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Magnets
A substance that attracts pieces of iron and steel is called a magnet and this property of the substance is called magnetism.
The basic property of magnets are summarized below :
When an ordinary bas magnet is dipped into a heap of iron filings, they cling to the ends of the magnet. These preferred regions of attractions are called poles of the magnet.
If a bar magnet is suspended by a thread, it will come to rest in a position close to the north-south direction. The end of the magnetic toward the north is called north pole (N-pole) of the magnet and the other end the south pole (S-pole).
The poles of a magnet cannot be separated. If a bar magnet is broken into two parts, each part will be a complete magnet with the poles at its ends. No matter how many times a magnet is broken; each piece will contain N-pole at one end and S-pole at the other.
The two poles of a magnet are of equal strengths. The pole strength is represented by m. The SI unit of pole strength is ampere meter (Am).
The distance between the two poles of a magnet is called magnetic length (2l) as shown in figure. It is slightly less than the geometrical length of the magnet.
Magnetic length  =  (6/7) x geometrical length
Like poles repel each other and unlike poles attract each other.
An isolated N-pole or S-pole does not exist. It is because electric current is the cause of magnetism in every case and other is no way to divide up a current and obtain a single magnetic pole.
When a piece of substance such as iron, steel, nickel etc is placed near a magnet, it acquires magnetism. The magnetism so acquired is called induced magnetism.