Demagnetizing Force.—It has already been mentioned that when a ferromagnetic body is placed in a magnetic field, the resultant magnetic force H, at a point within the body, is compounded of the force H0, due to the external field, and of another force, Hi, arising from the induced magnetization of the body. Since Hi generally tends to oppose the external force, thus making H less than H0, it may be called the demagnetizing force. Except in the few special cases when a uniform external field produces uniform magnetization, the value of the demagnetizing force cannot be calculated, and an exact determination of the actual magnetic force within the body is therefore impossible. An important instance in which the calculation can be made is that of an elongated ellipsoid of revolution placed in a uniform field H0, with its axis of revolution parallel to the lines of force. The magnetization at any point inside the ellipsoid will then be
| I = | κH0 |
| 1 + κN |
(29)
where
| N = 4π ( | 1 | − 1 ) ( | 1 | log | 1 + e | − 1 ), |
| e2 | 2e | 1 − e |
e being the eccentricity (see Maxwell’s Treatise, § 438). Since I = κH, we have
κH + κNI = κH0,
(30)
or
H = H0 − NI,