834-] AMPERE S THEORY. 419
833.] Still, however, we have arrived at no explanation of the nature of a magnetic molecule, that is, we have not recognized its likeness to any other thing of which we know more. We have therefore to consider the hypothesis of Ampere, that the magnetism of the molecule is due to an electric current constantly circulating in some closed path within it.
It is possible to produce an exact imitation of the action of any magnet on points external to it, by means of a sheet of electric currents properly distributed on its outer surface. But the action of the magnet on points in the interior is quite different from the action of the electric currents on corresponding points. Hence Am pere concluded that if magnetism is to be explained by means of electric currents, these currents must circulate within the molecules of the magnet, and must not flow from one molecule to another. As we cannot experimentally measure the magnetic action at a point in the interior of a molecule, this hypothesis cannot be dis proved in the same way that we can disprove the hypothesis of currents of sensible extent within the magnet.
Besides this, we know that an electric current, in passing from one part of a conductor to another, meets with resistance and gene rates heat ; so that if there were currents of the ordinary kind round portions of the magnet of sensible size, there would be a constant expenditure of energy required to maintain them, and a magnet would be a perpetual source of heat. By confining the circuits to the molecules, within which nothing is known about resistance, we may assert, without fear of contradiction, that the current, in cir culating within the molecule, meets with no resistance.
According to Ampere s theory, therefore, all the phenomena of magnetism are due to electric currents, and if we could make ob servations of the magnetic force in the interior of a magnetic mole cule, we should find that it obeyed exactly the same laws as the force in a region surrounded by any other electric circuit.
834.] In treating of the force in the interior of magnets, we have supposed the measurements to be made in a small crevasse hollowed out of the substance of the magnet, Art. 395. We were thus led to consider two different quantities, the magnetic force and the magnetic induction, both of which are supposed to be observed in a space from which the magnetic matter is removed. We were not supposed to be able to penetrate into the interior of a mag netic molecule and to observe the force within it.
If we adopt Ampere s theory, we consider a magnet, not as a
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