Anti-submarine indicator loop

In the first years of World War I submarines were fearful, one-sided weapons because they were invisible. In July 1915 Arthur Balfour replaced Winston Churchill as First Lord of the Admiralty. Balfour appreciated the importance of science, so he set up a Board of Invention and Research (BIR), made up of a three-man central committee supported by an eminent consulting panel.[1] The remits of Section II of the panel included submarines, its members included the eminent physicists Ernest Rutherford and William Henry Bragg.[2] [3] The panel concluded that the most promising approach was to listen for submarines, so they pushed work to improve hydrophones. Soon Bragg moved to the hydrophone research centre HMS Tarlair at Aberdour on the Firth of Forth (which later relocated to Harwich in Essex).

Underwater loop configuration

Independently from the BIR, in August 1915, a submerged cable was laid on the seabed of the Firth of Forth.[4] The idea originated with the Scottish physicist Alexander Crichton Mitchell, who was helped by the Royal Navy at HMS Tarlair.[5] He had shown that the passage of a submarine past a cable formed an induction loop which induced a voltage of a millivolt or so that could be detected by a sensitive galvanometer. However, voltages were also induced in the cable by random fluctuations in the earth's magnetic field and electrical noise from the Glasgow tram lines, so Mitchell installed an identical loop outside of the channel for vessels, the two loops were connected so that the random fluctuations cancelled each other out. A rheostat was used to give the two loops identical resistances, so that no current flowed until a vessel approached. Unfortunately his report to the BIR was misunderstood and his findings rejected as of no value.[6] Consequently, there was a hiatus in the installation of loops until their utility was demonstrated beyond question; then with Bragg's leadership a number were installed.[7] Later in World War I the tiny induced voltages were amplified by vacuum tube amplifiers. Even with this assistance a long loop installed to monitor traffic in the English Channel proved impractical.

The "Liverpool Cable" used for the loops consisted of four core, single strand 1.23 mm copper wire sheathed in 2-layer rubber insulation of diameter 3.7 mm that was wrapped in jute identification tape. The cores are separated by five strands of 36-thread cotton serving, wrapped in two layers of linen identification tape, all encased in a 12.8 mm diameter lead sheath that was wrapped in 18 strands of tarred hemp serving and armoured with 26 strand 2.0 mm steel wire to a final diameter of 18.8 mm.[8] The cores were wired together when the cable was used for a loop.

A notable operational use of a loop was at the Grand Fleet's anchorage at Scapa Flow.[9] The German submarine UB-116 captained by Lieutenant JJ Emsmann — who along with his crew had volunteered for a suicide mission — was detected by hydrophones at 21:21 on 28 October 1918 while entering the harbour via Hoxa Sound. There were no allied vessels in the harbour, therefore the indicator loops on the minefields were activated. 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 cable. Activation of the loop detonated mines in the field, which sank the submarine.[10] It was the last U-boat destroyed by enemy action before the Armistice, ironically when it had no prey. The wreck of UB-116 was raised in 1919 but floundered while being towed and its broken-up scraps fell back onto the seabed, where now they are popular viewing for scuba divers.[11]

After the First World 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.[12][13] For example, the Hoxa channel into Scapa Flow was provided with two guard loops followed by eight mine loops in echelon.[14]

An indicator loop gave the first warning of the 1942 attack on Sydney Harbour, when it detected the midget submarine M-14, but this was ignored owing to civilian traffic in the area. The submarine was soon afterwards spotted visually, after it became entangled in a submarine net and its bow broke the surface.

References
  1. Van der Kloot, William (2014). Great Scientists wage the Great War. Stroud: Fonthill. pp. 93–128.
  2. Andrade, E. N. de C.; . Lonsdale, K (1943). "William Henry Bragg, 1862-1942". Biogr. Mem. Fellows R. Soc. 4 (12): 277–300. doi:10.1098/rsbm.1943.0003.
  3. Van der Kloot 2014, pp. 129-161.
  4. Maxwell, Diana (2014). Listen Up. Aberdour: Aberdour Cultural Association. p. 58. ISBN 978-0-9929470-1-9.
  5. Mitchell, A C. On the vertical force changes during the "sudden commencement" of a magnetic storm. Proceedings of the Royal Society of Edinburgh Vol. 45, no. 26 (1925) pp. 297-301.
  6. Walding, R 'Bragg and Mitchell's Antisubmarine Loop', Australian Physics, 46 (2009), 140-145. Available online at https://www.academia.edu/6720520/Bragg_and_Mitchell_s_Antisubmarine_Loop
  7. Walding 2009, pp. 140-145
  8. Walding 2009, pp. 144
  9. Maxwell, Diana (2014). Listen Up. Aberdour: Aberdour Cultural Association. ISBN 978-0-9929470-1-9.
  10. Lecane, Philip (2005). Torpedoed! The R.M.S. Leinster Disaster. Periscope Publishing Ltd. p. 92. ISBN 978-1-904381-29-7.
  11. "UB.116". Submerged - Shipwrecks And Scuba Diving Around Devon And The World. Retrieved 2008-06-10.
  12. "Indicator Loops around the World". Indicatorloops.com. Archived from the original on 25 February 2009. Retrieved 2009-03-01.
  13. "What are Indicator Loops and how do they work?". Indicatorloops.com. Retrieved 2008-06-10.
  14. Hewison, W. S. (2002). This great harbor Scapa Flow. Edinburg: Birlinn. p. 243.
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