Page:The New International Encyclopædia 1st ed. v. 12.djvu/769

MAGNETISM. 687 MAGNETISM. since we have defined H as the number of lines of force per square centimeter when the coil has no core. Thus ji for air is equal to 1. The permeability fi differs widely in diflerent materia'ls, and in any one material may vary widely with the induction B. For in any one of the metals above mentioned with a uniformly increasing H, B increases at first slowly, then rapidly, then slowly again, entering a region in which the increments of B are exactly equal to the increments of H. In this region the material is said to be saturated. This process is shown in the curve A in Fig. 1. It will thus be noticed that with increasing values of H the ratio of B to H varies widely. In Fig. 2, a curve is given which shows the variation of the permeability in a given specimen of iron for different values of H ; as indicated, fi. increases rapidly, at first corresponding to the rapid increase in B, then decreases, corresponding to the portion of the BH curve in which the material is approaching saturation. The values of the permeability and so of the induction which may be attained in iron are greatly in excess of those obtainable in other materials, and this together with the fact that it is so much more common in nature has led to its widespread adoption in the construction of electric machinery and all apparatus in which it is necessary to have strong magnetic fields. 4 8 12 16 20 H Fig. 2. CURVE showisg variation in permeability. The behavior of iron when subjected to a mag- netizing force is usually represented by the curve between the induction B and the intensity of the force H mentioned above. Such a curve taken from a particular sample of soft iron is shown in curve A, Fig. 1. With iron in the neutral state, that is, starting from H=0, with increas- ing H, B increases slowly at first, then very rapidly, then more and more slowly, the curve approaching a slightly inclined straight line a little above 15,000. This is the region commonly spoken of as the saturated state; that is, al- though H may be continually increased, the cor- responding increase in B does not exceed that of H. If now H be decreased the values of B will not traverse the same path that they did for increasing H, but the values of B will be higher for the same value of H than those on the ascending curve. Mien H is reduced to zero again, B has still a considerable value; that is, about 11.000. There is thus within the iron a considerable amount of residual magnetism. If now H is increased in a reverse direction. B may be brought to zero, and on still further increase of H, begins to increase in the other direction; i.e. the lines of force now pass through the iron in the opposite direction. The value of H which is required to reduce the residual magnetism, that is, to bring B to the zero value, is called the coercive force. When H is increased in the reverse direction, the induction B traces the same form of curve as during the original ascent of H; that is to say, it increases rapidly for low values of H and then gradually approaches the region of saturation. If H is now decreased again there is found to be a residual magnetism of about the same amount, but of opposite sign to that before found. If H'is now increased in the same direction as at the starting of the process, the residual induction is reduced to zero only by a definite positive value of H. A further increase of H results in the rapid increase of B, and the curve finally merges into the original ascending curve. If now the same cycle of processes be gone through, the induction will traverse the same paths. The cycle then forms a closed curve of definite area. If at any point in the curve, such as tlie point C, H be de- creased, B does not decrease over the path of in- crease, but tends to hold its value, or lags behind the change in H. On increasing H again the lower part of loop C is formed, and on further increase the path joins the original curve. This tendency of B to hold its value, or to lag behind H and so to form closed loops, is called hysteresis. When iron is carried through the hysteresis cycle there is a loss of energy due to molecular friction and work done against other molecular forces consequent upon the re- versal of direction of magnetization. This energy loss is manifested in a heating of the iron. It may be shown that this loss in ergs per cycle is proportional to the area of the hysteresis curve. The magnetic properties of iron vary widely for different specimens. These variations depend in the first place on the past history of the material; that is to say, whether it has been annealed, hardened, stretched, twisted, or sub- jected to any treatment which would be likely to affect the molecular structure. In the second place, they depend on the chemical composition; that is to say, the presence or absence of such substances as manganese, carbon, etc. In general it may be stated that the .softer the iron the greater the values of B and n which may l)e obtained. The curve A, Fig. 1, is taken from a test on an average specimen of good soft iron. The maximum value of B is about 1.5,000. the maximum /t about 245, corresponding to 11=4.5. The whole curve in this case is unusually nar- row, and therefore incloses a small area, showing that in soft iron B follows H with greater readiness than in harder specimens, and that the loss due to hy.steresis is smaller. When Bub- jected to a magnetizing force the behavior of soft iron is particularly sensitive to any kind of mechanical disturbance. Thus if the specimen be lightly tapped while any given value of H is applied, the corresponding value of B may be largely increased. It retains considerable resid- ual magnetism if undisturbed, but this residual practically disappears if the iron is tapped or heated, or the molecular structure is disturbed in any way. If soft iron is mechanically hard- ened in any way, that is to say. if it is ham- mered, rolled, stretched, or twisted, its per- meability and value of residual magnetism are much lessened, and the coercive force is in- creased. If, while the iron is on the descending part of the hysteresis curve and so has the higher value of B for anv given value of H, an electric oscil-