Before being replaced by transistors and integrated circuits, vacuum tubes (thermionic valves) were predominately used in electronic devices such as televisions, radios, and computers. They are still in use today in a few specialized devices.
The invention of vacuum tubes dates back to the observation of the so-called Edison Effect, an observation made by Thomas Edison. Edison noted that current flows between an incandescent lamp's filament and a plate within the vacuum, when the plate is connected to the positive end of the filament.
Because the first computers ran on vacuum tubes instead of today's small computer chips, a single computer had to contain thousands of vacuum tubes, and could fill an entire room. Early stereo amplifiers also used vacuum tubes, and even today, some audiophiles prefer them because they produce less distortion. Vacuum tubes are still used in some electric guitar amplifiers. Also, tubes are still used in some military applications, because tube electronics are not affected by the radio waves made by atomic explosions.
Vacuum tubes are typically enclosed in a glass enclosure, although some tubes use ceramic or metal instead. In its most basic diode design, the tube, or envelope, is sealed tight to create a vacuum. Electrodes within the envelope are attached to leads, which protrude out of the envelope and plug into a socket. A basic vacuum tube contains filaments within the envelope, similar to those of a light bulb. The filaments are heated and then release electrons, creating a negatively-charged electron cloud. The electrons are drawn to an anode, or small metal plate, within the tube, which is positively charged, and a unidirectional flow is established between the filament and plate.
An additional electrode in the form of a small screen-like grid is sometimes contained in the tube, which is then called a triode, which is more efficient and able to amplify the voltage. As voltage is applied to the grid, the flow between the filament and plate can be varied. In addition to diodes and triodes, further innovations followed, including tetrodes, hexodes, heptodes and octodes, designed for a variety of specialty applications and to minimize distortion. Some vacuum tubes combine the function of two or more diodes or triodes in a single unit.
One of the major drawbacks of vacuum tubes is that the filament becomes unstable over time. In addition, if air leaks into the tube, oxygen will react with the hot filament and damage it. The properties of a tube will change with age, which is why early vacuum tube television sets had to be adjusted often to produce a good picture.