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Magnetism and Matter
Question
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A magnetic needle free to rotate in a vertical plane, orients itself with its axis vertical at a certain place on the earth. What are the values of (a) horizontal component of earth’s field? (b) angle of dip at this place?

Answer

The place is clearly the magnetic pole of earth as needle rotates in vertical plane and, not in horizontal plane we have,
                     BH = 0 . 
Hence, 
(a) Horizontal component of earth's magnetic field at the magnetic poles, BH = 0 .
(b) Angle of dip at the magnetic poles of earth, δ = 90°.

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Some More Questions From Magnetism and Matter Chapter

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Geologists claim that besides the main magnetic N-S poles, there are several local poles on the earth's surface oriented in different directions. How is such a thing possible at all?

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The earth's magnetic field varies from point to point in space. Does it also change with time? If so, on what time scale does it change appreciably?

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The earth's core is known to contain iron. Yet geologists do not regard this as a source of the earth's magnetism. Why?

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The charged currents in the outer conducting regions of the earth's core are thought to be responsible for earth's magnetism. What might be the ‘battery’ (i.e., the source of energy) to sustain these currents?

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The earth may have even reversed the direction of its field several times during its history of 4 to 5 billion years. How can geologists know about the earth's field in such distant past?

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The earth’s field departs from its dipole shape substantially at large distances (greater than about 30,000 km). What agencies may be responsible for this distortion?