Magnetism

It is known since long ago that a certain ore of iron possesses the property of attracting small pieces of iron. This ore is called magnetite as it was found in the province of Magnesia in a country then known as Asia Minor. Magnetism is called as property of attracting small pieces of iron and an object possessing this property is called magnet. A piece of magnetite is a natural magnet it possesses two important properties,

• If a piece of magnetite is dipped into iron filings, it is observed that iron filings mostly cling to two opposite ends of piece. This ends are called the poles.
• If this piece freely  suspended by the means of a thread, it comes to rest with one pole pointing towards the north and other pole pointing towards the south. The pole pointing towards the north is called the north pole and the pole pointing towards the south is called the south pole.

This property was used by ancient mariners for navigational purposes. For this reason magnetite was also called the leading stone or loadstone.

It was soon discovered that when a piece of iron is rubbed with loadstone, it acquires the property of a magnet. It is called an artificial magnet. The process of converting iron or its alloys into a magnet is called magnetization. Different ways of preparing artificial magnets have been found and artificial magnet having different shape such as bar magnet, magnetic needle, horse-shoe magnet, etc. are being commonly used for various purpose.

If a magnetic needle is brought near a magnet, it experiences a mechanical force. This shows a magnetic field is produced in the space surrounding a magnet. When two magnets are brought near each other, their like poles repel each other while unlike poles attract each other.

We cannot identify any single point of a magnet as a magnetic pole, because if a magnet cut into two pieces, magnetic poles appear at the broken ends, where previously there were no poles. Therefore we consider two points of the magnet where its magnetic properties are concentrated as two fictitious poles of the magnet.

A bar magnet is thus magnetic dipole consisting of two fictitious poles separated by a finite distance. The distance between the poles is called the magnetic length of the magnet. The fictitious poles are not situated exactly at the ends, but a little distance inside the magnet. Therefore the magnetic length is smaller than the geometric length of the magnet. The relation between the two is.

Magnetic length= ×geometriclength

The magnetic length is usually denoted by 2l, so the l denotes half the magnetic length i.e. the distance of each pole from the center of the magnet.