Page:Elementary Text-book of Physics (Anthony, 1897).djvu/276

262 The quotient of the magnetic moment of such a magnet by its volume, or the magnetic moment of unit of volume, is called the intensity of magnetization. Since any magnet may be divided into small magnets, each of which is uniformly magnetized, and for which by this definition a particular value of the intensity of magnetization can be found, it is clear that the magnetic condition of any magnet can be stated in terms of the intensity of magnetization of its parts.

The dimensions of magnetic moment and of intensity of magnetization follow from these definitions. They are respectively '''241. Distribution of Magnetism in a Magnet.'''—If we conceive of a single row of magnetic molecules with their unlike poles in contact, we can easily see that all the poles, except those at the ends, neutralize one another's action, and that such a row will have a free north pole at one end and a free south pole at the other. If a magnet be thought of as made up of a combination of such rows of different lengths, the action of their free poles may be represented by supposing it due to a distribution of equal quantities of two imaginary substances, called north and south magnetism. This distribution will be, in general, both on the surface and throughout the volume of the magnet. If the magnet be uniformly magnetized, the volume distribution becomes zero. The surface distribution of magnetism will sometimes be used to express the magnetization of a magnet, by the use of a concept called the magnetic density. It is defined as the ratio of the quantity of magnetism on ai element of surface to the area of that element. The magnetic density thus defined has the same numerical value as the intensity of magnetization which measures the real distribution. To illustrate this statement, we will consider an infinitely thin and uniformly magnetized bar, of which the length and cross-section are represented by $$l$$ and $$s$$ respectively. Its intensity of magnetization is $$\frac{ml}{ls}$$ or $$\frac{m}{s}\cdot$$ If, now, for the pole $$m$$ we