Page:Elementary Text-book of Physics (Anthony, 1897).djvu/291

§ 249] the greater or less intensity of magnetization of the surrounding medium.

Faraday discovered that many bodies while in a vacuum exhibit diamagnetic properties. In accordance with this explanation, we must conclude that a vacuum can have magnetic properties. It seemed to Faraday unlikely that this should be the case, and he therefore adopted the explanation which was first given. As it has since been shown that the ether which serves as a medium for the transmission of light, and which pervades every so-called vacuum, is also probably concerned in electrical and magnetic phenomena, there is no longer any reason for the opinion that the possession of magnetic properties by a vacuum is inherently improbable.

To classify bodies as paramagnetic or diamagnetic, we examine the energy existing in them when placed in a magnetic field. We will first assume that $$k,$$ the coefficient of magnetization, is so small that the resultant force in the region occupied by the body is not appreciably changed by the presence of the body. The value of $$k$$ for vacuum is assumed to be zero, and for air it is very slightly different from zero; hence the value of $$\mu$$ for air may be set equal to 1. Before a body is brought into the field, the energy per unit volume in the space finally occupied by it is $$\frac{N^2}{8\pi};$$ the energy per unit volume in the same space when the body is brought into the field is $$\frac{N^2}{\mu 8 \pi}\cdot$$ The increase of energy caused by the introduction of the body, on the assumption we have made that the field is not disturbed by the body, or that $$N$$ remains the same after the introduction of the body as it was before, is $$\frac{N^2}{8 \pi} \left( \frac{1 - \mu}{\mu} \right),$$ and this is positive or negative according as $$\mu$$ is less or greater than 1. Now a body free to move will move so as to diminish its potential energy, and therefore a body for which $$\mu > 1$$ will move so as to make $$N^2$$ as large as possible, or will move from a weaker to a stronger part of the field. Such a body is called a paramagnetic body. On the other hand, a body for which $$\mu < 1$$ will move so