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NOTE: I am talking about the period before electricity and magnetism were unified. So I am not seeking for answers based on Ampere atomic current model of magnets.

Who first figured out that one of the surfaces (pole) of a magnet attract or repel other surface (pole) of another magnet?

How did people at that time explain the following possibility:

One half of the magnet's volume is north pole and the other half is south pole (as shown in figure below). Do these "volumes" attract or repel each other, or do the "surfaces" attract or repel each other?

North and south magnetic poles' volumes

EDIT: I found the following highlighted statement from Maxwell's treatise Vol II, Article 373.

Maxwell's treatise Vol II, Article 373

Does it suggest that "it has been seen experimentally that surface end of a long thin magnet attract or repel surface end of another long thin magnet instead of half of magnet's volume attract or repel half of other magnet's volume"?

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    $\begingroup$ Basic properties of magnets were known long before Coulomb. The poles were introduced by Peter Peregrinus c. 1269, who identified them as points based on distribution of needle directions, see Magnetic Field: History. The possibility described (if I understand it right) could hardly occur to anybody even before him since splitting a magnet in two produces two magnets with both poles, Peregrinus stressed it explicitly. $\endgroup$ – Conifold Sep 7 '18 at 17:58
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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.

ref.-

http://www.daviddarling.info/encyclopedia/C/Coulombs_law_for_magnets.html https://en.wikipedia.org/wiki/Coulomb%27s_law http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/elemag.html

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  • $\begingroup$ Your answer is indeed informative, but is not relevant to the question. $\endgroup$ – Joe Sep 8 '18 at 0:55
  • $\begingroup$ @Joe- you need info about how coulomb separated the poles...but he did take a thin long magnet to 'make' a pole far off from the other one...and he could not have done anything better...one has to accept that. $\endgroup$ – drvrm Sep 8 '18 at 7:00
  • $\begingroup$ I completely agree with you but it is not what I asked. I asked how can we ensure that the rest of the magnet (apart from surface of poles) is free from magnetic action. $\endgroup$ – Joe Sep 8 '18 at 7:14
  • $\begingroup$ Thanks for your answer. Please try to add the answer to this question (how can we ensure that the rest of the magnet (apart from surface of poles) is free from magnetic action) in your informative answer. $\endgroup$ – Joe Sep 8 '18 at 7:19
  • $\begingroup$ @Joe- when we plot the lines of forces of a bar magnet -the poles becomes a major point from where the lines of forces emerges-their density define the magnitude of force- I have edited the answer. $\endgroup$ – drvrm Sep 8 '18 at 8:06
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Pierre de Maricourt (Peter the Pilgrim / Petrus Peregrinus) showed that one cannot isolate a magnetic pole because when a magnetic is divided, the two halves each have a north and south pole:

Petrus Peregrinus's broken magnet experiment See also: Nicolas Wipf, “Pierre Duhem (1861 - 1916) et la théorie du magnétisme fondée sur la thermodynamique” (Université Lille 1 Sciences et Technologies, 2011) p. 77.

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