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What does a compass measure in Earth’s magnetic field?
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geomagnetic field
The magnetic field of a bar magnet has a simple configuration known as a dipole field. Close to Earth's surface this field is a reasonable approximation of the actual field.
A compass is a device featuring a permanently magnetized needle that aligns itself with Earth’s geomagnetic field to determine direction.12
What a compass specifically measures depends on how it is balanced and oriented:
- Horizontal Direction: In a conventional compass, the needle is balanced to pivot in the horizontal plane, allowing it to align with the horizontal component of the magnetic field.2
- Field Direction: Generally, a compass is used to measure the direction of the magnetic field. When a magnetized needle is free from the influence of gravity and pivots in the presence of a magnetic field, it aligns exactly along the magnetic field vector.32
- Inclination: If a needle is pivoted and balanced about a horizontal axis—a device known as a dip meter—it can align with the total field vector to measure the inclination angle.2
- Field Magnitude: It is also possible to estimate the magnitude of the horizontal field by observing the oscillations of a compass needle, as the period of oscillation depends on the strength of the field.2