Anti-submarine Indicator Loops
At the beginning of the first World War, Scapa Flow was poorly defended. Block ships were sunk to deter submarines from entering the Flow and some guns had been landed and placed on temporary batteries covering both Hoxa Sound and Hoy Sound. Patrols made by destroyers surveyed the main entrances into the flow.
The most immediate threat facing the interned Home Fleet was from submarines. In September 1914 there was a major panic when it was thought that a U-boat had found its way into Scapa Flow. Although a false alarm, it brought the lack of defences in this area to the admiralty’s attention. Something needed to be done.
Induction loops were early warning devices to alert personnel to the presence of submarines. These were lines of mines and each was surrounded by an electrical indicating loop. If a submarine drew near, the loops picked up the changes created in the magnetic field and the changes would be noted by an operator watching a dial in the station on the shore. He would press the button and the lines of mines would explode. Thus alerted, a warning would go to gunners and searchlight operators, as well as to any ships in the area.
Induction loops were positioned across the entrances of Hoxa and Switha Sounds and the western approaches to Hoy Sound.
The first recorded use of these loops was in Scapa Flow. On 28 October 1918 a German submarine, the UB116, was detected by this method, attempting to enter Hoxa Sound and was blown up.