THE 'LL' SWEEP FOR MAGNETIC MINES - PART THREE - PRINCIPLES OF SWEEPING [Allocated Title]
Naval instructional film with extensive animated sequences (and a brief dramatised introduction.
A graphic display demonstrates the intensity of the vertical field generated during a LL seep. Location of 'red' and 'blue' fields. Significance of the dead area surrounding the tails of the sweep. Importance of coordinating the ships' speed with the length of the pulse cycle. Increase in the cyclic rate required for double pulsing. Procedure for sweeping in a crosstide. Effect of varying the distance between the sweep ships. Importance of good station keeping and accurate speed - slight errors can leave holidays in the sweep. Means of checking the ordered speed by timing engine revolutions. Degree of permissible error while station keeping a beam. Pulse synchronisation between ships. Connection of the sweep cables on single pulse ships to generate fields of different polarities - graphics show the wiring of a 'blue to starboard' ship. Pulse synchronisation using indicator lights. Manipulation of pulse contacts to correct the synchronism. Use of condenser slave gear. Pulse time controller. Slave gear allows greater time for station keeping, but does not absolve COs from responsibility for proper synchronisation.
The 'Blitz' – from the German term Blitzkrieg ('lightning war') – was the sustained campaign of aerial bombing attacks on British towns and cities carried out by the Luftwaffe (German Air Force) from September 1940 until May 1941.