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Title:THE 'LL' SWEEP FOR MAGNETIC MINES - PART ONE - INTRODUCTION [Allocated Title]
Film Number:ADM 199
Other titles:MAGNETIC MINESWEEPING - THE LL SWEEP [Series Title]
Summary: Naval Instructional Film with graphic sequences explaining magnetic mines and minesweeping theory.
Description: Vital rôle of minesweeping. Simple geomagnetic theory and the basic operating principles of magnetic mines. Explanation of the 'red' and 'blue' - North and South seeking - polarities of ship's induced magnetic fields. Degaussing - overcompensation produces a blue field in the northern hemisphere, and the enemy, anticipating this, laid blue as well as the normal red mines. A brief review of previous methods of sweeping magnetic mines - shipborne electro-magnets, towed skids, airborne degaussing rings - is followed by a dramatic sequence in which a new minesweeping officer is instructed in the essentials of the 'LL' sweep: the streaming of a minesweepers tail electrodes; current pulsing and storage in batteries or impulse units; and the use of two ships pulsing current in opposite directions, thus generating fields of the same polarity between their respective tails. The advent of blue mines necessitated double pulsing to ensure that the area between the cables was swept alternately by each polarity. An explanation of basic terms such as pulse length, pulse interval and cycle, precedes a graphic display, tinted to show the successive blue and red fields generated in a double pulsed sweep.