What are Indicator Loops and how do they work?

This page gives a very brief overview of the once top-secret anti-submarine harbour defence technology known as "Indicator Loops".
 

Bribie Island, Australia Oban Bay, Scotland Nahant, Massachussetts

Indicator Loops are different to Controlled Mining Loops, Guard Loops, Indicator Nets, Harbour Defence Asdic (HDA) Loops, Boom gates and Boom Cables, all of which are used in harbour defence. If you worked at one of the two-dozen indicator loop stations during WW2, or laid cable for them, or was involved in loop research at HMS Osprey Underwater Detection Establishment from 1914 to 1950, then please contact me:
  
If you have any feedback please email me:

Dr Richard Walding  
Research Fellow - School of Science
Griffith University
Brisbane, Australia
Email: waldingr49@yahoo.com.au

Anti-submarine detection systems
Numerous methods of locating submarines have been developed over the years. As well as time-honoured visual sightings, they include: radar (surface), ASDIC or sonar (underwater, electromagnetic radiation emissions, heat sensing, exhaust analysis, sea lions, pelicans and importantly, in the context of this article, magnetic sensing.

One method relying on magnetic properties is the anti-submarine indicator loop. It relies on the production of an induced current in a stationery loop of wire when a magnet (in this case, a submarine) moves overhead. Even if wiped or degaussed, submarines still have sufficient magnetism to produce a small current in a loop. The technology was developed by the British Royal Navy at HMS Osprey (Portland Naval Base) starting back in 1915. It was sent to various Commonwealth countries for deployment. For example, all of the loop materials used in Australia were of British origin ("Admiralty Pattern") although Australian-made equipment became available from about 1942 onwards. There was a great spirit of cooperation during the war. The RAN operated as part of the RN and there was a continuous exchange of officers as well as unhindered access to RN and USN technology. In the USA, the indicator loops were known as 'loop receiving stations'.
If an indicator loop indicated the presence of a vessel, the two possibilities were "sub" or "non-sub". If no surface ship were sighted usually a ship would be sent to drop depth charges. It had to be an enemy submarine because friendly submarines always entered port surfaced. In several cases, the loops were positioned beside controlled or 'set' minefields in which the mines were connected by electrical cable back to a mine control hut on shore and the mines could be detonated manually.


The first recorded use of indicator loops was at Scapa Flow, in the Orkney Islands at the northern tip of Scotland. Here the Royal Navy stationed its Grand Fleet and on 28th October 1918, German U-Boat UB-116 was destroyed in the controlled minefield at Hoxa Sound with the loss of 34 crew. In his book From The Dreadnought to Scapa Flow, historian Arthur Marder wrote: UB-116 was blown up in one of the loop minefields which were the inner defence. These were lines of mines, each surrounded by an electrical indicating loop. When the observer noticed the tell-tale needle of a loop deflected and saw that there was no surface ship there, he pressed the button and the line of mines went up. The Royal Navy had fortuitously installed these antisubmarine measures in December 1914, not long after the declaration of war with Germany.

Other places where indicator loops were installed include:

  • England: Straits of Dover, Portsmouth, Portland, Plymouth, Falmouth,
  • Scotland: Firth of Forth, Loch Long, Oban Bay, Rosyth and Cumbrae
  • Ireland: Berehaven, Queenstown, St George's Channel, North Channel
  • Other Royal Navy ports: Hong Kong, Singapore, Penang, Alexandria, Malta
  • Australia: Sydney, Darwin, Fremantle, Broken Bay, Newcastle, Moreton Bay, Brisbane River, Port Moresby (protectorate)
  • New Zealand: Auckland Harbour
  • Canada: Saint John, Digby and Prince Rupert
  • United States: The USN set up 'loop receiving stations' mostly in the Casco Bay area near Boston on the East Coast:
  • 1B Bailey Island, Casco Bay (Maine)
  • 1B South Portland at Cape Elizabeth (Maine)
  • 1C North Scituate (Massachusetts)
  • 1D Nahant, East Point (Massachusetts)
  • 1E Gloucester (Massachusetts)
  • 1F Peaks Island/Fort Williams (Maine)
  • 1G Portsmouth (NH)
  • 1I South Westport (Massachusetts)
  • 1X Argentia (Newfoundland)
  • 1X Woody Island (Alaska).
    •  The cable
    A submarine loop is made of a lead-sheathed single core (Admiralty Pattern No. 1989).

    SINGLE CORE LEAD LOADED CABLE - ADMIRALTY PATTERN 1989 (W. T. Henley Telegraph Works Co.)
    This is the cable that actually detected the crossings. No photos or samples are available anywhere in the world. It consists of a single core of 7 strands of 0.029" tinned copper wire covered with three layers of india rubber then a layer of waterproof tape and wound with jute yarn. This is then covered with hessian tape and spirally wound with a soft lead alloy wire. The lead is covered with more waterproof tape, a tarred jute serving, two more layers of hessian tape, 22 steel armour wires (each about 2 mm diameter) covered in lead. Then there is a braiding of dressed hemp yarn wrapped over hot pitch and resin, and finally a preservative coating. Final diameter (1.3") 33 mm. It weighed 6.09 tons per 2000 yard mile in air (6.8 lb per yard).  The cost in 1938 was ₤180 per 1000 yards. In some harbours the Royal Navy used ADM Pattern 13142, a 7-core lead-loaded cable.

     

    Left: The ADM Patt. 9610 was a 4 core, 7 strand, 0.029" cable used for loop tails. It was made by Johnson & Philips (UK) in 1941. It has a diameter of 25 mm. 
    Right: The longitudinal section shows the four central cores insulated with 5mm diameter rubber (3 are white, one black), surrounded by 18 mm diameter rubber bedding. Also visible is the 25 mm wide cotton gauze around the rubber; the central square white rubber core (not visible) and the 22 strand steel armour. This cable was supplied by Johnson & Philips. You can just make out the reversed name transferred on to the white rubber.
     

    How the Loop Works
    The most common loop arrangement was the 'three-legged loop' as shown in the figure below.


    The loop cable was typically 5000 yards long and 200 plus 200 yards wide. The centre leg joined the top cable in a waterproof junction box. In the lower junction box, the centre leg and the two outer legs were joined to the 'tail' to shore. The tail was usually four core, seven strand, the spare core being used for doubling-up the copper wires to reduce resistance. The right outer leg conductor was connected to a "Box, Balancing" (in Navy parlance) which was just a variable resistor used to equalize the resistance of both half-loops. This was located inside the Loop Control Hut on shore.

    MAGNETIC FIELD OF A SUBMARINE
    The two figures below show the Earth's magnetic field (left) and how the magnetic field "threads" through a ship being constructed in the Northern Hemisphere (at say Latitude 50 degrees - perhaps in England, Germany or Japan).

     

    Earth's magnetic field Ship in Earth's field (northern hemisphere)

    The magnetic field threading a submarine being built in the Northern hemisphere may be represented from the side using an arrow (figure, left) or from above using a cross (photo, right):
    A submarine built in the Northern Hemisphere becomes "North-down" Looking from above, the magnetic field of a North-down sub is represented by a cross (X) showing that the field is pointing away from the viewer.

    When a magnetised vessel passes over a conductor, an induced voltage is produced. This is recorded on a paper chart. The diagrams below show the successive stages of a submarine crossing a 3-legged loop from left to right - as depicted from above the submarine.

     


     

    The first figure shows the three-legged loop before a submarine approaches. No voltage is registered on the galvanometer.

     

    As the submarine crosses the outer leg, a voltage is induced (terminal "A" becomes positive with respect to "B" (which is earthed).

         		 As the submarine is in the centre of the outer loop, no voltage is induced as the sub's magnetic field is not cutting any leg of the loops (terminal "A" returns to zero with respect to "B" (which is earthed).

         		 As the submarine crosses the centre leg, a voltage is induced (terminal "A" becomes negative with respect to "B" (which is earthed). However, as the centre leg is common to both loops, double the voltage is produced. It may be easier to think of the centre leg as made up of two wires, one from each loop.
    As the submarine passes the centre of the inner loop, no voltage is induced as the sub's magnetic field is not cutting any leg of the loops (terminal "A" returns to zero with respect to "B". As the submarine crosses the inner leg, a voltage is induced (terminal "A" becomes positive with respect to "B". And as the sub continues to move away to the right from the loops the voltage returns to zero.


    The final waveform looks like this. It is called an inverted "William" Pattern.


    Sample chart from Casco Bay loop station in 1943
     

    The interpretation of a loop signature and some more examples from the United States can be found at my webpage: Reading Loop Signatures.

    The Loop Hut

    The tails from the loop cables entered the hut and were connected to the Box Balancing (No. 2 on the photo below). This was connected to the Box Adjusting (No. 3) followed by the Integrator (inside No. 1). Light from the Integrator shone on to Photo Electric Cells (inside No. 1) whose signal was fed into the Amplifier (no. 5) and on to the Recorder (No. 4). The Recorder was driven by a motor (No. 6). Morse code signals picked up by the loop cables were amplified by the Loop Indicating Signal Apparatus (LISA) (No. 9) and were fed into the LISA Loudspeaker (No. 8) to be heard. Power for the equipment came from the Input Transformer Box (7).

    The Box, Balancing

    The Box, Adjusting

    The Box, Balancing, Pattern No. 2327.
    The box, balancing, is a teak box fitted with a panel on which is mounted a double scale concentric potentiometer and an ebonite block carrying a series of terminals and a double pole change‑over switch. To eliminate thermo-e.m.f.s, the terminals and change‑over switch and the resistance coils of the potentiometer are all constructed of copper.

    The potentiometer resistance has a total value of 294 ohms, made up of 28 steps of 10 ohms and 28 steps of ‑1 ohm. The three upper terminals on the ebonite block are marked "Inner," "Middle","Outer," for connection to the corresponding legs of the loop. The change‑over switch allows the balancing resistance to be inserted in either half of the loop, or the loop to be isolated for testing purposes. A two‑way cable socket marked "A" is fitted in the bottom of the box, which is connected to the adjusting box by a two‑way cable connector labelled "A" which plugs into this socket. The box is secured by three lugs to porcelain insulators mounted on the battens. These lugs lie flat in recesses in the box for transport.

    The Box, Adjusting, Pattern No. 2324.
    The box, galvanometer adjusting, is a teak box fitted with a panel on which are mounted the following components:
    (a) An arrangement of double scale potentiometer and resistances connected to an inert cell, and a change‑over switch for the purpose of injecting known e.m.f.s into the loop circuit. This is known as the e.m.f. injector.
    (b) A single scale potentiometer for shunting the loop. This is known as the shunt.
    (c) An arrangement consisting of a fixed coil and a movable magnet for the purpose of linking known numbers of lines of force with the loop circuit. This is known as the coil injector.

    To eliminate thermo-e.m.f.s all parts which enter into the loop circuit are made ‑of copper or gold‑silver alloy.  Connections to the balancing box and the integrator are made by means of two‑way cable connectors labelled "A" and "F" which plug into sockets marked "A" and "B" respectively.

    The Integrator A/S 91.
    The integrator is in reality a fluxmeter, which may be defined as a moving coil galvanometer specially designed for the measurement of magnetic fields, its deflections being determined solely by the total quantity of electricity discharged through the moving coil and being practically independent of the rate at which the discharge takes place. The special design of the integrator is such that its accuracy for measuring changes in flux linkage in the loop is very high, even when the change takes place very slowly, such as when a vessel crosses at only one or two knots. This means that the size of the signature obtained from a crossing vessel will be approximately the same no matter at what speed the vessel is travelling, since the integrator measures the actual field emanating from the vessel and this does not depend on its speed.

    The constants of the integrator are as follows:
    Periodic time: Approximately 80 seconds.
    Flux sensitivity: 600 lines of force per I millimeter deflection at 100 centimetres scale distance.
    Direct current sensitivity: 12,000 millimetres per microampere at 100 centimetres scale distance.

    The main constructional details of the integrator are shown in Figure 12. A coil of fine copper wire (1) has two mirrors (2) and (3) attached to its upper edge, The upper or main mirror is convex and is used for reflecting a beam of light on to two photo‑electric cells. The lower, or auxiliary mirror, is plain and is tilted slightly upwards ; it reflects light from the same source on to a small scale. The whole is suspended by a robust phosphor bronze strip (4) so that the coil lies between the curved pole faces of a strong permanent magnet (5) and is free to rotate in the small air gap between the pole faces and a cylindrical core (6). The upper end of the suspension strip is soldered to a brass arm (7) mounted on a disc (8) secured to the upper face of an adjusting sleeve (9) clamped in a fixed collar (10). Rotation of the disc permits regulation of the torsion of the suspension strip. The complete upper anchorage assembly is known as the torsion head and is protected by a soft iron cover (11). Current is led into and out of the moving coil by two fine copper spirals (12) connected to two terminals (13) at the base of the instrument, the back terminal being earthed to the casing (14) and marked " +ve " Attached to the upper end of the main mirror is a glass tube (15) across the top of which is mounted a very small magnetised needle (16).

    The needle lies in the uniform field produced by two auxiliary magnets (17) the strength of this field can be adjusted by altering the distance between the magnets by means of the knurled discs (18). The auxiliary magnets are mounted in a soft iron shell (19) closed at the top by the soft iron cover and at the bottom by two semi‑circular soft iron discs (20) bored to give clearance to the adjusting sleeve. The magnets are locked in their correct setting by the set screws (21), and this setting should on no account be disturbed. The shell ‑and its associated magnets, cover, etc. (the whole known as the magnetic head) is rotatable relative to the lower half of the integrator by means of the knurled disc (22) operating the worm (23) and thread (24) ; it is held in place by three collared keep screws (25), which do not, however, prevent its rotation. Rotation of the magnetic head alters the zero of the integrator and is used for zero adjustment should this be necessary on first setting up the integrator on its table.

    A clamp is fitted to provide a means of securing the coil system for transport purposes to eliminate danger of breakage. The clamp consists of a cork pad (26) free to slide clear of the projecting boss of the lower copper coil clip (27) ; the pad is mounted on a movable springy phosphor bronze arm (28), whose other end bears upon an eccentric (29). Rotation of the eccentric by a lever (30) raises the arm and forces the pad to travel upwards until it lifts the coil and just holds it against the core piece of the main magnet. In this position the projecting boss of the upper coil clip (31) just fits into a coned cork pad (32) carried on a fixed brass arm (33), thus preventing vibration of the mirror, glass tube, etc., mounted on the upper edge of the coil. Damage due to possible vibration of the upper end of the glass tube is prevented by a cork ring (34), secured inside the adjusting sleeve, and through which the tube passes with a little clearance. The eccentric is so designed that when locking the coil it has to rotate past its dead centre, which prevents it from unlocking of its own accord during transport. A plain glass window (35) is let into the front of the instrument.

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