Alexander Crichton MitchellFRSE was a Scottish physicist with a special interest in geomagnetics who worked for many years in India as a professor and head of a meteorological observatory before returning to Scotland. He then worked with the Royal Navy to devise a system, known as an anti-submarine indicator loop, for detecting submarines by detecting currents induced in a loop of wire on the sea floor.
In 1890, he went toTrivandrum, in India, where he taught physics at the Maharaja's College He also took up the position of director of the Trivandrum Observatory which had been established by John Caldecott and later headed by John Allan Broun before it fell into disuse. He married Agnes Farquharson Robertson on 2 May 1892 and the couple had three children, all of whom were born in Trivandrum. By 1893 he became a Principal of the college and also served as Director of Public instruction in the State of Travancore. The work involved inspecting schools across the region which he did on motorcycle. He once had an accident in Mavelikkara at a place later called as Mitchell Junction. Resigning from the college in 1912, he returned to Scotland and became a Honorary Research Fellow at Edinburgh University.
Later work
During the First World War, German U-Boats were operational and a strategy was proposed to destroy the British Empire by blocking key ports such as the Firth of Forth. The Royal Society of Edinburgh set up a War Committee that discussed how science could be applied to defence. Mitchell visited the West Pier at Leith in June 1915 and on 1 August he tried a loop of wire at the end of Leith Pier and found that it could detect a passing trawler through the induced current. He later placed the loop horizontally on the harbour floor and found that it was too sensitive but could detect all passing ships. He solved the sensitivity problem by placing the loop in a figure of eight pattern. In 1916, he became the superintendent of the Eskdalemuir Observatory in 1916. In 1922 he headed the Edinburgh office of the meteorological department which was created after the dissolution of the Scottish Meteorological Society. He was the curator of the Royal Society of Edinburgh of the library, and a Vice President from 1926 to 1929. A major publication of Mitchell's was a review and history of the study of terrestrial magnetism published in three parts. His work earned him the Keith Prize for 1931–33.
