World of Scientific Discovery on Edward Victor Appleton
When Guglielmo Marconi (1874-1937) made his remarkable transatlantic Morse code transmission on December 11, 1901, the world was stunned. Several experts had said such a thing was impossible.
Radio waves, like light waves, are a part of the electromagnetic spectrum. Like light waves, they should travel in a straight line, making their transmission beyond the curvature of the earth impossible. At the time it was believed that transmission as far as 180 miles (300 km) was the absolute limit. Yet Marconi had been successful; the question now was why. Arthur Kennelly (1861-1939) and Oliver Heaviside (1850-1925) had theorized that the upper atmosphere, or ionosphere, might contain a layer of charged particles that could reflect radio waves. The existence of a Kennelly-Heaviside layer was ultimately proven by Edward Appleton. He was born on September 6, 1892, at Bradford, Yorkshire. After graduation from Cambridge, he served as a radio officer with the Royal Engineers in World War I, which sparked his interest in radios. Appleton noticed radio signals faded out at night and, after the war, he decided to find out why.
Commercial broadcasting began in England in 1922, giving Appleton numerous signals with which to experiment. He surmised that the fading could be caused by reflections of the signal off the upper atmosphere. This would cause the signals to reach the receiver slightly out of phase, causing them to cancel each other.
Using a transmitter and receiver that were located 70 miles (112 km) apart, Appleton, aided by Miles A. Barnett, was able to tune various wavelengths and discover the minimum height of the source of the reflection in 1924; the Kennelly-Heaviside layer turned out to exist at an altitude of nearly 62 miles (100 km). This layer dispersed at dawn, but Appleton discovered yet a higher source of reflection. In 1926 he was able to determine this source was nearly 155 miles (250 km) high. Not surprisingly, this layer became known as the Appleton layer.
Further study showed that the behavior of the Appleton layer was directly related to solar activity in general, and sunspots in particular. Charged particles emitted by the sun interact with the Earth's upper atmosphere. Because there is a high concentration of ions at this level, it seemed logical to rename it the ionosphere, a name first suggested by Appleton's colleague Robert Watson-Watt. The ionosphere is transparent to light waves, but it bends radio waves, making it directly responsible for Marconi's triumph.
Appleton was later involved in the development of radar and the atomic bomb during World War II, and he was awarded the Nobel Prize in physics in 1947. On April 21, 1965, he died in Edinburgh.
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