The existence and location of an ionized reflecting layer in the Earth's atmosphere was identified by Edward Appleton, whose research responded to the first long distance radio messages transmitted across the Atlantic Ocean. After Guglielmo Marconi made his successful transmission in 1901, two physicists, Oliver Heaviside and Arthur E. Kennelly postulated the existence of an ionized band inthe atmosphere. More than twenty years later, Appleton, then Wheatstone Professor of Physics at King's College at the University of London verified that hypothesis and later discovered two more layers, one of which is now named in his honor. For his work on atmospheric structure, Appleton was awarded the 1947 Nobel Prize in physics.
Edward Victor Appleton was born in Bradford, Yorkshire, England, on September 6, 1892. His parents were Peter and Mary (Wilcock) Appleton. Appleton attended Barkerend Elementary School from 1899 to 1903 and Hanson Secondary School from 1903 to 1911. He gave evidence of a brilliant future at an early age, passing the entrance examination at the University of London with first class honors at the minimum age permitted of 16. Two years later he won a scholarship at St. John's College, Cambridge, from which he graduated, again, with honors in physics in 1913. Upon graduation, Appleton continued his studies in crystallography under William Henry Bragg , but his work was interrupted by the outbreak of World War I. In August of 1914, he enlisted in the British infantry and eventually rose to the rank of captain in the Royal Engineers. After his discharge, Appleton returned to Cambridge and became first a fellow at St. John's College and later demonstrator in physics at the Cavendish Laboratories. His major area of interest was vacuum tubes, a topic he first learned about during the war. Some years later, he was to publish a monograph on this research, Thermionic Vacuum Tubes.
In 1924, at the age of 32, Appleton was appointed Wheatstone Professor of Physics at King's College at the University of London. The first major research topic to which he turned his attention was the propagation of radio signals. While serving his country, he had become aware of the problem that radio signals have--a tendency to fade out at various times of the day. That same year, assisted by graduate student Miles Barnett, he designed and carried out a series of experiments to elucidate the mechanism by which radio waves are transmitted in the atmosphere.
Appleton and Barnett were able to convince the British Broadcasting Corporation (BBC) to allow them to use the network's radio signals outside of regular broadcasting hours. They varied the frequency of the BBC signal on a regular basis, looking for points at which interference occurred between direct waves from the ground and reflected waves from the atmosphere. Their results indicated the existence of a reflecting layer in the atmosphere approximately 60 mi (100 km) above the Earth's surface. The layer corresponded to a region that had been predicted more than twenty years earlier by Britain's Oliver Heaviside and American physicist A. E. Kennelly. That region was later named the Heaviside-Kennelly, or "E," layer. It is now known to be the lower boundary of the ionosphere.
Appleton continued his research on the atmosphere throughout his life. In 1926, he discovered a second reflecting layer above the E-layer, established as the F- or Appleton-layer. He calculated the lower boundary of this region to be about 150 mi (230 km) above the Earth's surface. The discovery of these reflecting bands in the ionosphere made possible later developments in radio, shortwave, and radar technologies. Appleton's research also led him to discover a number of important characteristics of the reflecting layers. In 1927, for example, his observations of a solar eclipse convinced him that the existence of such layers are the consequence of solar radiation bombarding the Earth's atmosphere.
Investigative studies on the atmosphere brought Appleton a number of honors. He was elected a fellow of the Royal Society in 1927, was knighted in 1941, and received the Nobel Prize in physics in 1947 for "his work on the physical properties of the upper atmosphere, and especially for the discovery of the so-called Appleton layer." In addition, he was awarded many honorary doctorates, including those from the universities of Aberdeen, Glasgow, London, Oxford, and Cambridge.
In 1936, Appleton succeeded to the Jacksonian Chair of Natural Philosophy at Cambridge, left vacant two years earlier by the retirement of Charles Thomson Rees Wilson. He brought with him most of his research staff from King's College, began the construction of a new field laboratory for atmospheric research, and initiated an active program for the study of ionized gases at the Cavendish Laboratories at Cambridge.
Appleton remained at Cambridge for only three years, however, before accepting an appointment as secretary of the Department of Scientific and Industrial Research (DSIR). In that post, he rapidly converted the DSIR from a civilian research agency to one making preparations for the war then looming on the horizon in Europe. Included among the DSIR's later military activities was Great Britain's first research on nuclear weapons. At the war's conclusion, Appleton returned to academia, becoming Principal and Vice-Chancellor of the University of Edinburgh, where he remained until his death on April 21, 1965.
Appleton married Jessie Longson in 1916, with whom he had two daughters, Marjery and Rosalind. Lady Appleton died in 1964 from a stroke suffered three years earlier. A month before he died, Appleton was married for a second time, to Mrs. Helen Allison, his secretary of more than 13 years. His biographer in Biographical Memoirs of Fellows of the Royal Society, J. A. Ratcliffe, says that Appleton was known for many admirable qualities, among which were "his wide humanity, his ability as a public speaker, and his continuing dedication to his scientific researches."
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