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Magnetism

Angle of inclination (dip angle) ranges from 0° at the magnetic equator to
If we position a compass on its side so 90° at the magnetic poles. It is measured
that its needle can rotate within a vertical using a dip needle, which is a magnetised
plane, the needle will align itself with the needle mounted on a pivot allowing it to
Earth’s magnetic field. rotate freely in a vertical plane, as shown
in Figure 3.53.
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The angle formed between the direction
of the resulting magnetic field and its
horizontal component is known as
the angle of inclination, or dip angle,
denoted by I.

This angle is positive when the magnetic
field vector points downward (as in the
Northern Hemisphere) and negative
when it points upward (as in the Southern
Hemisphere). The angle of inclination Figure 3.53: Dip needle

In practice, a compass is used to align the dip needle with the horizontal component
of the magnetic field, and the dip needle rotates vertically to align with the total field.
The magnetic dip produces a vertical moment, causing the needle in a compass to
rotate vertically. This produces friction in the pivot, which hinders the free rotation of
the needle. To compensate for the dip, a small mass is added to one end of the needle
to produce an opposite gravitational moment so as to maintain the needle in vertical
equilibrium. A compass to be used in the northern hemisphere, whereby the vertical
component points down, has the mass added to the tail end of the needle. A compass to
be used in the southern hemisphere, whereby the vertical component points up, has the
mass added to the head end of the needle. Figure 3.54 (b) shows the various magnetic
field components in the northern and southern hemispheres.
Geographic
Geographic north pole
axis Dip 90 Magnetic south pole
Angle of Magnetic
declination axis
Lines of force

Equator
Dip zero

Earth

Figure 3.54: (a) Compass showing angle of declination (b) Compass showing dip angle

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Physics Form 2 Final.indd 119 25/10/2025 10:27