Page:Elementary Text-book of Physics (Anthony, 1897).djvu/292

278 as to make $$N^2$$ as small as possible, that is, will move from a stronger to a weaker part of the field. Such a body is a diamagnetic body. These motions are not necessarily along the lines of force, but are in the directions in which $$N^2$$ changes most rapidly.

In general, the introduction of a body into a field changes the arrangement of the tubes of induction and the force in the field; it has already been remarked that the resultant force at a point is diminished by the presence of a paramagnetic body around that point, because the induced distribution acts against the forces in the field. Since the energy per unit length of the tubes of induction is equal to $$\frac{R}{8\pi},$$ where $$R$$ is the resultant force, a tube of induction within a paramagnetic body possesses less energy per unit length than it does outside that body. In accordance with the tendency of the potential energy to become a minimum, the tubes of induction will therefore move into a paramagnetic body. On the other hand, the resultant force at a point being increased by the presence of a diamagnetic body around that point, the tubes of induction will move out from a diamagnetic body. This movement of the tubes of induction, into or out of bodies in the field, ceases when the loss of potential energy due to their movement into or out of the body is balanced by the gain in potential energy due to the lengthening of those tubes of the field which do not pass through the body. This change in the arrangement of the tubes of induction does not invalidate the former conclusion that paramagnetic bodies move from a place of weaker to a place of stronger magnetic force, while diamagnetic bodies move from a place of stronger to a place of weaker magnetic force. A complete discussion of the behavior of bodies in a magnetic field is outside the scope of this work.

'''250. Changes in Magnetization. Hysteresis.'''—When a magnetizable body is placed in a powerful magnetic field, it often receives, temporarily, a more intense magnetization than it can retain when removed. It is said to be saturated, or magnetized to saturation, when the intensity of its magnetization has its highest