FROM "HANDBUCH DER RADIOLOGIE" (AKA. DEMISCHE VERLAGSGESELLSCHAFT) Fig. 39.-SUSCEPTIBILITIES OF IRON, NICKEL AND COBALT ABOVE Variation of Magnetization with Field.—From a technical point of view, a knowledge of the variation of the magnetization of different ferromagnetic materials with the external field at ordi nary temperatures is of great importance, and most extensive ex perimental data have been collected. It is impossible, within the scope of this article, to do more than indicate the general charac ter of the results, details being considered only for a few repre sentative substances.
The manner in which the magnetization varies as the field changes, for a typical specimen of steel, is shown in fig. 40. The initial, unmagnetized state of the specimen is represented by the point A. As the field increases from zero, the intensity of mag netization increases at first slowly (A —B). For this part of the curve The sus ceptibility K , is thus given by K = =a+bH.
The value of K when the field is very small (a in the above expression) is known as the initial susceptibility, Ko. The corresponding initial permeability, (uo= 1+4 rico), varies greatly for different substances, being about 4o for hard steels, 200 for low carbon dynamo steel, 500 for silicon steel, and as much as 8,000 for iron-nickel alloys.
As the field increases, the intensity increases much more rap idly, giving the steep portion BC of the curve, the permeability rising to a maximum value and then decreasing. The maximum permeability is roughly inversely proportional to the coercive field for ordinary iron and steel; it is of the order of too for hard steel, 200-700 for cast iron, 3,000-8,000 for electrolytic iron. For pure silicon alloys values of over 6o,000 have been obtained, and for permalloy (an iron nickel alloy with 78.5% nickel) values of over 8o,000. In strong fields the saturation value of the magne tization appropriate to the temperature is gradually approached. Saturation is reached at lower field values the higher the initial susceptibility. Thus saturation is practically attained at about 2,000 gauss for soft iron and at about 7,000 gauss for cast iron, while permalloy only requires about coo gauss.
The manner in which the intensity of magnetization varies with the field when the field varies periodically between definite limits is in general quite different from that indicated by the curve ABCD (fig 4o). On decreasing the field to zero after the state D, for example, has been reached, the magnetization is not re duced to zero, but there is a certain residual magnetization which is greater the greater the maximum intensity. The term "re manence" is used for the residual magnetization when the field is reduced to zero from a strength sufficient to magnetize the speci men to the saturation value, and is specified by the remanent intensity or induction .13, (Br =47r Zr). To reduce the magne
tization to zero (point F of the curve) it is necessary to apply an opposing field, which also depends on the maximum intensity which has been reached, and in the limiting case is termed the "coercivity," which is specified by the strength of the coercive field H,. In fig. 4o only the up per part of the magnetization curves are shown (corresponding to the magnetization in the direction of the positive field). Some typical shapes for complete hysteresis curves are shown in fig. 41.
The curves of fig. 41 corre- Fig 41.—SOME TYPES OF HYSTER spond to the variation of the field ESIS CURVES between definite limits +H and —H. If these values of H are sufficient to produce saturation the limiting hysteresis cycle is obtained; the curves for smaller maximum values of the field lie within the limiting cycle. If the specimen has oeen brought into a cyclic condition by reversals of the maximum field, the de scending and ascending branches of the hysteresis curve are simi lar in form. (In fig. 4o, GD is the upper half of the ascending branch.) The intensity of magnetization of a ferromagnetic does not de pend simply on the field that is applied at the time, but also on the manner in which it has been magnetized before, i.e., on the previous history of the specimen. This may be attributed to the occurrence of both reversible and irreversible processes during magnetization, in the manner suggested both by the Ewing and Weiss theories. It is possible experimentally, to a certain extent, to separate the two processes. Thus, in fig. 4o, when the specimen has been brought to the state represented by the point M, if the field is slightly decreased by a small amount OH, the path MB is not retraced, but the intensity decreases by an amount I, as in dicated by the short line MP. On increasing the field again by OH the path PM is retraced—the process is reversible. Similar re versible changes occur when the field is increased (or the field in the negative direction decreased) at N. The ratio 8//oH under these conditions is known as the reversible susceptibility K,.. It is found that Kr depends only on the intensity of magnetization (being, for example, the same when the specimen is in the states represented by M and N) ; and that it is a maximum when the specimen is unmagnetized (being then the same as the initial susceptibility K,,) and decreases towards zero as the saturation value is approached. A theoretical treatment has led R. Gans to the conclusion that should vary with (where is the saturation intensity) in the same way for all ferromagnetics, which seems to be approximately borne out by the experiments which have been made.