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28 with which we have been concerned hitherto; there is a relation between the two, as will be explained in Chapter III, but they are not identical.

The same reasoning as above applied to electric attraction and repulsion may also be applied to forces produced by magnetism, but if we attempt an experimental verification of the general law of forces acting through space we encounter some difficulty. When dealing with electricity it is quite easy to isolate a positive from a negative charge each on its own conductor, or, as we may also term it, it is possible to accumulate free electricity of one sign on a conductor. It is not possible to accumulate only north magnetic matter, or only south magnetic matter on one piece of steel. We always get magnetic matter of both signs simultaneously on the steel. If this has the form of a bar we can, by stroking it with a loadstone, make one end of the bar a north pole and the other a south pole, but if we break the bar in halves we do not get one half all north and the other all south. Each half again shows north at one end and south at the other. In experimenting on magnetic forces we are, therefore, always disturbed by the presence of magnetic