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 H o, due to the external field, and of another force, Hi, arising from the induced magnetization of the body.
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.
Since I =KH, we have KH I-KNI =KHo, (30) or H =Ho-NI, NI being the demagnetizing force H i.
Du Bois (Magnetic Circuit, p. 33), the demagnetizing factor, and the ratio of the length of the ellipsoid 2c to its equatorial diameter 2a (=c/a), the dimensional ratio, denoted by the symbol nt.
The demagnetizing force inside a cylindrical rod placed longitudinally in a uniform field Ho is not uniform, being greatest at the ends and least in the middle part.
Denoting its mean value by Hti, and that of the demagnetizing factor by N, we have H=Ho-Hti=Ho-NI.
Du Bois has deduced the corresponding mean demagnetizing factors.
P. 41): Demagnetizing Factors.
Retained by a bar of ferromagnetic metal after it has been removed from the influence of an external field produces a demagnetizing force NI T, which is greater the smaller the dimensional ratio..
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.
The magnetometric method, except when employed in connexion with ellipsoids, for which the demagnetizing factors are [[[Magnetic Measurements]] accurately known, is generally less satisfactory for the exact determination of induction or magnetization than the ballistic method.
With these arrangements there is no demagnetizing force to be considered, for the ring has not any ends to produce one, and the force due to the ends of a rod 400 or 500 diameters in length is quite insensible at the middle portion; H therefore is equal to Ho.
19 shows the apparatus by which the ends of the bar are prevented from exerting any material demagnetizing force, while the permeance of the magnetic circuit is at the same time increased.
Demagnetizing action which might be due to the two transverse cuts, it is probable that they are somewhat too high.
In all such measurements a correction should be made in respect of the demagnetizing force due to the joint, and unless the fit is very accurate the demagnetizing action will be variable.
Since the demagnetizing factor was o 052, the strongest field due to the coil was about 1340; but though arrangements were pro vided for cooling the apparatus by means of o ice, great difficulty was experienced owing to heating.