Permanent Magnets

Author: E.E. Kimberly

If an annular ring of hardened steel or special alloy is magnetized to the point b in Fig. 7-7 (a) and the mmf is then removed, the magnetic strength decreases to point c. Such a structure would have no practical value because all of the flux would be inside the metal and would not be available for use. To make the flux available it is necessary to cut a portion of the metal out to provide an air-gap, as in Fig. 7-6. The air-gap causes an increase in the reluctance of the circuit and, hence, a reduction of remanent flux. The result is the same as though a reversed mmf had been applied to the ring and had driven the flux farther down along the hysteresis loop to a point such as m in Fig. 7-9, which shows only the demagnetization portion of Fig. 7-7 (a); permanent-magnet operation is confined to that portion. The distance to m from c is determined by the length of the air-gap cut in the ring.

Fig. 7-9. Demagnetization Curve and Minor Hysteresis Loop

If the metal of the air-gap were to be replaced,the flux density would not return to point c but would return to point n along what is called a minor hysteresis loop. If the air-gap metal were again removed, the density would return to m along the upper portion of the minor hysteresis loop.

If a permanent magnet is to be used in a device, such as a direct-current measuring instrument, from which it may be removed at intervals to facilitate repair of the device, its magnetism must be stabilized at some value below that at which it is expected to work after it is assembled. This stabilization is usually accomplished by "knocking down" the magnetism by applying a small amount of demagnetizing mmf.