The invention: A sealed glass tube from which air and gas have been removed to permit electrons to move more freely, the vacuum tube was the heart of electronic systems until it was displaced by transistors.
The people behind the invention:
Sir John Ambrose Fleming (1849-1945), an English physicist
and professor of electrical engineering Thomas Alva Edison (1847-1931), an American inventor Lee de Forest (1873-1961), an American scientist and inventor Arthur Wehnelt (1871-1944), a German inventor
A Solution in Search of a Problem
The vacuum tube is a sealed tube or container from which almost all the air has been pumped out, thus creating a near vacuum. When the tube is in operation, currents of electricity are made to travel through it. The most widely used vacuum tubes are cathode-ray tubes (television picture tubes).
The most important discovery leading to the invention of the vacuum tube was the Edison effect by Thomas Alva Edison in 1884. While studying why the inner glass surface of light bulbs blackened, Edison inserted a metal plate near the filament of one of his light bulbs. He discovered that electricity would flow from the positive side of the filament to the plate, but not from the negative side to the plate. Edison offered no explanation for the effect.
Edison had, in fact, invented the first vacuum tube, which was later termed the diode; at that time there was no use for this device. Therefore, the discovery was not recognized for its true significance. A diode converts electricity that alternates in direction (alternating current) to electricity that flows in the same direction (direct current). Since Edison was more concerned with producing direct current in generators, and not household electric lamps, he essentially ignored this aspect of his discovery. Like many other inventions or discoveries that were ahead of their time—such as the laser—for a number of years, the Edison effect was “a solution in search of a problem.”
The explanation for why this phenomenon occurred would not come until after the discovery of the electron in 1897 by Sir Joseph John Thomson, an English physicist. In retrospect, the Edison effect can be identified as one of the first observations of thermionic emission, the freeing up of electrons by the application of heat. Electrons were attracted to the positive charges and would collect on the positively charged plate, thus providing current; but they were repelled from the plate when it was made negative, meaning that no current was produced. Since the diode permitted the electrical current to flow in only one direction, it was compared to a valve that allowed a liquid to flow in only one direction. This analogy is popular since the behavior of water has often been used as an analogy for electricity, and this is the reason that the term valves became popular for vacuum tubes.
Same Device, Different Application
Sir John Ambrose Fleming, acting as adviser to the Edison Electric Light Company, had studied the light bulb and the Edison effect starting in the early 1880′s, before the days of radio. Many years later, he came up with an application for the Edison effect as a radio detector when he was a consultant for the Marconi Wireless Telegraph Company. Detectors (devices that conduct electricity in one direction only, just as the diode does, but at higher frequencies) were required to make the high-frequency radio waves audible by converting them from alternating current to direct current. Fleming was able to detect radio waves quite effectively by using the Edison effect. Fleming used essentially the same device that Edison had created, but for a different purpose. Fleming applied for a patent on his detector on November 16, 1904.
In 1906, Lee de Forest refined Fleming’s invention by adding a zigzag piece of wire between the metal plate and the filament of the vacuum tube. The zigzag piece of wire was later replaced by a screen called a “grid.” The grid allowed a small voltage to control a larger voltage between the filament and plate. It was the first com-
John Ambrose Fleming
John Ambrose Fleming had a remarkably long and fruitful scientific career. He was born in Lancaster, England, in 1849, the eldest son of a minister. When he was a boy, the family moved to London, which remained his home for the rest of his life. An outstanding student, Fleming matriculated at University College, London, graduating in 1870 with honors. Scholarships took him to other colleges until his skill with electrical experiments earned him a job as a lab instructor at Cambridge University in 1880. In 1885, he returned to University College, London, as professor of electrical technology. He taught there for the following forty-one years, occasionally taking time off to serve as a consultant for such electronics industry leaders as Thomas Edison and Guglielmo Marconi.
Fleming’s passion was electricity and electronics, and he was sought after as a teacher with a knack for memorable explanations. For instance, he thought up the “right-hand” rule (also called Fleming’s rule) to illustrate the relation of electromagnetic forces during induction: When the thumb, index finger, and middle finger of a human hand are held at right angles to one another so that the thumb points in the direction of motion through a magnetic field—which is indicated by the index finger—then the middle finger shows the direction of induced current. During his extensive research, Fleming investigated transformers, high-voltage transmitters, electrical conduction, cryogenic electrical effects, radio, and television, and also invented the vacuum tube.
Advanced age hardly slowed him down. He wrote three topics and more than one hundred articles and remarried at eighty-four. He also delivered public lectures—to audiences at the Royal Institution and the Royal Society among other venues— until he was ninety. He died in 1945, ninety-five years old, having helped give birth to telecommunications.
plete vacuum tube and the first device ever constructed capable of amplifying a signal—that is, taking a small-voltage signal and making it much larger. He named it the “audion” and was granted a U.S. patent in 1907.
In 1907-1908, the American Navy carried radios equipped with de Forest’s audion in its goodwill tour around the world. While useful as an amplifier of the weak radio signals, it was not useful at this point for the more powerful signals of the telephone. Other developments were made quickly as the importance of the emerging fields of radio and telephony were realized.
Impact
With many industrial laboratories working on vacuum tubes, improvements came quickly. For example, tantalum and tungsten filaments quickly replaced the early carbon filaments. In 1904, Arthur Wehnelt, a German inventor, discovered that if metals were coated with certain materials such as metal oxides, they emitted far more electrons at a given temperature. These materials enabled electrons to escape the surface of the metal oxides more easily. Thermionic emission and, therefore, tube efficiencies were greatly improved by this method.
Another important improvement in the vacuum tube came with the work of the American chemist Irving Langmuir of the General Electric Research Laboratory, starting in 1909, and Harold D. Arnold of Bell Telephone Laboratories. They used new devices such as the mercury diffusion pump to achieve higher vacuums. Working independently, Langmuir and Arnold discovered that very high vacuum used with higher voltages increased the power these tubes could handle from small fractions of a watt to hundreds of watts. The de Forest tube was now useful for the higher-power audio signals of the telephone. This resulted in the introduction of the first transamerican speech transmission in 1914, followed by the first transatlantic communication in 1915.
The invention of the transistor in 1948 by the American physicists William Shockley, Walter H. Brattain, and John Bardeen ultimately led to the downfall of the tube. With the exception of the cathode-ray tube, transistors could accomplish the jobs of nearly all vacuum tubes much more efficiently. Also, the development of the integrated circuit allowed the creation of small, efficient, highly complex devices that would be impossible with radio tubes. By 1977, the major producers of the vacuum tube had stopped making it.
See also Color television; FM radio; Radar; Radio; Radio crystal sets; Television; Transistor; Transistor radio.
