Sir Edward Victor Appleton
- the man interested in radio who proved the ionosphere existed and
enabled an understanding of world wide radio signal propagation
Sir Edward Victor Appleton was one of the key figures of the
twentieth century who contributed to the knowledge of radio and the ionosphere
and hence improved our knowledge of the way in which radio waves propagate in
the HF portion of the spectrum. Edward Appleton received a Nobel prize in 1947
for his work, the technique for which laid the foundations for the development
of radar. He became Sir Edward Appleton when he was knighted in 1941.
Early years
Edward Victor Appleton was born on 6th September 1892 in Bradford England. The
city with is in Yorkshire was famous for its wool mills and was a centre of
industry. Edward Appleton was the son of Peter and Mary Appleton.
The young Appleton received his early education at Hanson
Grammar School in Bradford. Initially he showed little interest in anything
apart from music and cricket, although at the age of 18 he won a scholarship to
St John's College, Cambridge University where he studied under famous names
including Sir J J Thomson and Lord Rutherford. Appleton was very successful and
not only did he win prizes for his work and ultimately gained a first class
degree in Natural Sciences.
Outbreak of War
With the outbreak of the First World War, Edward Appleton joined the armed
forces, initially with West Riding Regiment, but later transferring to the Royal
Engineers. While in the army, he trained on the relatively new technology of
radio or as it was called then "wireless". This obviously interested him
considerably because after the cessation of hostilities he returned to Cambridge
in 1920 and took up research on radio waves. Here Appleton started as an
assistant demonstrator of physics under J J Thomson. He soon developed an
interest in wireless or radio valves, as well as in the propagation of wireless
or radio signals.
Research begins
In 1924 Edward Appleton was appointed Professor of Physics at King's College of
London University. He held this post for 12 years and it was during this time
that he undertook much of his work on what was termed the Kennelly-Heaviside
layer. This was a layer in the upper atmosphere that reflected radio signals,
enabling the radio signals to be heard over great distances. This work was to
lay not only the foundations for much of our knowledge of the ionosphere, but
also for the later development of radar.
Much of the work Appleton undertook at Kings was based at
their campus on the Strand in London. However his experiments caused
interference to many others in the locality, and eventually his work was
transferred to another Campus opened by the college in Hampstead in the outer
suburbs of London. There was more space in this area and fewer radio users.
Accordingly the interference to others was kept within acceptable limits.
The idea of a layer in the upper reaches of the atmosphere
that could reflect radio signals had been postulated for some years. In 1901
Marconi made the first transatlantic radio transmission and this made it obvious
that there must have been some mechanism to "bend" the radio signals. Then in
1902 Oliver Heaviside and A.E.Kennelly independently postulated the idea of the
presence of a conducting layer. This was termed the Kennelly-Heaviside Layer.
Additionally Appleton had observed that the strength of the
radio signal from a transmitter a on a frequency such as the medium wave band
and over a path of a hundred miles or so was constant during the day but varied
during the night, rising and falling in a regular manner. This lead to him to
believe that it was possible that two radio signals were being received, one
traveling along the ground, and another reflected by a layer in the upper
atmosphere. The fading or variation in strength of the overall radio signal
received resulted from the interference pattern of the two signals. The
variation, he postulated, was caused by small changes in the reflecting medium
causing the path length to change and hence the way in which the two radio
signals interfered. Sometimes this would be constructive interference when the
two radio signals would add together, and at other times it would be destructive
when the two signals would tend to cancel each other out.
Appleton used the British Broadcasting Corporation (BBC)
radio broadcast transmitter at Bournemouth England and transmitted a signal
towards the upper layers in the atmosphere. He received the radio signals near
Cambridge, proving they were being reflected. By making a periodic change to the
frequency of the broadcast radio signal he was able to measure the time taken
for the signals to travel to the layers in the upper atmosphere and back. In
this way he was able to calculate that the height of the reflecting layer was 60
miles above the ground. The technique he used is now known as frequency
modulation radar, and the layer in the ionosphere was the first item to be
located using a radar technique.
Appleton realized that the reflections in this experiment
could have conceivably been caused by reflections from distant hills or other
objects, although if this were so it would not explain the fading of the radio
signals that was observed. To ensure that this was not the case, he repeated the
experiments some months later but used a directional radio antenna, thereby
proving that the reflected signal was indeed emanating from the upper reaches of
the atmosphere. In this way he eliminated any doubt about the mechanism of the
way in which the radio signals propagated.
Further researches
Appleton continued his work on the ionosphere and in 1926 discovered a further
layer above the lower Kennelly-Heaviside layer. This one at an altitude of
between 250 and 350 kilometres was named the Appleton layer.
Later Appleton refined his method of measuring the height and
nature of the ionosphere using a transmitter that sent out pulses of energy. The
results of the reflected signals could then be displayed on an oscilloscope,
giving a visual image.
Appleton found that the more he was discovering about what
was now termed the ionosphere was posing more questions. For example it had been
noticed that the ionosphere varied over time, but it was not understood what
caused the changes. Accordingly Appleton continued his researches. A solar
eclipse on 29th June 1927 provided a unique opportunity to investigate the
effect of the Sun on the ionosphere. He discovered that as soon as the Sun was
hidden by the Moon, the effective height of the Appleton layer increased. This
suggested that the Sun had a direct effect on the layer and that radiation from
the Sun was required to ionise the upper atmosphere. From this and other
researches, the Appleton-Hartreee equation was developed. This showed that the
charges that caused the "reflection" were free negatively charged electrons.
There were other discoveries Appleton made. He found that the
height of the ionospheric layers was affected by the Moon as well as the Sun,
and that they were strongly influenced by the earth's magnetic field and linked
tot his Appleton discovered that the polar blackouts were caused by magnetic
storms.
Career
Edward Appleton was clearly a very gifted research scientist. After undertaking
much of his research at the University of London between the years of 1924 and
1936. During this period he was elected Vice President of the American Institute
of Radio Engineers. After his time at London University he took up the post of
Professor of Natural Philosophy at Cambridge University between 1936 and 1939.
After hostilities broke out in 1939, Appleton was appointed as the secretary of
the Dept. of Scientific and Industrial Research. In this position he had
considerable responsibility in defining the scientific research that was
undertaken within Britain.
War years
One of the main achievements of the early war years was radar which was used for
the early detection of aircraft. This was based on the original scheme used by
Appleton for the detection of the ionospheric layers. The development of the
radar system was undertaken by Robert (later Sir Robert) Watson-Watt. He stated
that had it not been for the work undertaken by Appleton, then radar would have
been developed too late for use in the Battle of Britain.
As a result of his work, Appleton was knighted in 1941. He
also became a member of the Scientific Advisor Committee of the War Cabinet. In
this capacity he was one of the committee that advised the War Cabinet on the
feasibility of creating an Atomic Bomb.
Despite the heavy work load associated with his position in
the Scientific Advisor Committee and with his involvement in the development of
the Atomic Bomb, Appleton still found some time to continue with his researches
into radio signal propagation and the ionosphere. He also discovered that the
condition of the ionosphere was heavily dependent upon the number of sunspots on
the sun, and when working with Dr. J S Hey he discovered that these sunspots
were powerful emitters of radio signals.
Awards
In view of the tremendous contributions Edward Appleton had made to the
advancement of radio and science in general he was given many awards. He was
awarded the Nobel Prize for Physics in 1947. He was also awarded the Medal of
Merit, the highest civilian award given by the United States. France made him an
Officer of the French Legion of Honour, and Norway, the Norwegian Cross of
Freedom, and in 1948 the Pope appointed him to the Pontifical Academy of
Science.
These are but a few of the awards given to Sir Edward
Appleton. However in 1949 he moved to the University of Edinburgh to become
Principal and Vice-Chancellor, a position that he held for the rest of his life.
However Appleton died on 21st April 1965
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