Vacuum Tube

An electron tube where virtually all the air has been removed (creating a vacuum), thus permitting electrons to move freely, with low interaction with any remaining air molecules. Early tubes had two electrodes (cathode and anode, or plate) and were known as diodes. Later (1907) a grid (or control grid) was added, creating what is known as a triode. The grid provided real-time control of output levels, where a small voltage applied could regulate the current passing from the cathode to the plate, creating the first practical amplifier.