Summary: Training film on the origins, construction and uses of two types of RAF magnetic compass, 1920.
Description: The film starts coherently, but degenerates progressively into pullbacks and misplaced scenes until the later reels are not always comprehensible. There is virtually no actuality material, all the points being explained by diagrams, models and workbench demonstrations. (Reel 1) "Your friend the compass", explains the cardinal and quadrantal points and the lubber line. The fact that a compass is a magnet is dealt with in some detail, and the principles of magnetism outlined (Reel 2) by means of bench demonstrations with iron filings and soft iron by an RAF tutor. (Reel 4 - there is no Reel 3) A magnet dips with the earth's magnetic field, and that magnetic variation is noted and dated on every chart. A Dip Circle is used to measure the angle of dip, which at London is 67 degrees. This dip affects the compass needle. The difference between the magnetic and true meridian is explained, along with the angle of variation between the two and the fact that the line of no variation is the Agonic Line. A map projection of the earth gives a double Agonic Line. The variation at London in 1920 is 14 degrees west, that of Paris 13 degrees west. The magnetic pole moves slightly so that "about 1976" the variation at London will be zero. The date and change in variation is used to correct the figure for magnetic north given on a chart. (Reel 5) The construction of the RAF Flat Type 253 Aero Compass is shown, compared to a standard ship's compass, and the evolution of the compass from the ancient type through to the present day. (Reel 6) More of the construction of the Type 253, followed by the construction of the RAF Upright Type 5/17 Aero Compass. The general principles of compass construction are described. A compass card (or needle) oscillates slightly in the manner of a pendulum swing. The Type 253 has an oscillation period of 25 seconds, the Type 5/17 an oscillation period of 12 seconds. The card should be allowed to oscillate gently in flight as attempts to correct each oscillation only produce a zig-zag flight course which increases, rather than decreases, the swing of the compass. (Reel 7) In a tight turn the aircraft floor tilts to be almost perpendicular to the ground, whereupon the compass acts as a Dip Needle and swings into the line of dip. Not until the aircraft has flattened out does the compass respond properly to the direction of the turn. On turning east or west from a southerly course the compass exaggerates the extent of the turn. On turning east or west from a northerly course the compass fails to show the turn until it is complete. The film ends abruptly at this point.
