Field

FIELD H Fig. 40.—CURVES SHOWING THE VARIATION OF INTENSITY OF MAG NETIZATION WITH FIELD. FOR A TYPICAL SPECIMEN a :a etio, and also a strength of the final intensity be steady held, rim. In this tsis is obtained. The interesting from the origin ttersity of rug. Ilapplied field. it lends strong agnetizatioll of an irreversible ithe Weiss the 100taBtOUS nag' his changes are leitatian cot id flitude of the through ities in the ;sea 1 a coil.

intious ch4ges .

ick actiag mag has cre 1froP of d chall!e 0.

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Jr stl 1 -c hot z; ttt the co,* s'Peutuez j, if the Path ME the t ratio is in the states rut when thE' the initial the saturation led R. Gans to (where I, is ferrotnapetics, the sliFeriments verse field (see fig. 4o, ENF on the curve), as it does when the co ercivity H, is small, the apparent remanence may be much less than the true remanence. The higher the coercive field the more nearly will the apparent remanence approach the true remanence. A good approximate measure of the effectiveness of a material for a permanent magnet is given by the product of the remnant induction and the coercive field The requirements are the opposite of those for materials in which a small hysteresis loss is required. Many materials in which the remanent magnetism is high (such as dynamo steel) are quite unsuited for permanent magnets owing to the small value of the coercive field.

In general high remanence is associated with small coercivity; the production of materials with high remanence and high co ercivity has presented a most interesting problem. The materials most used have been hardened high carbon steels, tungsten steels, and the remarkable cobalt steels, some of which have a coercivity of as much as 24o gauss. Some constants of different materials are given below. It is of course also necessary that material for permanent magnets should not be liable to gradual changes of a chemical nature; that it should not be affected by mechanical shocks; and that its magnetic characteristics should change as little as possible with temperature variations. Magnets are gener

ally stabilised by an artificial ageing process involving thermal and mechanical treatment.

Magnetic Characteristics of Particular Materials.—The way in which the intensity of magnetization varies as the field is increased from zero, the specimen being initially demagnetized, is shown for a number of materials in fig. 42. The form of the initial curve, and of the hysteresis curves can be markedly influenced by mechanical and thermal treatment of the specimen. Even in the case of the purest form of iron produced by elec trolysis, the remanence is much smaller after rapid cooling from the annealing temperature than after slow cooling. For this rea son the shapes of the curves of fig. 42 should be regarded as typi 1600 t 1200 a t 800 400 Fig. 42.--CURVES SHOWING THE VARIATION OF MAGNETIZATION WITH INCREASING FIELD FOR INITIALLY UNMAGNETIZED SPECIMENS cal of specimens which have undergone a particular treatment. The saturation intensity, however, depends only on the chemical constitution.

The word "iron" is used not only for the pure metal, but also for commercial iron, in which there may be a large amount of purities, depending on the method of manufacture. The impurities present in crude cast iron (pig iron) account for the much lower saturation intensity than that of dynamo steel, which contains about 99.5% iron. The impurities in cast iron may amount to about no%, but owing to their low specific gravity, the reduction hysteresis cycle is then different from the cycle taken slowly. While there may be quite generally a very slight viscosity effect, it is interesting that it is prominent only in the case of materials which are magnetically soft, having a low co ercivity, and that it then occurs for the relatively small fields corresponding to the steep part of the final magnetization curve.