AGNETIC PROPERTIES OF IRON. We have seen that when an electric current is sent through a coil or spiral of wire the coil is found to have the properties of a magnet; that is to say, it will at tract bits of iron, deflect a magnetic needle, etc. The region around the spiral when the current is flowing in it is called a magnetic field. The intensity or strength of the field is defined as the force which would act on a unit north pole, and so varies from point to point in the neighborhood of the spiral. In describing this field the con vention has been adopted of imagining the field traversed by lines of force which indicate by their direction the path the unit north pole would follow if free to move. When the field has unit strength it is said to contain one line of force per square centimeter perpendicular to the direction of the line. It may be shown that the intensity of the field, or the number of lines of force per square centimeter within the spiral carrying the current, is represented by the formula H= 0.4rroni, H being the intensity of the n the number of turns per unit length of the spiral. and i the current strength in amperes.
If now the coil is wound on a core of iron, nickel, or cobalt, its properties as a magnet are found to be greatly intensified; that is to say, the number of lines of force per square centi meter is increased; i.e. the core has become mag netized. The number of lines of force per square centimeter within the core is called the t ion. and is usually designated by the letter B. The ratio of the induction B to the intensity of the field H is called the-permeability, i.e. B = pH. For air B is evidently equal to II, since we have defined H as the number of lines of force per square centimeter when the coil has no core. Thus g for air is equal to 1.
The permeability ,a differs widely in different materials, 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. I. It will thus be noticed that with increasing values of H the ratio of B to 11 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, ,a increases rapidly, at first corresponding to the rapid increase in B, then decreases, corresponding to the portion of the BIT 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.
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. I. 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. When 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 II 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 the 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.