By suspending a piece of magnetic material at the Earth’s surface, we can identify and mark its north pole (the end that points roughly in the direction of the Earth’s north geographic pole) and it’s South Pole (the opposite end). Let us test and mark two pieces of magnetic material in this way. We can then directly study the magnetic force that one of these pieces expert on the other in various orientations. In particular, we can study the force that one North Pole exerts on another north pole or on a south pole. From experiments of this type we deduce the following rule of the interactions of magnetic poles.

Like poles repel, and unlike poles attract.

That is, a south pole attracts a north pole, but two north poles or two south poles repel one another. This rule is very similar to the rule for the interaction of electric charges. Applying this rule to the behavior of a magnetic compass on the Earth’s surface. We conclude that to attract the north pole of the compass there must be a magnetic south pole near the Earth’s geographic North Pole.