Magnetic Induction

There are some substances in which the action of magnetic forces produces a magnetic state which lasts only as long as the magnetic forces are acting. Such substances, of which iron is the most marked example, become themselves temporary magnets when placed in a magnetic field. They are said to be magnetised by induction. They lose nearly all their magnetic property when the magnetising forces cease to act. In most specimens of iron a certain amount of this remains as permanent magnetism after the cessation of the magnetising forces. In very soft iron the amount is very small; in steel, on the other hand, the greater portion remains permanently. We shall call such substances magnetic.

The attraction between a magnet and a magnetic substance is due to this induction.

Wherever a line of force from a magnet enters a magnetic substance it produces by its action a south pole. Where it leaves the substance it produces a north pole. Thus, if a magnetic body be brought near a north pole, those portions of the surface of the body which are turned towards the body become endued generally with south polar properties; those parts of the surface which are away from the north pole acquire north polar properties. An attraction is set up between the north pole of the magnet and the south polar side of the induced magnet, a repulsion of weaker amount between the north pole and the north polar side, so that on the whole the magnetic body is attracted to the north pole. This may even be the case sometimes when the magnetic body is itself a somewhat weak magnet, with its north pole turned to the given north pole. These two north poles would naturally repel each other; but, under the circumstances, the given pole will induce south polar properties in the north end of the weak magnet, and this south polarity may be greater than the original north polarity of the magnet, so that the two, the given north pole and the north end of the given magnet, may actually attract each other.