(35)
Du Bois has shown that when the dimensional ratio m (= length/diameter) exceeds 100, Nm² = constant = 45, and hence for long thin rods
N = 45 / m².
(36)
From an analysis of a number of experiments made with rods of different dimensions H. du Bois has deduced the corresponding mean demagnetizing factors. These, together with values of m²N for cylindrical rods, and of N and m²N for ellipsoids of revolution, are given in the following useful table (loc. cit. p. 41):—
Demagnetizing Factors.
| m. | Cylinder. | Ellipsoid. | ||
| N. | m²N. | N. | m²N. | |
| 0 | 12.5664 | 0 | 12.5664 | 0 |
| 0.5 | — | — | 6.5864 | — |
| 1 | — | — | 4.1888 | — |
| 5 | — | — | 0.7015 | — |
| 10 | 0.2160 | 21.6 | 0.2549 | 25.5 |
| 15 | 0.1206 | 27.1 | 0.1350 | 30.5 |
| 20 | 0.0775 | 31.0 | 0.0848 | 34.0 |
| 25 | 0.0533 | 33.4 | 0.0579 | 36.2 |
| 30 | 0.0393 | 35.4 | 0.0432 | 38.8 |
| 40 | 0.0238 | 38.7 | 0.0266 | 42.5 |
| 50 | 0.0162 | 40.5 | 0.0181 | 45.3 |
| 60 | 0.0118 | 42.4 | 0.0132 | 47.5 |
| 70 | 0.0089 | 43.7 | 0.0101 | 49.5 |
| 80 | 0.0069 | 44.4 | 0.0080 | 51.2 |
| 90 | 0.0055 | 44.8 | 0.0065 | 52.5 |
| 100 | 0.0045 | 45.0 | 0.0054 | 54.0 |
| 150 | 0.0020 | 45.0 | 0.0026 | 58.3 |
| 200 | 0.0011 | 45.0 | 0.0016 | 64.0 |
| 300 | 0.00050 | 45.0 | 0.00075 | 67.5 |
| 400 | 0.00028 | 45.0 | 0.00045 | 72.0 |
| 500 | 0.00018 | 45.0 | 0.00030 | 75.0 |
| 1000 | 0.00005 | 45.0 | 0.00008 | 80.0 |
In the middle part of a rod which has a length of 400 or 500 diameters the effect of the ends is insensible; but for many experiments the condition of endlessness may be best secured by giving the metal the shape of a ring of uniform section, the magnetic field being produced by an electric current through a coil of wire evenly wound round the ring. In such cases Hi = 0 and H = H0.
The residual magnetization Ir retained by a bar of ferromagnetic metal after it has been removed from the influence of an external field produces a demagnetizing force NIr, which is greater the smaller the dimensional ratio. Hence the difficulty of imparting any considerable permanent magnetization to a short thick bar not possessed of great coercive force. The magnetization retained by a long thin rod, even when its coercive force is small, is sometimes little less than that which was produced by the direct action of the field.
Demagnetization by Reversals.—In the course of an experiment it is often desired to eliminate the effects of previous magnetization, and, as far as possible, wipe out the magnetic history of a specimen. In order to attain this result it was formerly the practice to raise the metal to a bright red heat, and allow it to cool while carefully guarded from magnetic influence. This operation, besides being very troublesome, was open to the objection that it was almost sure to produce a material but uncertain change in the physical constitution of the metal, so that, in fact, the results of experiments made before and after the treatment were not comparable. Ewing introduced the method (Phil. Trans. clxxvi. 539) of demagnetizing a specimen by subjecting it to a succession of magnetic forces which alternated in direction and gradually diminished in strength from a high value to zero. By means of a simple arrangement, which will be described farther on, this process can be carried out in a few seconds, and the metal can be brought as often as desired to a definite condition, which, if not quite identical with the virgin state, at least closely approximates to it.