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The vacuum tube has been described as the most important single piece of equipment introduced into electrical engineering
during the twentieth century. Its development has produced a new engineering art called electronics, which has given us radio,
long distance telephones, sound motion pictures, public address systems, television, radar, electronic computers, and industrial
automation. Many improvements have been made in vacuum tubes since the invention of the first crude models. Today vacuum tubes
are used in electronic circuits to perform many different functions:
As rectifiers, they convert alternating current to direct current.
As mixers, they combine separate signals to produce a different signal.
As detectors, they separate the useful component from a complex signal.
As amplifiers, they increase the strength of a signal.
As oscillators, they convert a direct current to an alternating current of desired frequency.
As wave shapers, they change a voltage waveform into a desired shape for special use.
Vacuum tube operation presents three inherent problems.
One. Vacuum tubes are power-consuming devices. The power output of a vacuum tube circuit is always considerably less
than the total power input.
Two. The size of vacuum tubes limits reduction in size of electronic circuits although tubes used today are smaller than
early types.
Three. The performance of a particular vacuum-tube circuit is limited to a relatively small range of frequencies. A circuit
that is capable of amplifying one signal may produce a loss of signal strength at another frequency.
Transistors may solve the first two problems associated with vacuum tube circuits. They require very little power for
operation in an electronic circuit, and occupy a much smaller space than vacuum tubes. However, vacuum tubes continue to be
the major components in many electronic circuits.
A vacuum tube usually contains a cathode as a source of electrons, an anode (commonly called a plate) which attracts
electrons from the cathode, and one or more grids for controlling the flow electrons between the cathode and plate. These
electrons are enclosed in a highly evacuated, gas-tight envelope. Modified forms of the vacuum tube may contain only a cathode
and a plate, or two independent sets of electrodes in a single envelope. In some, a small amount of a particular gas is introduced
to obtain special operating characteristics.
Common tubes are classified as diodes, triodes, tetrodes, and pentodes, according to the number of electrodes. High vacuum
tubes are known as hard tubes and those containing gas under very low pressure are known as soft tubes. Special tubes have
special names. For example, the cathode ray tube is a visual indicating tube used in cathode-ray oscilloscopes. It serves
also as the picture tube in television sets, and as the indicator in radar equipment.
While experimenting with the first electric lamp in 1883, Thomas Edison was plagued by the frequent burning out of the
carbon filament with an accompanying black deposit inside the bulb. While trying to correct this difficulty he sealed a metal
plate inside the lamp near the filament and connected it through a galvanometer to the battery. He observed a deflection on
the galvanometer when the plate was connected to the positive terminal of the battery, but no deflection when the plate was
connected to the negative terminal. Edison recorded these observations and continued his lamp experiments without attempting
to explain the phenomenon.
Several years later, Sir J. J. Thomson discovered the electron and provided the explanation for the Edison effect. Electrons
escaped the heated carbon filament and moved unimpeded through the evacuated space to the plate when it was connected to the
positive terminal of the filament battery. The negatively charged plate, on the other hand, repelled the escaping electrons
and prevented an electron current in the galvanometer circuit.
Near the end of 1901, wireless telegraphy, the forerunner of modern radio communications, was successfully used to transmit
radio signals over long distances. In the early days of radio, crystals of semiconductors were used as receivers to detect
(remove the intelligence from) the transmitted signals, but these crystals were unsatisfactory in many ways. A better method
of detection was clearly needed.
In 1904, Sir J. A. Flemming, an English physicist, patented the first diode, called the Flemming valve. He used the Edison
effect to develop a crude detector for radio signals. Flemmings valve was so insensitive that it found little immediate application,
yet it was an important link in the evolution of the vacuum tube.
Dr. Lee De Forest was one of Americas pioneers in wireless telegraphy, radio telephone, sound movies, and the development
of the vacuum tube. In 1906 he succeeded in amplifying feeble radio signals which neither the Flemming valve nor the crystal
detectors were sensitive enough to detect.
De Forest placed a third electrode, consisting of a grid of thin wire, between the filament and plate of the diode. He
found that a small variation in voltage applied to the grid produced a large variation in the current in the plate circuit
of the tube. Thus the feeble radio signals from an antenna could be amplified by De Forests triode and then successfully detected
by the Flemming valve to provide the varying direct current signal necessary to operate headphones. The success of the De
Forest triode led to the development of modern vacuum tubes.
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