History of the vacuum tube
- or to give it its Eupoean name - the thermionic valve
The invention of the vacuum tube or thermionic valve brought the dawn of the
age of electronics. Its invention enabled the wireless technology of the day to
move forward. Many new and exciting applications were found for these devices,
first as telephone repeater amplifiers and then many other applications that
were not always linked to wireless and as a result the new area of electronics
was born.
Beginnings
The first vacuum tube was not made until the beginning of the 20th Century, but
the foundations for its discovery were laid many years before. Professor Guthrie
made one of the first discoveries in 1873. He was investigating effects
associated with charged objects and he showed that a red-hot iron sphere that
was negatively charged would become discharged. He also found that the same did
not happen if the sphere was positively charged.
The American inventor named Thomas Edison took the next major step in 1883.
Edison was developing electric light systems and one of the major problems that
he was facing was their short life. Although the filament life was a problem,
the main limiting factor was that the bulbs quickly became blackened. Initially
it was thought that this was caused by atoms of carbon from the element hitting
the glass. As it was known that the particles leaving the element were
negatively charged, experiments were carried out to prevent them hitting the
glass. One method that Edison tried involved placing a second element into the
envelope. He reasoned that if he placed a positive charge on the second
electrode, particles could be attracted away from hitting the glass of the bulb.
Edison experimented with the polarity of the charge on the second electrode and
he noticed that when the second element was made positive with respect to the
filament then a current flowed in the circuit. When the potentials were reversed
he noticed that this did not happen. Edison was fascinated by the effect but
uncharacteristically he did not find a use for it. Even so it became known as
the Edison Effect.
Over the years Edison demonstrated the effect to many other leading
scientific personalities including Preece, a well known British electrical
engineer and more importantly to Ambrose Fleming, the professor of electrical
engineering at University College London. Although no developments were made for
a number of years the seed had been sown for later discoveries.
More Developments
Like Edison, Fleming was also fascinated by the effect and performed some
experiments around the idea. For example in 1889 he had some bulbs made up for
him by the Ediswan Company in the UK. Using these bulbs he reproduced the Edison
Effect, although again this was performed using a steady state charge. It was
not until a few years later that he observed that if an alternating current with
a frequency between 80 and 100 Hz was passed through the bulb, then only one
half of the cycle was passed. In other words it was rectified to produce a
direct current.
At this time there was a lack of understanding about the operation of the
device and this prevented further progress from being made. However the
situation improved when Sir Joseph Thomson discovered that atoms were made from
even smaller particles, one of which was a negatively charged particle, an
electron. Accordingly it was quickly realised that it was electrons that were
being emitted from the heated filament in the bulb, and it also provided the
reason why they were attracted to an electrode with a positive charge.
Fleming's oscillation valve
In addition to his work at University College London, Fleming also acted as a
consultant to the Marconi, who at this time was rapidly increasing the distances
over which wireless signals could be used for communication. For example, in
1901 he made the first transatlantic transmission, and then sought to improve
the performance that could be achieved. Fleming rightly saw that the major
limitation in the sensitivity of the receiving equipment was caused back the
lack of sensitivity of the detector. At the time coherers and magnetic detectors
were used, and both of these instruments were very inefficient.
Fleming decided that he needed to seek ways of improving this situation, and
in November 1904 whilst he was walking along Gower Street in the West End of
London, he had what he called "sudden very happy thought". He wondered if the
Edison Effect could be used to rectify what he called the "feeble to and fro
motions of electricity from an aerial wire". Fleming instructed his assistant to
set up an experiment and to their great exhilaration they were quickly able to
prove that the idea worked.
![The concept of a diode vacuum tube](/images/electronics/diodediag.gif)
Concept of the diode vacuum tube
Fleming called his new invention an "oscillation valve" because it acted in a
similar way to a valve in a pump that allows gas or water to move in only one
direction. He patented the idea that was clearly a major step forwards in
wireless technology. Even though the vacuum tube was still in its infancy it was
still a major improvement over the coherer or magnetic detectors that were
available at the time.
Despite its clear advantage over other detectors, Fleming's oscillation valve
or vacuum tube was not widely used. Valves or tubes were difficult and expensive
to make and their heaters consumed large amounts of power and this had to be
supplied by expensive batteries. Additionally some cheaper devices were
discovered in 1906. Devices that were forerunners of the Cat's Whisker detectors
that were used in crystal sets until the mid-1920s were discovered. In fact two
different patents were filed, one by Ferdinand Braun for a crystal detector
using hydrated crystals of manganese oxide and the other by H. Dunwoody for a
crystal detector using carborundum. These devices had many limitations but they
were very much cheaper than Fleming's oscillation valve and as a result they
were quickly adopted.
The Audion
Even though crystal detectors were very successful, several people continued to
investigate whether they could develop thermionic or vacuum tube technology
whilst avoiding any infringement of Fleming's patent. It was de Forest, an
American who had been working on a variety of areas associated with wireless who
made the next and crucial vacuum tube development. He had been researching
Fleming's diode valve and having investigated the idea he took out some patents
for improvements in 1905 and 1906 where he introduced a third electrode. However
in 1907 he took out a patent for a three-electrode device where the additional
electrode which was placed between the anode and cathode had a fine grid
structure. He called this device his Audion which he used as a leaky grid
detector, not realising its full potential.
It was not until 1911 that the vacuum tube was used as an amplifier. After
this discovery people were quick to try to exploit it. De Forest built an
amplifier using three Audions and demonstrated it to the telephone company A.T &
T. Although the performance was poor they saw its potential and soon started to
build repeaters using vacuum tubes which they had improved. Naturally as soon as
the tube was used as an amplifier, people were quickly able to use it as an
oscillator. Indeed, one of the problems soon encountered was difficulties in
preventing oscillations in view of the high values of grid anode capacitance.
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