Machine 43Make a magnetic compass← → step through the guidechart · north arrow · iron 15 cm west
Box turned000°
Needlenorth
Card reads000°
Iron15 cm
Swing2.4 s
The north end of each magnetised needleParts already in placeShen Kuo, writing in 1088, magnetised steel needles by rubbing them on lodestone, floated them or hung them from a single silk thread fixed with wax, and judged the hung needle best; he also saw that it points a little off true north and south. The first record of a compass at sea is from 1117. A magnetised needle swings about north like a pendulum, T = 2π√(I / mB): for a 6 cm steel needle, with the Earth's horizontal field taken as 20 µT (typical at mid-latitudes) and an assumed magnetisation of 4 × 10⁵ A/m, T ≈ 2.4 s, and friction at the jewel is assumed to damp it with a ratio of 0.25. Iron is modelled as pulling the nearer end of the needle with a pull that falls with the cube of distance and is assumed to equal the Earth's at 9 cm for this 8 cm bar; where it rests, 15 cm due west and lying north–south, it pulls both ends of the needle alike and does not turn it. Drawn at 1 unit = 2.5 mm, a tenth of the other guides' scale, and from 35° up so the card and the water read. This is a guide to how the parts go together, not engineering drawings. Check it against a known bearing, correct for your local declination, and keep iron, steel tools and magnets well away from it when you read it.
