A vacuum tube is a hollow glass cylinder from which as much air as possible has been removed. The cylinder also contains two metal electrodes: the cathode, or negative electrode, and the anode, or positive electrode. Current flows within a vacuum tube from the cathode, which has an excess of electrons, to the anode, which has a deficiency of electrons.
Vacuum tubes were a subject of great interest among both scientists and inventors at the end of the nineteenth century. Among scientists, vacuum tubes were used to study the basic nature of matter. Among inventors, vacuum tubes were used as a means of controlling the flow of electric current within an electrical system.
One of the first practical vacuum tubes was invented by English electrical engineer John Ambrose Fleming (1849–1945). Fleming's device permitted the flow of electric current in one direction (from cathode to anode) but not in the other (from anode to cathode). It was, therefore, one of the first devices that could be used to control the direction of flow of electric current. Because it consisted of two parts, Fleming's invention is called a diode. Fleming himself referred to the device as a thermionic valve because, like a water valve, it controlled the flow of electricity.
In 1906, American inventor Lee de Forest (1873–1961) discovered a way to improve the efficiency with which vacuum tubes operate. He installed a third element in the diode: a metal screen between the anode and cathode. This modification of the diode was given the name triode because it consists of three parts rather than two.
For more than half a century, the vacuum tube had an enormous number of applications in research and communications. They were usedin radio receivers as well as in early digital computers. Incorporated intophoto tubes, they were used in sound equipment, making it possible torecord and retrieve audio from motion picture film. In the form of cathode-ray tubes, they were used to focus an electron beam, leading to theinvention of oscilloscopes (which measure changes in voltage over time),televisions, and cameras. As microwave tubes, they were used in radar, early space communication, and microwave ovens. When modified asstorage tubes, they could be used to store and retrieve data and, thus, wereessential in the advancement of computers.
Despite their many advantages, vacuum tubes had many drawbacks.They are extremely fragile, have a limited life, are fairly large, and require a lot of power to operate. The successor to the vacuum tube, thetransistor, invented by Walter Brattain, John Bardeen, and William Shockley in 1948, overcame these drawbacks. After 1960, small, lightweight, low-voltage transistors became commercially available and replaced vacuum tubes in most applications. With the creation of microscopic vacuumtubes (microtubes) in the 1990s, however, vacuum tubes are again beingused in electronic devices.
Anode: Also known as target electrode; the positively charged electrode in an X-ray tube.
Cathode: The negatively charged electrode in an X-ray tube.
Cathode-ray tube (CRT): A form of vacuum tube in which a beam of electrons is projected onto a screen covered with a fluorescent material in order to produce a visible picture.
Electrode: A material that will conduct an electrical current, usually a metal, used to carry electrons into or out of an electrochemical cell.
Transistor: A device capable of amplifying and switching electrical signals.