A compass is a deceptively simple device containing a magnetized needle that aligns itself with the Earth’s magnetic field lines, indicating direction. The instrument converts the relatively weak signal from the planet’s magnetic core into a readable direction for navigation. While fundamentally reliable, a compass can give an inaccurate reading because various forces can distort or overwhelm the magnetic field it measures. These inaccuracies range from temporary, immediate interference to permanent, geographically fixed errors. Understanding these factors is paramount for anyone relying on a compass for accurate orientation.
Local Sources of Magnetic Interference
The most common cause of a compass displaying a false reading is local magnetic interference, often called “magnetic noise,” which is stronger than the Earth’s background field. The Earth’s magnetic field at the surface averages about 0.5 Gauss, meaning even small, nearby magnetic fields can easily overpower the needle’s alignment. Ferrous metals, which contain iron, are particularly disruptive because they can become temporarily magnetized and attract the compass needle, an effect known as deviation.
Everyday items carried by a person can pull the needle away from the true magnetic direction. These include steel-cased watches, belt buckles, knives, and the small magnets found in electronic devices like smartphones and tablets. Magnetic fields generated by electric currents are also a source of noise. Power lines, car engines, and large electrical transformers create electromagnetic fields that can render a compass useless when in close proximity.
Large metal infrastructure, such as steel bridges, railroad tracks, metal fences, and rebar in concrete buildings, can distort the magnetic field across a significant area. Geological formations containing high concentrations of magnetic minerals, like magnetite, also create localized magnetic anomalies. These local errors are temporary; the compass will return to an accurate reading once it is moved a few meters away from the source of the interference.
Understanding Magnetic Declination
A permanent, geographic difference known as magnetic declination is another source of inaccuracy. Declination is the angle between True North and Magnetic North. True North is the fixed direction toward the geographic North Pole, where the Earth’s axis of rotation intersects the surface. Magnetic North, in contrast, is the constantly moving point where the magnetic field lines plunge vertically into the planet.
The compass needle aligns itself with the local magnetic field lines, pointing toward Magnetic North, not True North. Declination is the angular difference between these two points, and its value varies significantly depending on the user’s location. For example, in parts of the United States, the difference can be as much as 20 degrees East or West.
Since all maps are oriented to True North, a compass reading is inaccurate for navigation unless declination is corrected. The Magnetic North Pole slowly shifts over time (geomagnetic secular variation), meaning the declination value for any given location is not static. Navigators must use current declination charts or tools to determine the precise correction angle for their specific location and adjust their bearing accordingly.
Mechanical Failures and Environmental Factors
The physical condition of the compass and the environment also cause inaccurate readings.
Mechanical Failures
Many compasses are liquid-filled to dampen the needle’s movement and help it settle quickly. A common issue is the formation of an air bubble within the capsule, often a sign of a fluid leak caused by heat or damage. A bubble impedes the free and accurate rotation of the needle on its pivot.
The magnetic needle can lose its precise alignment due to demagnetization, which occurs from prolonged exposure to high heat or a strong magnetic field. If the needle’s magnetism is weakened, it becomes less responsive to the Earth’s magnetic field and may not point accurately. Furthermore, the pivot on which the needle rests, often a hardened steel point on a jewel bearing, can become dull or damaged from physical shock or wear, causing the needle to stick or move erratically.
Environmental Factors
Extreme temperatures affect the dampening fluid’s viscosity (resistance to flow). In extreme cold, the fluid thickens, making the needle sluggish and slow to settle. High heat causes the fluid to expand and leak, potentially leading to bubble formation. Although rare, massive solar flares or geomagnetic storms can also temporarily distort the Earth’s magnetic field, resulting in erratic compass readings over a wide area.