What is a magnetic compass and how does it work?

What is a magnetic compass and how does it work?

The Basic Concepts of All Magnetic Compasses

Using a magnetic compass connects you with the world on foot, by ship, in the air, and by many other means that dates back centuries and covers the globe. The basic principle behind this enduring technology has stood the test of time, and is still required in many cases even when sophisticated satellite-based navigation is available. Here's a simple magnetic compass definition according to the Encyclopedia Britannica: "An instrument for determining direction on the surface of Earth by means of a magnetic pointer that aligns itself with Earth's magnetic field."

The operation of a magnetic compass is a basic principle that's been in use to facilitate travel and trade for about a thousand years. When combined with today's comprehensive navigational charts, it can be used for oceangoing travel as well as making port anywhere in the world. The key to the modern compass is understanding its accuracy limitations and why they exist. With that knowledge, precision compasses using several basic principles including magnetic fields can be built, used, and maintained for safe voyages.

  • A magnetic compass originally indicated direction because of the alignment of the magnetized metallic "needle," typically a piece of iron or steel.
  • A magnetic compass indicates direction because it is free to rotate and align with Earth's magnetic field.
  • Card compasses use magnets attached to a card marked with directions that rotates on a pivot.
  • Modern magnetic sensors used in electronics can be used to create magnetic compass devices.
  • Advantages and disadvantages of magnetic compass technology have made it a universal navigation instrument but more complex to use in modern times, in particular on metal ships.

Unlike most modern technologies, it's hard to say who invented the magnetic compass, since the use of a simple magnetized pointing device was recorded in Han Dynasty China over 2000 years ago. The magnetic compass meaning evolved from words in middle English, Latin, and other languages, referencing a circle, mathematical instrument, and route or journey.


Magnetometer-based compasses

These days, many smartphones contain a type of magnetic compass sensor called a magnetometer, that detects magnetic fields and supplies data to computers. It's possible to load a magnetic compass app for Android and Apple phones. Numbers don't necessarily indicate accuracy, so it's important to note the true precision and deviation sources nearby.

  • Digital compass vs magnetic compass differences include ease of use and data availability for navigation equipment.
  • The digital fluxgate magnetic compass is not the same as a magnetometer, employing magnets with wire coils to accurately sense magnetic fields.
  • A direct reading magnetic compass is a special suspension used for aircraft to provide usable readings during flight, different from digital devices.
  • Magnetometer-based compass devices are being included as part of mobile robotic systems, even STEM school-based inventions, so students are becoming familiar with them.


Gyroscope-Based Compasses

Not every compass you see on a ship or aircraft is a magnetic compass, or the electronic equivalent using magnetometer or fluxgate technology. Aviation magnetic compass errors can arise from the construction of the aircraft and nearby instruments, as well as the effect of the motions of flight on the indicator. In particular, the concept of "dip" can interfere with a standard compass in flight, or even in small craft traversing stormy seas. The gyro compass is a precision instrument, so cost is a factor in its use, but the advantages of gyro compass vs magnetic compass navigation can be significant. Examples:

  • Gyro compass, based on gyroscope stability, is not affected by nearby metal.

  • Aircraft magnetic compass

  • In the northern hemisphere a magnetic compass will normally indicate a northerly turn when an east-west flight decelerates.

  • A gyro compass needs very little "settling time" due to vessel or aircraft motion before it can be read.

  • Aircraft magnetic compass calibration can be complex due to its location

  • Gyro compass relies on Earth's rotation, rather than magnetic field.

  • Gyromagnetic compasses are available that combine both principles.

Geology and Physics Behind Magnetic Compass Operation

As society's reach extends to the moon and other planets, you might ask, "would a magnetic compass work on the moon?" Celestial bodies have some principles in common such as gravity, and some major differences as well. Earth's magnetic field is not based in particular on its size or shape, it's a specific effect of the outer core of our planet which is fluid. The molten material which flows slowly as the planet rotates has a high iron content, which generates electricity and from that, a magnetic field. It's called a "self-exciting dynamo effect," where a dynamo is a simple type of electric generator.

  • Molten iron is not the only factor in the Earth's magnetic field, the crust, ionosphere, and magnetosphere also can influence it.
  • Variation error of the magnetic compass is caused by changes over time in all these factors.
  • From the time of the first Chinese magnetic compass to today, there have been significant changes in the magnetic field it interprets.
  • As an aviator, mariner, or even a hiker travels, it's important to consider magnetic compass deviation as well, based on local effects on compass readings.

Variation and Deviation Would be Problems – But They're Correctable

On navigational charts, you'll see notations for variation, which change over time in a large-scale manner, and deviation, which is specifically noted for a given area on the chart. Deviation can even be noted by heading, with a different deviation in magnetic compass readings for southerly travel versus westerly, for example. For navigation, both variation and deviation need to be applied to the compass heading to determine the true heading on the chart. An important note: it's easy to confuse the arithmetic when calculating true heading from compass, or the reverse, factoring in whether the variation or deviation is east or west.

  • A deviation table for magnetic compass readings provides the deviation for each heading in a quick-reference fashion.
  • A deviation card for magnetic compass is the name typically used for references used in aircraft.
  • When professionals are performing magnetic compass adjustment, they make reference to a deviation curve of magnetic compass data.

Local Deviation: How Magnetism on the Ship Affects Compass Operation

From the compass mount to the ship's construction, nearby metals, typically iron and steel and generated magnetic fields can have a serious effect on the compass accuracy. Some types of metals such as brass and copper are safe to use for compass mounts and nearby because they don't carry magnetic fields. If a magnetic compass is placed near an insulated copper wire with current flowing, however, a magnetic field can be generated that affects the compass needle. Ferrous and other magnetic metals should be assumed to have potential influence on compass operation, which may vary over time. They may even be magnetized during construction processes and lose that effect over time.

  • A magnetic compass airplane mount must be carefully located and the compass adjusted to compensate for nearby magnetic field influences.
  • In some cases, such as gyromagnetic compasses, the device is located at an aircraft extremity, such as near a wing tip, for isolation.
  • While variation and deviation information is available for general navigation, a ship or airplane also faces its own magnetic compass deviation sources, and crew need relevant information.

Swinging the Vessel

"Swinging" a vessel, is a practical magnetic compass calibration method, a required practice typically every two years. It verifies the accuracy of the compass in cardinal and intercardinal directions. The process can be done by pleasure craft and larger vessels, taking transit lines of known magnetic bearing as referenced on a chart, and comparing them with the vessel's compass reading. This produces a list of deviations between known bearings and compass bearings for eight compass points, which are kept on the deviation card or table. It's important to note that these values can change significantly due to configuration changes on the vessel, including loading metallic stores such as canned goods, or cargo.

  • Swinging the vessel provides an accurate set of vessel deviation values for navigation use.
  • Changes in rigging, stores, cargo, and equipment, especially metallic or electronic, can influence deviation values.

Use and Maintenance of Binnacle and Wet Compass

The binnacle of a ship is typically a small stand containing the ship's compass. Its name derives from the Latin for "little house," which is quite appropriate. Binnacles are carefully constructed to survive their existence on the ship, and placed where the navigator has ready reference to the ship's bearing when needed. Materials used are usually wood or brass to avoid interference with magnetic compass operation, and the compass is mounted in a gimbal to preserve its orientation as seas toss the ship during stormy weather.

Even if the ship has gyro compass technology and data relayed as needed or other technologies, a binnacle magnetic compass requires no power. The best magnetic compass models like the Ritchie magnetic compass are highly precise, and can be used both when electronics have failed, and as a precision real-time heading in combination with GPS position information. Wet compass construction includes a liquid in addition to gimbal mount. It's important that the binnacle magnetic compass stay properly positioned to provide an accurate reading of Earth's magnetic field, and also have its motion damped enough to be easily read.

  • Magnetic compass bowls are filled with a liquid to serve as a kind of "shock absorber" that makes them more readable.
  • The construction of magnetic compass mounts such as in a binnacle should be non-magnetic, such as brass or wood.

Dry Card Compass Isn't for Steering

One of the advantages of magnetic compass design is that it's easily constructed as the ancient Chinese did, using natural magnetism, floating or suspended, to provide directional information. Many outdoor enthusiasts carry a combined map measurer and magnetic compass designed for hiking use. Dry and liquid-filled compasses are available for handheld use, but experts recommend liquid-filled for the same reason maritime compasses use viscous liquid: stability. A dry card compass is especially poor for steering when in motion, because it's quite difficult to obtain an accurate heading as the needle or card reacts to shock and vibration.

  • Making an emergency magnetic compass, try to use a liquid support.
  • Steering while in motion, a liquid-filled magnetic compass is the only way to go.
  • Bringing your hiking compass for emergencies while day sailing, think about liquid-filled versions and vessel-based deviation issues to avoid significant errors.

Three Dimensional Compass Movement: the "dip"

Since the compass needle is aligning with the earth's magnetic field, it's actually only level at the equator. The rest of the time, it has a "dip" that reflects the curvature of the field along with the earth itself. Compass behavior varies as you travel north to south, based on Earth's magnetic field. It's most accurate in reflecting north and south bearings at the equator, where magnetic field lines are parallel to the Earth and horizontal. This relationship changes as you travel towards the poles, causing the magnetic compass needle to "dip" with the magnetic field lines and not remain level.

  • Shipboard compasses are designed to compensate for the dip using weights and other design factors.
  • Aviation magnetic compass errors are more common. A Cessna 172 magnetic compass experiencing relatively slow flight speeds can experience dip-related errors.
  • Jet aircraft will experience more extreme problems during banked turns and changes in airspeed.
  • Aircraft pilot training includes information about changes in compass readings such as northerly turn indicated during east-west flight due to deceleration.

Buying, Mounting, Managing, and Adjusting Your Steering Compass

As a legally required, essential navigation tool for maritime and aviation use, a proper magnetic compass needs to be fit for the job. When you buy magnetic compass components, you should have a design in mind that supports, protects, and illuminates the compass, such as a binnacle. When constructing your own, don't forget to use brass, wood, and other non-magnetic materials. Supports should also provide a gimbal mount to adapt in motion. Most essential, don't consider your compass mount complete without a magnetic compass calibration. It's only with precise, professional calibration that compasses, from a digital magnetic compass to an antique magnetic compass as well as modern models, will serve you well.

  • Magnetic compasses don't require power, and are essential for safe navigation.
  • Professional calibration ensures that your compass is ready for all the places you travel.
  • Find a high-quality ship or aircraft magnetic compass for sale to customize your navigational instruments.

The Magnetic Compass: Changing History

In the early days of land and sea exploration, adventurers recorded their travels and experiences and established descriptions and then charts that provided a track for others to follow. Trade routes were developed using those directions. As basic magnetic compasses came into use, they first appeared in China two thousand years ago but as a more decorative than practical device. On a more widespread and practical basis they became popular around the 11th century, and travel became more reliable and efficient.

The simple magnetic compass played a large role in the expansion of people's knowledge of the world, and in the commerce that followed. Sailing trade routes brought exotic goods and spices, and the Silk Road trade route was enhanced by use of the compass. The Silk Road also impacted bringing the device to many new cultures, and in a sweeping leap to today, it's still making a large difference in travel and trade of all kinds.

An ancient magnetic compass is still usable today, and there are lots of places online where you can learn how to make a magnetic compass for emergencies. As noted, there's likely to even be a version available in your smartphone. Just remember, for safe and reliable navigation, the quality and precision of aviation-quality and maritime-quality compasses is still essential.

Without a Compass, or In Space, How Do You Navigate?

Earth provides many clues about your position on it and direction of travel, as it has done since people began to explore the globe. The sextant and celestial navigation is enjoying a revival as a non-electronic and fascinating way to navigate, even on the high seas. In the cabin of some yachts, you'll find them kept as decorative items, with the understanding that it's also ready for use, or a romantic night under the stars taking sights and calculating positions. With charts in sight of shore or navigational aids, taking transit lines as is done during "swinging the vessel" can provide information as well. Welcome to the ancient world of navigation, and the modern world of precision compass use.

We're glad to provide magnetic compass adjustment services and access to quality compasses for vessels of all sizes. Contact us for more information.