Vacuum Tube

The outer part of a vacuum tube consists of a tubelike glass or metal shell. Inside the shell are specially designed wires and small metal plates that control the electronic signals. The vacuum tube gets its name from the fact that almost all the air must be removed from the tube for it to work. A partial vacuum is created inside the tube by pumping out as much air as possible.

Vacuum tubes were essential to the development of the science and technology of electronics. From the 1920's to the 1950's, all electronic equipment used vacuum tubes. Since that time, the vacuum tube has been replaced in most kinds of electronic equipment by a newer device called the transistor. Transistors do the same jobs as vacuum tubes. But they are smaller and more reliable and consume less power (see Transistor). Certain electronic equipment still uses various types of vacuum tubes. For example, the screen of a television set is one end of a large vacuum tube called a cathode-ray tube. See Electronics.

How a vacuum tube works. The outer part of most common vacuum tubes is a glass or metal container called an envelope or bulb. The envelope encloses two or more metal parts called electrodes. The electrodes create and control a flow of electrons within the tube. This flow corresponds to the electronic signal being controlled by the tube. The electrodes usually are connected to electric circuits outside the tube by wires that pass through the base of the envelope.

Two basic electrodes in a vacuum tube are the emitter, or cathode, and the collector, or anode. The emitter gives off electrons. These electrons flow to the collector, which in most tubes surrounds the emitter. A coating on the emitter gives off electrons when heated. Close to the emitter is a filament (fine wire) much like that of a light bulb. Electric current from outside the tube flows through the filament and heats it. The filament heats the emitter, causing it to give off electrons.

The emitter usually has a negative electric charge and the collector usually has a positive charge. The electrodes get their charges from a battery or other source of direct current. The emitter's negative charge helps to push away the electrons it produces. This happens because electrons have a negative charge, and two negative charges--or two positive charges--always push away from each other. But a negative and a positive charge always attract each other. Thus, the positive collector attracts the negative electrons. In this way, a current of electrons flows from the emitter to the collector.

Another basic vacuum tube electrode is the grid. It consists of a wire mesh located between the emitter and the collector. The grid controls the amount of electrons flowing through the tube. A strong negative charge on the grid prevents many of the electrons from reaching the collector. If the negative charge becomes weaker, more electrons get past the grid and reach the collector. The strength of the charge on the grid corresponds to the strength of the electronic signal entering the vacuum tube.

A vacuum tube may have several other parts between the emitter and collector. It may also have charged metal plates that can "bend" a stream of electrons created in the tube. Magnets outside the tube can also bend the stream of electrons.

Kinds of vacuum tubes. There are many hundreds of vacuum tubes having various sizes and functions. But electrical engineers classify all tubes into a few basic types. Receiving tubes, the kind once widely used in radio and television receiving sets, are classified by the number of electrodes they have. Receiving tubes include (1) diodes (two-electrode tubes), (2) triodes (three-electrode tubes), and (3) multielectrode tubes. Other types of tubes include (1) cathode-ray tubes, (2) microwave tubes, and (3) gas-filled tubes.

Diodes have only an emitter and a collector. These tubes are used chiefly as rectifiers. A rectifier changes alternating current into direct current. An alternating current is one that keeps reversing its direction of flow. An electrode connected to a source of alternating current gets a charge that constantly changes from positive to negative and back again. If an alternating current is sent to a diode, the tube will pass the current only when the emitter has a negative charge. This happens only when the current is flowing in one direction. Thus, the current leaving the tube is direct current.

Diodes were used in receiving sets as rectifiers and also as detectors. A detector changes the weak alternating current of the radio waves into direct current. The receiver produces a sound or a picture from this direct current. See Electronics.

Triodes have a grid, as well as an emitter and a collector. The triode amplifies (strengthens) weak signals. A weak signal connected to the grid controls the much larger current flowing from the emitter to the collector. The large current thus becomes a stronger copy of the signal on the grid. A triode can also produce an alternating current without using an outside signal if some of the large current is directed back to the grid. When the triode operates in this way, it is called an oscillator.

A triode vacuum tube creates and controls a flow of electrons in a vacuum. Electrons leave the cathode when a source of direct current is connected to the tube. The electrons flow through the grid to the anode. The voltage applied to the grid controls the number of electrons that reach the anode. From The World Book™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved. World Book illustration by Arthur Grebetz.

Multielectrode tubes have more than one grid between the emitter and collector. Two important multielectrode tubes are the tetrode and pentode. A tetrode contains two grids--the basic one and a second grid called the screen. The screen prevents the tube from producing unwanted oscillations. A pentode has a third grid, called the suppressor. The suppressor improves the multielectrode tube's amplifying power. Other multielectrode tubes have even more grids, but such tubes have limited use.

Cathode-ray tubes (CRT's) are used in electronic equipment to display pictures or other information. The picture tube of a television set is a CRT. In a radar set, a CRT shows tiny spots of light that locate the position of ships or airplanes. A CRT in an electronic instrument called an oscilloscope may display wavy-line "pictures" of electronic signals.

All CRT's basically work the same way. The tube has a round or rectangular screen at one end. The tube tapers from the screen to a narrow neck at the opposite end. In the neck, the emitter and other electrodes are arranged to form an electron gun. The electron gun "shoots" a beam of electrons toward the screen. Wherever the beam strikes the screen, it causes a special coating to glow. Electrically charged metal plates inside the CRT, or electromagnets outside the CRT, move the beam across the screen. The beam thus "paints" a picture on the screen with spots of light. See Television.

Microwave tubes produce or control radio waves of extremely high frequencies. Radar sets, long distance telephone and television systems, and microwave ovens use such waves. Three types of microwave tubes are klystrons, magnetrons, and the traveling-wave tube.

Gas-filled tubes contain a small amount of such gases as argon, mercury vapor, and neon. These gases increase the amount of current that can flow through a tube. The atoms of gas become ionized by losing some of their electrons and thus becoming positively charged. The ionized atoms can carry much more electrical current than would otherwise flow through the tube. A typical gas-filled tube is the thyratron.

Development of the vacuum tube. Electrical experimenters began working with devices that resembled vacuum tubes during the mid-1800's. These devices were glass tubes with the air partially removed. The experimenters noticed a glow around the tubes, as well as other unusual effects, when electricity flowed through the tubes. See Electronics.

The American inventor Thomas A. Edison invented the first electronic vacuum tube. But he did not realize the importance of his invention. In the early 1880's, Edison sealed an extra electrode into an electric light. When the light was on, Edison found that a current flowed from the filament to the extra electrode if this electrode was positively charged. This phenomenon became known as the Edison effect. Edison made no use of his discovery, which was actually a diode vacuum tube.

A British scientist, John Ambrose Fleming, experimented with the Edison effect. His experiments led him to develop a diode in 1904 to detect "wireless" radio signals. Fleming's valve, as he called his device, was the first practical radio tube.

In 1906, the American inventor Lee De Forest patented a two-electrode tube much like the Fleming valve. De Forest called his tube an audion. In 1907, De Forest patented an audion with a zigzag wire between the other two electrodes. This tube was the first triode.

In 1912, Harold D. Arnold, an American physicist, began experimenting with the audion. He changed its internal construction and removed as much air as possible to create a partial vacuum in the tube. In 1914, De Forest's audion worked successfully as an amplifier on a long-distance telephone line. Beginning in 1912, several inventors contributed to the development of the audion as an oscillator. They included De Forest and the American radio pioneer Edwin H. Armstrong. During World War I (1914-1918), Walter Schottky, a German physicist, invented an experimental four-element tube. From this tube, Albert W. Hull, an American engineer, developed a practical tetrode tube in 1924. A Dutch engineer, Benjamin D. H. Tellegen, invented the pentode in 1926.

The work of two inventors during the 1920's led to the development of electronic television. The Russian-born American scientist Vladimir Zworykin invented a TV camera tube called the iconoscope, an electronic tube that converts light rays into electric signals. Philo T. Farnsworth, an American inventor, also developed a TV camera tube called the image dissector.

Attention turned away from vacuum tubes after the development of the transistor in the 1950's and the integrated circuit in the 1960's. These solid-state devices, in which electronic signals flow through a solid material instead of through a vacuum, have largely replaced vacuum tubes.

Contributor: Bernard S. Finn, Ph.D., Curator, National Museum of American History.

See also Electronics and its list of Related Articles.

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