Alexander Crichton Mitchell

Alexander Crichton Mitchell FRSE (1 July 1864 – 15 April 1952) 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.

Alexander was born in Edinburgh to James Mitchell and Isabella Mitchell née Crichton and studied physics at Edinburgh University before going to Trivandrum in 1890 where he taught physics at the Maharaja's College there. 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. 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. Retiring in 1912 he returned to Scotland and was a Honorary Research Fellow at Edinburgh University initially and later became superintendent of the Eskdalemuir Observatory in 1916 and in 1922 he headed the Edinburgh office of the meteorological department which was created after the dissolution of the Scottish Meteorological Society.

Around this time, German U-Boats became 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.

A major publication of Mitchell was a review and history of the study of terrestrial magnetism published in three parts.

He married Agnes Farquharson Robertson (d. 1948) on 2 May 1892 and they had three children who were born in Trivandrum. He was elected Fellow of the Royal Society of Edinburgh in 1889 and was curator of the library for long and a Vice President from 1926 to 1929. His work on the variations in the terrestrial magnetic field earned him the Keith Prize for 1931-33.