an early electronics device. there were several types.
diodes consisted of a plate and cathode.
triodes consisted of a plate 1 grid, and a cathode.
tetrodes consisted of a plate, 2 grids, and a cathode.
pentodes consisted of a plate, 3 grids, and a cathode. all types had a heater to heat the cathode. these devices required relatively high plate voltages, 350 volts not being uncommon.
Vacuum tubes were used prior to the invention of the transistor. This meant that computers were less powerful than your average $2 pocket calculator is today, but nevertheless took up whole floors of buildings and sucked electricity like it was going out of style. But they still beat using an abacus.
the cathode was heated until it glowed red, then a positive potential was applied to the plate. this would attract the electrons emitted by the glowing cathode.
- purpose of the grid
the grid was introduced to control the flow of electrons to the plate. if the grid was suffeciently negative it would cut off the flow completely. so by varying the grid potential a larger plate current could be controlled.