Compass

• 1. Introduction

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  A compass is a device that shows the cardinal directions, but its oldest surviving ancestor was never meant to guide anyone anywhere. In the second century BC, Chinese geomancers set a spoon carved from lodestone onto a smooth bronze plate, and the spoon would invariably swing around to rest on a north-south line. That divining tool predates the first ships steered by magnetism by more than a thousand years. So how did a device built for fortune-telling become the instrument that carried explorers across oceans, guided armies through open country, and eventually shrank to fit inside a wristwatch or a phone? And why, in an age of satellites, does anyone still trust a magnetized needle at all?

• 2. Origins and Early History

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  The earliest known reference to a lodestone's pull on iron came from Thales of Miletus, a Greek philosopher of the sixth century BC whom the ancient Greeks credited with discovering the attraction between lodestones. The name magnet itself may trace back to lodestones found at Magnesia, in Anatolia. In India, the medical text Sushruta Samhita describes surgeons using a lodestone's magnetism to remove arrows embedded in a wounded body.

  China's literary trail runs even further back. The fourth-century BC Book of the Devil Valley Master contains the earliest Chinese reference to magnetism, and the second-century BC chronicle Lüshi Chunqiu states plainly that the lodestone attracts iron. A text composed sometime between 20 and 100 AD, the Lunheng, records the first mention of a needle drawn toward a lodestone, decades before geomancers began experimenting with the material for divination.

  Iron needles, magnetized by striking them against a lodestone, became the first true navigational compasses in China by 1088, during the Song dynasty, as the scholar Shen Kuo described. Sailors in Western Europe first recorded using a compass around 1190, and the Islamic world adopted the instrument by the 13th century. Dry compasses, sealed in a box rather than floated in water, began appearing around 1300 in both regions. Some historians point to a 4th-century vessel in Southern India that used an iron fish floating in oil to find north, though others consider any evidence of compasses before the 11th century too weak to trust.

  Historians count the compass among China's Four Great Inventions, tracing its lineage back to a divining tool used since the founding of the Han dynasty around 206 BC, centuries before the Song dynasty put a magnetized needle to work finding a ship's way rather than a fortune.

• 3. How the Needle Finds North

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  A magnetized needle at the heart of a compass aligns itself with the horizontal component of Earth's magnetic field, and the field exerts a torque that pulls the needle's north-seeking end toward the planet's North magnetic pole. The needle rests on a low-friction pivot, a jewel bearing in higher-quality instruments, so it can turn freely. Held level, the needle oscillates for a few seconds before settling into equilibrium, pointing along the local magnetic meridian.

  A compass dial reads its heading as an angle running clockwise from 0 degrees, so north is 0, east is 90, south is 180, and west is 270; navigators use those same clockwise degree readings to state an azimuth or bearing toward any target.

  Sailors and hikers need true north, the direction of the geographic pole and Earth's rotation axis, not magnetic north. The angle between the two, called magnetic declination, changes with location, and most maps print the local declination so a compass can be lined up with true north. The magnetic poles also drift over time, a process called geomagnetic secular variation, so a map's declination figure can go stale. Some compasses let a user manually dial in the correction so the needle reads a true bearing instead of a magnetic one.

• 4. Building and Refining the Device

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  Most modern compass needles float inside a capsule completely filled with liquid, commonly lamp oil, mineral oil, white spirits, or purified kerosene, which damps the needle's swing and shortens the time it takes to settle. Older capsule designs commonly built in a flexible rubber diaphragm or a small airspace to absorb the liquid's volume changes as temperature or altitude shifted, while some modern compasses instead shrink the housing itself or use a flexible capsule material to the same end. Phosphorescent or self-luminous markings on the needle let it be read in the dark, and because the fill liquid cannot be compressed, an ordinary liquid-filled compass keeps working accurately at considerable depth underwater.

  Orienteering compasses add a transparent baseplate and a rotating bezel marked in degrees, letting a hiker take a bearing straight off a paper map without doing arithmetic. A related design, the thumb compass, strips away most of the degree markings and adds an oversized needle so an orienteer can glance at it while holding a map in the same hand; the best versions use rare-earth magnets to settle in a second or less.

  The United States Army still issues field compasses with a magnetized card instead of a needle, read through an optical or prismatic sight so a soldier can align the compass with a target while reading the bearing, though this card design normally needs a separate protractor tool to take a bearing directly off a map. The M-1950 lensatic compass uses electromagnetic induction rather than liquid to damp its card, and self-luminous versions of it contain 120 millicuries of tritium for night reading. Tritium has a half-life of about 12 years, so a compass with 120 millicuries when new holds only 60 after 12 years and 30 after 24, and its glow fades accordingly.

  Mariners' compasses mount two or more magnets on a card that floats freely inside a glass-covered bowl, suspended in a gimbal within the ship's binnacle to keep it level regardless of the vessel's roll. A fixed marker called a lubber line shows the ship's actual heading against the card. Boats whose angle shifts constantly often use special marine damping fluids, such as isopar M or isopar L, to keep that rapid rocking from throwing the needle into constant fluctuation.

• 5. Angles and Specialty Designs

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  Early compasses were marked only with the cardinal points, but Chinese instrument-makers eventually divided the dial into 24 equally spaced points, while Europeans settled on 32. Modern civilian compasses instead use 360 degrees, but 19th-century Europeans experimented with a grad or gon system that made a right angle equal to 100 grads, and armies sometimes divided grads into tenths for 4,000 decigrades per circle. Most militaries today use the French millieme system, an approximation of the milliradian that spaces the dial into 6,400 mils, useful because one angular mil subtends roughly one metre at a distance of one kilometre. Imperial Russia divided a circle into 600 units by chord length, and the Soviet Union split those into tenths for a 6,000-unit circle still used across the former Warsaw Pact.

  Because Earth's magnetic field varies in strength and angle by latitude, manufacturers balance compass needles for one of five geographic zones, from zone 1 covering most of the Northern Hemisphere to zone 5 covering Australia and the southern oceans, to stop the needle dipping and sticking. Some compasses instead carry a small sliding counterweight called a rider that can be adjusted to correct the balance if the instrument travels to a different zone. A basic magnetic rod, meanwhile, can be made simply by rubbing an iron rod repeatedly against a lodestone; older still, a magnetized needle stuck through a cork and floated in a bowl of water served as a compass before the box-like dry compass appeared around 1300.

  The Qibla compass split off from the navigational mainstream for a single purpose: helping Muslims find the direction of Mecca for prayer. The optical or prismatic compass, fitted with a built-in sight and often a jeweled bearing, lets surveyors, cave explorers, foresters, and geologists take bearings accurate to fractions of a degree. The trough compass, mounted in a rectangular box several times longer than it is wide, has been used for land surveying with plane tables for centuries, and the luopan serves feng shui practitioners in China. Small electronic compasses built into phones and clocks use solid-state microelectromechanical sensors, typically two or three magnetic field sensors feeding a microprocessor, to output a digital or analog heading instead of a swinging needle.

  A vertical card magnetic compass, installed in some aircraft cockpits, uses a set of gears driven by a magnet mounted on a shaft to turn its dial, which cuts down on the dipping errors that make a simple needle harder to read in flight; an eddy current induced into a small damping cup further calms the magnet's oscillation. A related instrument, the earth inductor compass, dispenses with a magnetized needle altogether, instead treating Earth's own magnetic field as the induction field for a small electric generator and reading direction off the generator's varying output.

• 6. Reading a Bearing

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  A magnetic compass points toward the magnetic north pole, roughly 1,000 miles from the true geographic North Pole, so a navigator must correct for both variation, the gap between true and magnetic north, and deviation, the local distortion caused by iron and electric currents nearby. Variation values for most of the world's oceans had been calculated and published by 1914, letting mariners consult tables rather than work it out from scratch on every voyage.

  Casual hikers rarely need this level of precision, since it is really only mariners who worry about correcting for the last fraction of a degree. Except in areas where magnetic declination varies by 20 degrees or more, ignoring the gap between true and magnetic north still keeps a walker from veering seriously off course over short, flat distances, provided the terrain stays fairly level and visibility holds up. To take a map bearing, the edge of a protractor compass is laid so it connects the current position to the destination, the dial's orienting lines are rotated to true north using a line of longitude, and the resulting bearing is read off the degree indicator, ignoring the needle itself until the moment a magnetic bearing is wanted. A direction-of-travel arrow on the baseplate then lets a hiker check progress by aiming the arrow at a visible landmark, such as a distant mountain, and reading the fresh bearing back off the dial.

  A compass has to sit level to work; tilted, the needle can drag against its casing and give a false reading, so users are advised to watch the needle sway freely from side to side and tilt the housing gently until it does. Earth's own magnetic field is weak, measuring only about 0.5 gauss, so nearby magnets or household electronics can easily overpower a needle and should be kept away, and rocks rich in magnetic minerals such as magnetite, often visible as a dark, metallic-looking surface, can throw a reading off if the user lingers too close.

• 7. Errors and Limits

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  Near the magnetic poles, a compass becomes useless. As it is carried closer to a pole, the angle between true north and magnetic north grows until the needle stops indicating any particular direction and simply drifts. The needle also begins tilting up or down near the poles because of magnetic inclination, and compasses with poor bearings can stick and give a false reading as a result.

  Iron and steel bodies, running electric motors, MRI machines, and even magnetic mineral deposits in local rock can all throw off a compass reading. Acceleration or deceleration in a car or airplane can tilt the needle toward or away from its compensating magnets, distorting the reading further, and whether the indicated heading swings too high or too low depends on which hemisphere the compass is in and whether the vehicle is speeding up or slowing down. Turning through a heading of east or west makes a mechanical compass lag behind or lead ahead of the actual turn.

  Ships correct for this deviation by having the vessel swung, meaning rotated about a fixed point while its heading is checked against known landmarks onshore, producing a deviation card the navigator can consult. Correction happens in stages: the lubber line is realigned with the ship's actual direction of travel, small magnets inside the compass case counter permanent magnetism, and two iron balls mounted on the binnacle, paired with a Flinders bar, cancel out the effect of ferromagnetic materials nearby. Light aircraft use a similar deviation card mounted near the instrument panel, though fluxgate electronic compasses can now calibrate themselves automatically.

• 8. Beyond Magnetism

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  Navigators count seven distinct ways to find north, and magnetism supplies only one of them; two of the remaining six also travel under the name compass, the gyrocompass and the GPS compass, neither of which relies on a magnetized needle at all. A gyrocompass dispenses with magnetism entirely, using an electrically powered, fast-spinning wheel and friction forces to exploit Earth's own rotation and find true north directly. Unlike a magnetic compass, it is unaffected by the iron, steel, cobalt, or nickel in a ship's hull, which is why large vessels have long relied on a gyrocompass and kept a magnetic compass only as backup.

  GPS receivers using two or more antennas, blended with an inertial motion unit, can now determine heading to within 0.02 degrees and start up in seconds rather than the hours a gyrocompass needs. Even a single handheld GPS receiver can work out a direction of travel just by tracking its own changing position every few seconds while moving, distinguishing that from the direction its nose happens to be pointing if there is a crosswind or a tidal current.

  GPS compasses are cheaper than gyrocompasses, work better near the poles, and start up far faster, but they depend entirely on functioning satellite signals that a solar storm or deliberate jamming could disrupt. Submarines, where both magnetic and GPS signals are useless underwater, still rely on gyrocompasses for that reason, even as GPS has replaced gyroscopic systems almost everywhere else in civilian use.