential by the B battery so that it will attract the negatively charged electrons. The grid is a screen and is the secret to the triode. If the grid has a sufficiently negative potential, it repels the
negatively charges electrons and will keep all the electrons from passing through it. However, as the screen grid potential becomes more positive by adjusting the C battery though its potentiometer, more and more electrons are accelerated through the grid and are collected by the plate and measured by the plate milliammeter as current. Thus the grid acts as a valve to regulate the flow of current in the tube. Small changes in the grid voltage can cause large changes in the plate current. In the appropriate circuit, this property is used to cause amplification of small signals applied to the grid. In this demonstration, you can use your
cursor to move the potentiometer arrow right or left to change the
potential on the grid and see how it effects the tube function. Many of these same
principles explain the function of transistors.
Adapted from "The Radio Manual", by George E. Sterling, 1929.