What was the early recognition of Earth's magnetic field? State the dynamo effect.

Answer the following questions:
$(a)$ 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?
$(b)$ The earth's core is known to contain iron. Yet geologists do not regard this as a source of the earth's magnetism. Why?
$(c)$ 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?
$(d)$ 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?
$(e)$ 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?
$(f)$ Interstellar space has an extremely weak magnetic field of the order of $10^{-12}\; T$. Can such a weak field be of any significant consequence? Explain.

The angle of dip at a certain place is $30^o$. If the horizontal component of the earth's magnetic field is $H$,the intensity of the total magnetic field is:

$A$ magnetic compass needle oscillates $30$ times per minute at a place where the angle of dip is $45^{\circ}$,and $40$ times per minute where the angle of dip is $30^{\circ}$. If $B_1$ and $B_2$ are respectively the total magnetic field due to the earth at the two places,then the ratio $B_1/B_2$ is best given by:

At a point $A$ on the earth's surface, the angle of dip is $\delta = +25^{\circ}$. At a point $B$ on the earth's surface, the angle of dip is $\delta = -25^{\circ}$. We can interpret that:

Which of the following relations is correct in the context of Earth's magnetism?