MEASUREMENT OF MAGNETIZATION AND INDUCTION The high magnetic permeability of ferromagnetics permits the use of a number of methods, not generally applicable, for deter mining their magnetic characteristics. Much experimental work has been carried out owing to the importance of a knowledge of the magnetic properties of different types of iron and steel for technical purposes. It will only be possible here to indicate the essential features of some of the main methods of magnetic test ing. In general the variation of the induction B, or the intensity of magnetization I with the magnetizing force H is required.
The specimen should be in the form of an ellipsoid, or a rod or wire whose length is some 30o times its diameter. It is placed vertically (A, fig. ro). The mirror magnetometer M is placed as in fig. 8 with its needle opposite the upper end of the wire. L and S are a lamp and scale by means of which the deflections are measured. The specimen at A is surrounded by a solenoid which is somewhat longer than the rod. This solenoid is sup plied by current from the battery via the potentiometer arrangement DF, a reversing key K, a switch H, a galvanometer G, and a subsidiary coil C. Outside the inner solenoid is a second coil supplied with current by whose object is to produce a field to neutralize the vertical field of the earth. C is a compen sating coil whose position can be adjusted so as to neutralize any direct effect of the magnetizing coil on the magnetometer. By changing the position of E the current can be varied. The mag netometer readings are taken corresponding to different values of the current, from which the field inside the magnetizing coil can be calculated. Let 0 be the deflection, corresponding to a field and let HE be the controlling horizontal field of the earth.
Let I be the intensity of magnetization of the specimen, v the volume, I the distance between the poles, S the cross section. Then, from the equation given above, the major and minor axes are equal to 20 and 2C, and the mag netization is along the major axis, the ellipsoid behaves externally as though the poles were situated at a distance *a from the centre. For cylindrical and rectangular bars, no general statement may be made as to the distribution of magnetism, but from experi mental investigations, according to Kohlrausch, for rods with a length to diameter ratio of io to 3o the distance between the equivalent poles is approximately five-sixths the length of the rod.
The resultant magnetic force inside a magnetized body placed in a magnetic field is made up of the force due to the external field, Ho, and that arising from the magnetization of the body. If H is the resultant force, The value of N, the demagnetizing factor, can be exactly calculated only when the magnetization is uniform. For an ellipsoid of revolution, with its axis of revolution parallel to the lines of force, and of ec centricity e, may be demagnetized by gradually reducing the current from its maximum value to zero (by moving E from F to D), at the same time continually reversing it by means of the key K. Starting from the condition of zero magnetization, the magnetiza tion changes with changing field in the way indicated in fig. 3.
The "end-on" and "broadside-on" positions may also be used in measuring the magnetization, but in these it is necessary to know more accurately the distance between the poles of the specimen. It may be shown that uniform magnetization is only possible if the form of the body is ellipsoidal. In this case, if Owing to the uncertainties in connection with rods, the mag netometer method is less accurate than the ballistic, except when ellipsoids can be employed. The end correction for the demag netization may be conveniently applied by drawing on the I, Ho diagram a line through the origin inclined at an angle equal to N to the I axis. This inclined line then forms an axis from which the corrected H may be measured.