in 1785, the French physicist Charles-Augustin de Coulomb published his first three reports of electricity and magnetism where he stated his law.
The conclusion that there were two types of magnetism, was a natural consequence of the discovery that the poles of a magnet either attract or repel one another. The north (i.e. north-seeking) pole was said to be positive, whereas the south pole was negative.
Attempts were made to separate the positive and negative poles of a magnet by breaking the magnet into two pieces, but it was always found that negative and positive poles, respectively, appeared on the two sides of the break.
It was, therefore, reluctantly accepted that the privilege of independent experience, enjoyed by electric charges, is not a prerogative of magnetic poles.
Whenever a positive pole is discovered in nature, it is inevitably associated with a negative pole of equal strength.
The problem of obtaining "free" poles, in order to determine the force between them, was overcome by using long, thin magnets.
The positive pole was sufficiently far removed from its associated negative pole as to be practically isolated from it.
The law of force between poles was investigated by Charles Coulomb, using the same torsion balance with which he established the law of force between electric charges, and was found to be similar in form to that for charges.
The torsion balance consists of a bar suspended from its middle by a thin fiber. The fiber acts as a very weak torsion spring.
As regards the suggestion that one half of the bar magnet may be taken as one type of pole of the magnetic field- let us see the plot of lines of forces
The density of lines of force is much smaller in the region from the poles to the center of the magnet. (see diagrams in hyperphysics ref.given below))
In fact, the magnetic dipoles in the material of the magnet form a circuital line and are well connected inside- as this array reaches the ends the lines of force emerge and get connected to any 'magnetic material' outside the magnet- if one traces back these lines- a pole gets defined.
Therefore if ane takes a thin long magnet the effect of the force from poles will be much larger than from surfaces along the length- In the torsion
balance experiment Coulomb actually minimized the effects of lateral lines of forces and could measure the deflection due to polar action.