561,455. Signalling the proximity of submarines. COOPER, S. E., and PALMER, W. A. March 6, 1941, No. 3055. [Class 118 (ii)] A minefield for the defence of harbours and the like against underwater attack comprises a forward zone of detectors G1-7, Fig. 1, with a corresponding indicator for each detector at a control point D, an intermediate zone of mines A1-7 explodable as desired from the control point, and a rearward zone of detectors G8-14 with each detector also connected to the control point. The forward detectors serve to indicate the approximate position of a submarine as it crosses the zone, and the inner detectors indicate the further course of the submarine, thus enabling an operator to explode the mine or mines nearest the submarine. Each detector comprises an electrical soundsensitive device such as a microphone, and each detector in a row is connected to one of a number of contacts M, Fig. 2, arranged in a circle. A revolving arm L sweeps over the contacts and puts each detector in turn in circuit with an indicator, such as a sound reproducer O. The operator, by listening to or observing the indicator, can decide which detector is closest to the submarine. The mines may be controlled also by contact devices.
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