China's compass began as a Han-dynasty divination tool
A compass shows cardinal directions, usually via a magnetized needle aligning with Earth's magnetic field. Degrees often run clockwise from north at zero. Among China's Four Great Inventions, magnetic pointing began for divination by the Han era from about 206 BCE before guiding travelers.
The needle feels a twist from the horizontal part of Earth's magnetic field, which swings its north end toward the magnetic pole. Maps, though, are drawn to true north, the axis of rotation, and the angle between the two, called declination, varies from place to place, so most maps print the local value. Near the magnetic poles declination grows so large that the needle simply drifts. Iron and steel, ore deposits, electric motors, current-carrying wires and MRI machines all pull it off course, and in cars or aircraft speeding up, slowing down or turning from east or west makes the reading lag or lead.
Thales of Miletus, in the 6th century BC, was credited by the Greeks with noticing that lodestone attracts iron, and the word magnet may come from lodestones found at Magnesia in Anatolia. India's Sushruta Samhita describes using lodestone to pull arrows out of wounds. China's Guiguzi mentions magnetism in the 4th century BC, the Lüshi Chunqiu states plainly that lodestone attracts iron, and the Lunheng of the 1st century AD notes that it draws a needle. Geomancers meanwhile spun lodestone spoons on bronze plates to point south.
Shen Kuo described navigational compasses of lodestone-rubbed iron needles by 1088, under the Song dynasty. Western Europe records one around 1190 and the Islamic world in the 13th century; dry compasses spread in both around 1300, and liquid-filled ones replaced them in the early 20th century. Some historians argue for an iron fish floating in oil in 4th-century southern India, though others find evidence before the 11th century very thin.
Modern hikers' baseplate compasses pair a fluid-damped needle with a rotating bezel and a transparent base for map work. The US Army's M-1950 lensatic model instead damps its card by electromagnetic induction, tolerates 8 degrees of tilt, and glows from 120 millicuries of radioactive tritium. Ships' compasses traditionally split the card into 32 points called rhumbs and sit in gimbals so they stay level at sea.
Source: Compass
