A freely suspended compass needle on the earth's surface settles in a plane called----------

Analyze the question: It asks for the name of the vertical plane in which a freely suspended magnet (compass needle) comes to rest under the earth's magnetic field.

Concept:

• Geographic meridian: the vertical plane passing through the geographic north and south poles and the point of observation.
• Magnetic meridian: the vertical plane containing the earth's magnetic field at a place, which is the plane in which a freely suspended magnet rests.
• Magnetic declination (variation): the angle between the geographic meridian and the magnetic meridian. It is an angle, not a plane.
• Isogonals: lines on a map joining places that have the same magnetic declination. (The option is spelled "isoganals", but this is what it means.)

Solution:

1. A freely suspended needle aligns itself along the earth's magnetic field direction.
2. The vertical plane containing this needle is, by definition, the magnetic meridian.
3. Option A is wrong because the geographic meridian is defined by the geographic poles. The needle only lines up with it where the declination is zero.
4. Option C is wrong because declination is an angle, not a plane.
5. Option D is wrong because isogonals are lines on a map, not a plane.

**Correct answer: B, magnetic meridian.

Tip: Remember "Magnet rests in Magnetic meridian." Also remember that declination is the angle between the two meridians, and dip (inclination) is the angle the field makes with the horizontal.