Page:Elementary Text-book of Physics (Anthony, 1897).djvu/290

276 in the cylinder is therefore $$\frac{N^2 ls}{\mu 8 \pi} = \frac{\mu R^2 ls}{8\pi},$$ and the energy contained in unit volume is $$\frac{N^2}{\mu 8 \pi} = \frac{\mu R^2}{8\pi} \cdot$$

249. Paramagnetism and Diamagnetism.—It was discovered by Faraday that all bodies are affected when brought into a magnetic field: some of them, such as iron, nickel, cobalt, and oxygen, are attracted by the magnet setting up the field; others, such as bismuth, copper, most organic substances, and nitrogen, are repelled from the magnet. The former are said to he ferromagnetic or paramagnetic, the latter diamagnetic.

The most obvious explanation of these phenomena, and the one adopted by Faraday, is to ascribe them to a distribution of the induced magnetization in paramagnetic bodies, in an opposite direction from that in diamagnetic bodies. If a paramagnetic body be brought between two opposite magnet poles, a north pole is induced in it near the external south pole, and a south pole near the external north pole. The magnetic separation is then said to be in the direction of the lines of force. According to this explanation, then the separation of the induced magnetization in a diamagnetic body is in a direction opposite to that of the lines of force. In other words, if a diamagnetic body be brought between two opposite magnet poles, the explanation asserts that a north pole is induced in it near the external north pole, and a south pole near the external south pole.

One of Faraday's experiments, however, indicates that the different behavior of bodies of these two classes may be due only to a more or less intense manifestation of the same action. He found that a solution of ferrous sulphate, sealed in a glass tube, behaves, immersed in a weaker solution of the same salt, as a paramagnetic body; but, when immersed in a stronger solution, as a diamagnetic body. It may from this experiment be concluded that the direction of the induced magnetization is the same for all bodies, and that the exhibition of diamagnetic or paramagnetic properties depends, not upon the direction of induced magnetization, but upon