Magnetism

Magnetism is the property of attracting iron exhibited in varying degree by certain metals such as iron, nickel, and cobalt. Lodestone, a form of magnetite (an ore of iron), is naturally magnetic, and pieces of lodestone are called natural magnets. It is likely that magnetism was first observed by the ancients in lodestone.

Other substances can be magnetized artificially either by induction, i.e., by bringing them in contact with an existing magnet, or by placing them in a coil of wire (solenoid) through which a direct electric current runs. Some substances can be magnetized more easily than others. Soft iron, for example, is easily magnetized when inside a solenoid, but quickly loses its magnetisme when taken out again. Steel is more difficult to magnetize, but retains its magnetism for a long time. Magnets that retain their magnetism are called "permanent".

Magnetic permeability is a measure for the ease by which a material can be magnetized. Substances differ greatly in their permeability, those with a high value can be highly magnetized, i.e., can become strong magnets.

The first scientific study of magnets was performed by William Gilbert in the late sixteenth century. He found magnets to have their strength concentrated in small regions called poles. If a bar of iron is magnetized (e.g., in an electric coil), magnetic poles occur at the two ends of the bar. These are called north and south pole. If one hangs the bar magnet from a rope, the magnet will direct itself with one pole pointing to the North. The pole that points to the North is the magnetic north pole, the other end of the bar contains the south pole. It is a remarkable phenomenon that when a magnet with two poles is cut in half, each half will then itself have two poles.

Like poles repel each other, whereas opposite poles attract. The force by which they repel orr attract is given by Coulomb's law named after the French physicist Charles-Augustin de Coulomb.

Physicists find it convenient to picture the magnetic force as being transmitted through a magnetic field. That is, instead of saying that one magnet exerts a force on the other, they say that the first magnet creates a magnetic field in the space around it and that the second magnet feels a force because it finds itself in a magnetic field. The magnetic field of a magnet consists of lines of force running from its north pole to its south pole.