Ere long the needle was mounted on a pivot and provided with a moving card showing the principal directions.  The variation of the needle from the true north and south was certainly known in China during the twelfth, and in Europe during the thirteenth century.  Columbus also found that the variation changed its value as he sailed towards America on his memorable voyage of 1492.  Moreover, in 1576, Norman, a compass maker in London, showed that the north-seeking end of the needle dipped below the horizontal.

In these early days it was supposed that lodestone in the pole-star, that is to say, the “lodestar” of the poets or in mountains of the far north, attracted the trembling needle; but in the year 1600, Dr. Gilbert, the founder of electric science, demonstrated beyond a doubt that the whole earth was a great magnet.  A magnet, as is well known, has, like an electric battery, always two poles or centres of attraction, which are situated near its extremities.  Sometimes, indeed, when the magnet is imperfect, there are “consequent poles” of weaker force between them.  One of the poles is called the “north,” and the other the “south,” because if the magnet were freely pivotted like a compass needle, the former would turn to the north and the latter to the south.

Either pole will attract iron, but soft or annealed iron does not retain the magnetism nearly so well as steel.  Hence a boy’s test for the steel of his knife is only efficacious when the blade itself becomes magnetic after being touched with the magnet.  A piece of steel is readily magnetised by stroking it from end to end in one direction with the pole of a magnet, and in this way compass needles and powerful bar magnets can be made.

The poles attract iron at a distance by “induction,” just as a charge of electricity, be it positive or negative, will attract a neutral pith ball; and Dr. Gilbert showed that a north pole always repels another north pole and attracts a south pole, while, on the other hand, a south pole always repels a south pole and attracts a north pole.  This can be proved by suspending a magnetic needle like a pithball, and approaching another towards it, as illustrated in figure 26, where the north pole N attracts the south S. Obviously there are two opposite kinds of magnetic poles, as of electricity, which always appear together, and like magnetic poles repel, unlike magnetic poles attract each other.

It follows that the magnetic pole of the compass needle which turns to the north must be unlike the north and like the south magnetic pole of the earth.  Instead of calling it the “north,” it would be less confusing to call it the “north-seeking” pole of the needle.