Page:A Treatise on Electricity and Magnetism - Volume 2.djvu/115

 Rh These results are laid down in Fig. 10.

Fig. 10.

The curve of temporary magnetization is at first a straight line from X = 0 to X = L. It then rises more rapidly till X = 2), and as X increases it approaches its horizontal asymptote.

The curve of residual magnetization begins when X = L, and approaches an asymptote at a distance = .81 M.

It must be remembered that the residual magnetism thus found corresponds to the case in which, when the external force is removed, there is no demagnetizing force arising from the distribution of magnetism in the body itself. The calculations are therefore applicable only to very elongated bodies magnetized longitudinally. In the case of short, thick bodies the residual magnetism will be diminished by the reaction of the free magnetism in the same way as if an external reversed magnetizing force were made to act upon it.

446.] The scientific value of a theory of this kind, in which we make so many assumptions, and introduce so many adjustable constants, cannot be estimated merely by its numerical agreement with certain sets of experiments. If it has any value it is because it enables us to form a mental image of what takes place in a piece of iron during magnetization. To test the theory, we shall apply it to the case in which a piece of iron, after being subjected to a magnetizing force X, is again subjected to a magnetizing force X1.

If the new force Xl acts in the same direction in which X acted, which we shall call the positive direction, then, if X1 is less than X0, it will produce no permanent set of the molecules, and when Xl is removed the residual magnetization will be the same as