The study of properties of magnets, magnetic substances and of associated
phenomena is the subject matter of magnetism. Earlier it was limited to the properties
of load stone,(a magnetic oxide of iron, Fe3 O4
A magnet (natural or artificial) is one which exhibits two important properties
(i) attractive property and
(ii) directive property. Magnets attract small pieces of substances like iron or steel. This property of attraction is found to be greatest near the two ends of the magnet. The points near the ends and lying inside the magnet
where magnetism appears to be concentrated are called the poles of the magnet. When
a magnet is freely suspended in a horizontal plane, it always comes to rest approximately in the North-South direction. The pole of the magnet pointing towards
north is the North pole. The pole pointing South is the South pole.
In 1750 John Michael using a torsion balance showed that magnetic poles exert
attractive or repulsive forces on each other. He also showed that these forces vary
inversely as the square of the separation between the poles. Hence, the force between
two magnetic poles is similar to the force between two electric charges. However, there
is an important difference. Electric charges can be isolated while magnetic poles
cannot be isolated. Magnetic poles always occur in pairs. All attempts till today to
isolate poles i.e. to detect an isolated magnetic pole have been unsuccessful.
The discovery of magnetic effect of electric current by the Danish scientist Hans
Oersted and the phenomenon of electromagnetic induction by Michael Faraday and
independently by Joseph Henry clearly established that magnetism is ultimately
electrical in nature.
The electrical field at a point in space is defined as the net electric force acting on
a unit positive charge placed at that point. Similarly, the gravitational field at a point
in space is the net gravitational force per unit mass. In the same way the magnetic
field at a point in space (also called magnetic flux density or magnetic induction) is
defined in terms of the magnetic force acting on a moving charge.