Vacuum Tubes
A vaccum tube is an evacuated electron conducting chamber (tube) designed to eliminate or minimize the influence of atmospheric gas. More commonly, a vacuum tube is an electronic mechanism used primarily to convert alternating current into direct current.
The vacuum tube consists of a glass encasement and two or more electrodes between which electrons flow freely. One of these electrodes, called the cathode, consists of a filament heated to the degree that it releases electrons, a process called thermionic emission. The other electrode, called the anode, consists of a metal plate attached to a filament. The electrons released by the cathode are attracted to the anode when the anode is positively charged, thus completing an electrical circuit. However, the electrons will not flow to the anode during the phase in an alternating current when the anode is negatively charged. In constructing the vacuum tube, all air and gas is removed from the encasement and the tube is sealed. The flow of electrons inside the tube has no interference from the presence of any gas.
The first vacuum tube was invented by British electrical engineer John Fleming who was investigating a principle discovered by American scientist Thomas Edison. Edison had accidentally discovered that a metal plate prevents electrons from flowing from the anode in an alternating current. Edison treated this discovery as only a scientific oddity. Fleming, who had once been an assistant to Edison, realized the practical applications of the Edison effect. Fleming, while working with Italian physicist Guglielmo Marconi on perfecting the radio, recognized that the Edison principle provided the means of rectifying an alternating current into a direct current (i.e., the process of rectification). The direct current alleviated some of the interference found in radio transmission and reception. Fleming's vacuum tube on two electrodes is now referred to as a diode. Years later, an American inventor, Lee De Forest, developed a triode, a vacuum tube with three electrodes. The important difference in De Forest's tube was the introduction of a grid made up of fine wire mesh. This grid, usually surrounding the cathode, can impede or amplify the flow of electrons depending on slight changes in voltage. Other types of vacuum tubes, with as many as eight electrodes, have been developed for a variety of uses such as frequency converters in radios.
In modern times, the transistor has largely replaced the vacuum tube. The advantages of the transistor over the vacuum tube include durability, compactness and greater utility. Furthermore, the vacuum tube is not as energy efficient as the transistor. The vacuum tube requires a greater energy supply and releases much unusable heat in thermionic emission.
In spite of the benefits and advancements offered by the transistor, there remain some functions that are better accomplished through a vacuum tube. One of these functions is harden military equipment that may be exposed to the electromagnetic pulse associated with an atmospheric nuclear explosion. Such an explosion produces a voltage pulse that vacuum tubes can withstand but which can destroy transistors.
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