Saturday, September 17, 2005

Sir Edward Appleton (1892 - 1965)

Born in Bradford, Appleton initially showed little more than a passion for music and cricket. However, aged 18 he won a scholarship to Cambridge, where in 1913 he received a first class degree in Natural Sciences. The outbreak of war in August 1914 saw him join the Royal Engineers; while there he was trained in Marconi's recent invention, radio technology.
After the war, he returned to Cambridge. He started work under J.J. Thomson, the discoverer of the electron, before becoming an assistant demonstrator of physics. His interest centred upon the way valves created radio waves, the problem of atmospherics and interruptions of radio signals, and the propagation of radio waves.
In April 1924 he began research into the strength of the radio signals received at Cambridge from the BBC station in London. He soon discovered that the strength of the signal was constant during the day but varied during the night, rising and falling in an almost regular manner. He conjectured that, at night, the Cambridge apparatus was receiving not one but two waves, one travelling directly and the other being reflected by the atmosphere. The fading was a result of the interference.
The existence of a reflecting layer had first been postulated around forty years earlier by Balfour Stewart, who suggested that the small daily changes detected in the earth's magnetic field might be due to electric currents in the upper atmosphere. Marconi had then demonstrated that, as radio signals could be sent from Cornwall to Newfoundland, something must cause them to 'bend' around the earth. In 1902 Oliver Heaviside and A.E.Kennelly had independently postulated the theory of a conducting layer of the atmosphere: the Kennelly-Heaviside Layer.
Appleton conducted an experiment to prove whether the Heaviside layer existed, and its height. Using the BBC transmitter in Bournemouth, he moved the receiver from Cambridge to Oxford so that the strength of the signal along the ground was comparable to the reflected wave. At midnight on 11th December 1924 the BBC in Bournemouth emitted the first of the special signals. It was soon clear that the experiment was working. There was some natural fading, but they deduced from the interference patterns that the height of the reflection was approximately 100 kilometres. To check that the signal was not being bounced off hills, Appleton returned some months later with highly directional antennae and showed that some of the signal was indeed coming down from the sky. Thus the ionosphere was the first 'object' detected by radio-location, a technique that developed into the radar under Robert Watson-Watt.
However, the experiments demonstrating the existence of a reflecting layer produced more questions than answers: what was the nature of the layer, did it stretch all round the earth, did it change during the day or seasons, and did its height vary? During experiments designed to tackle some of these questions, Appleton discovered another layer 250-350 kilometres high. He found that this layer reflected back shorter wavelengths in daytime as well as at night, and that they were reflected back with greater strength than the Heaviside layer. Appleton realised that this was the layer responsible for reflecting short wave radio round the world - 'the Appleton layer' - that now enables communication with Australia and America.
In 29th June 1927, an eclipse provided an opportunity to study the effect of sun on the ionosphere. As soon as the rays of light were cut off, the height of the reflecting layer increased, suggesting that the sun's radiation was required to create the ions in the atmosphere that reflected the radio waves. The resulting Appleton-Hartree equation showed that the charges that actually caused the reflection consist of free electrons.
Appleton went on to discover that the moon as well as the sun affected the height of the layers, and further showed that the layer was strongly influenced by the earth's magnetic field and that the polar blackouts were caused by magnetic storms. Knighted in 1941, in 1947 he was awarded the Nobel Prize for physics and, two years later, moved to the University of Edinburgh to become Principal and Vice-Chancellor, a position that he held for the rest of his life.


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