Vacuum tube is a bulky device once widely used in electronic equipment to create, strengthen, combine, or separate electrical signals. Electrical signals are variations in an electric current or voltage that can be used to carry information. Radios, television sets, and computers rely on such signals for their operation. Vacuum tubes played an essential role in the development of the science and technology of electronics. Most vacuum tubes have been replaced by newer technology. Yet even today, people still use vacuum tubes for certain specialized applications.
The outer part of a vacuum tube consists of a glass or metal shell called the 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. The flow of electrons creates an electrical signal. Wires passing through the base of the envelope connect the electrodes to electric circuits outside, enabling electric current to enter and leave the tube. The vacuum tube gets its name from the fact that almost all the air must be removed from the envelope for the tube to work, creating a partial vacuum. Many vacuum tubes resemble light bulbs. In fact, they were made in the same factories for many years.
From the early 1900’s to the 1950’s, most complex electronic equipment used vacuum tubes. Since that time, vacuum tubes have been mostly replaced by transistors. Transistors do many of the same jobs as vacuum tubes, but are smaller and consumer less power.
People today use vacuum tubes to generate microwaves in microwave ovens. Some musicians use vacuum tube amplifiers for instruments, such as the electric guitar. In addition, the screen of an older television set or computer monitor is one end of a large vacuum tube called a cathode-ray tube.
How a vacuum tube works
The two basic electrodes in a vacuum tube are the emitter (or cathode) and the collector (or anode). When connected to an outside source of current, such as a battery, the emitter has a negative electric charge and the collector has a positive charge. Heating the emitter—or an attached filament (fine wire)—with an electric current causes the emitter to give off electrons.
The emitter’s negative charge pushes away the electrons the emitter produces. This happens because electrons have a negative charge, and two negative charges (or two positive charges) always repel each other. But a negative and a positive charge always attract each other. Thus, the positively charged collector attracts the negative electrons. The electrons thus flow off the emitter, through the vacuum space, and onto the collector, forming an electric current.
The arrangement of electrodes in a vacuum tube controls the flow of current from the emitter to the collector. For example, many vacuum tubes have a third electrode called the grid, a wire mesh or other structure between the emitter and the collector. When a strong negative charge is placed on the grid, it repels most of the electrons, allowing only a few to pass through and reach the collector. A weak negative charge on the grid results in more electrons passing through to reach the collector. Varying the charge on the grid thus varies the flow of current from emitter to collector, creating or altering an electrical signal.
Kinds of vacuum tubes
Inventors have developed many hundreds of kinds of vacuum tubes with different sizes and functions. Electrical engineers classify all tubes into a few basic types.
Receiving tubes,
once widely used to receive radio and television signals, are classified by their number of electrodes. Receiving tubes include diodes (two-electrode tubes) and triodes (three-electrode tubes).
Diodes
have only an emitter and a collector. Rectifiers and detectors are the most common diodes.
Rectifiers change alternating current into direct current. Alternating current is electric current that regularly reverses its direction of flow. Direct current flows in only one direction. When an alternating current is sent to a diode, electrons flow from emitter to collector only when the emitter has a negative charge. When the current reverses, the emitter takes on a positive charge, and no electrons flow from it. Thus, the current leaving the rectifier flows in one direction and is direct current.
Detectors pick up radio waves and transfer signals carried by the waves to an electric current. Many wireless telegraph systems—the earliest radios—used detectors and rectifiers to receive messages in Morse code. The detector converted radio wave bursts into bursts of alternating current, which the rectifier changed into direct current “dots” and “dashes.” Later radios used detectors to receive more complex signals that carried words or music.
Triodes
have a grid between the emitter and the collector. A triode can amplify (strengthen) weak signals by passing them through the grid. The current passing through the grid controls the much larger current flowing from the emitter to the collector, generating a stronger copy of the signal.
A triode can also produce an alternating current if some of the large current is directed back to the grid. A triode operating in this way is called an oscillator.
Cathode-ray tubes,
commonly called CRT’s, are used in electronic equipment to display pictures or other information. Older televisions and computer monitors used CRT’s. In a radar display, a CRT shows tiny spots of light that locate the position of ships or airplanes. CRT’s have largely been replaced with other display technologies, such as liquid crystal displays (LCD’s) and light-emitting diodes (LED’s).
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 far end. The neck contains an emitter and other electrodes arranged to form an electron gun. The electron gun “shoots” a focused beam of electrons toward the screen. Wherever the beam strikes the screen, it causes a special chemical coating to glow. Electrically charged metal plates inside the CRT, or electromagnets outside the CRT, quickly change the shape of the beam, sending electrons to different areas of the screen. The beam thus “paints” a picture on the screen with electrons that create spots of light.
Microwave tubes
produce or control radio waves of extremely high frequencies. Radar sets bounce such waves off objects to detect them. Microwave ovens use klystrons, magnetrons, and traveling-wave tubes to make waves that cook food.
History
In the mid-1800’s, experimenters began working with sealed glass tubes that resembled vacuum tubes. When electric current flowed through the tubes, experimenters noticed a glow around the tubes and other unusual effects. See Electronics (Early experiments).
The American inventor Thomas Edison built an early diode but did not realize the importance of his invention. In the early 1880’s, Edison sealed an extra electrode into one of his electric light bulbs. When the light was on, Edison found that a current flowed from the hot filament to the extra electrode, but only if this electrode was positively charged. No current flowed if the extra electrode was negatively charged. This phenomenon became known as the Edison effect.
The early wireless telegraph industry inspired many experiments with vacuum tubes. Historians often mention three different people when discussing the invention of radio tubes, vacuum tubes used in early radios. In 1904, the British scientist John Ambrose Fleming invented a diode he called a valve. In 1905, the American inventor Lee De Forest produced a similar device and called it an audion. Beginning in 1903, the German physicist Arthur Wehnelt performed many experiments using diodes. In late 1906, De Forest added a third electrode to the audion. Though it was an inefficient amplifier, this tube was the first triode.
Scientists and inventors from the United States and Europe made many advancements in vacuum tubes in the following years. These advancements included improvements in internal design and the introduction of stronger vacuums. In 1915, AT&T used vacuum tube technology in the first transcontinental telephone line.
With the growth of radio, vacuum tubes became the standard component used to build electronic devices. Researchers in many countries invented specialized vacuum tubes for different applications. These specialty tubes included the four-electrode tetrode, the five-electrode pentode, and so on, up to nine electrodes. Advances in vacuum tubes enabled the development of the first television sets in the 1920’s. Some of the earliest electronic computers, built in the 1940’s and 1950’s, used thousands of vacuum tubes to perform calculations.
Vacuum tubes lost their central place in electronics after the development of the transistor in the 1950’s and of the integrated circuit or microchip in the 1960’s. In these solid-state devices, electronic signals flow through a solid material instead of a vacuum.
