An anti-submarine indicator loop was a submerged cable laid on the sea bed and used to detect the passage of enemy submarines. The first practical application of the science was in the Firth of Forth in August 1915, by the Scottish physicist Alexander Crichton Mitchell, with the help of the Royal Navy research establishment at HMS Tarlair (Aberdour). Unfortunately his report to the Board of Investigation and Research was misunderstood and his findings rejected as of no value. The scientist William Bragg of BIR was pursuing related research in the BIR, but as BIR was independent of Royal Navy control, which the navy resented, he faced significant obstacles, even when he moved to HMS Tarlair. Bragg moved to the BIR experimental station at Harwich, Essex, England. Re-examination of Mitchell's work, at Bragg's suggestion, in 1917 renewed interest in loops, leading to the successful development at Harwich of an operational system by mid-1918. Loops were extensively used by the Allies during World War II to protect harbours against submarine attack.
They worked as induction loops – the submarine's magnetism induced a current in the cable as the submarine passed across it. The first operational use was at the Grand Fleet's anchorage at Scapa Flow. The German submarine UB-116 was detected by hydrophones at 21:21 on 28 October 1918 attempting to enter the harbour and anchorage via Hoxa Sound. Two hours later (at 23:32) current was detected in an indicator loop laid in a remotely controlled minefield, induced by the submarine as it passed over the cables. Mines around the loop were detonated by remote control, sinking the submarine. It was the last U-boat destroyed by enemy action before the Armistice. After the war, indicator loop devices were further developed by the Admiralty's research divisions at HMS Vernon and HMS Osprey (Portland Naval Base). In WWII indicator loops were used by the Allies for harbour defence in the UK and its dominions and protectorates, as well as by the US Navy.