28 July 2015

Vacuum tube



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 neg
ative 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.
Adiode 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 in-
ventions 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-
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.


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
books 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.


See also: Color television; FM radio; Radar; Radio; Radio crystal
sets; Television; Transistor; Transistor radio.


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