HYSTERESIS

lagging of the magnetization of a ferromagnetic material, such as iron, behind variations of the magnetizing field. When ferromagnetic materials are placed within a coil of wire carrying an electric current, the magnetizing field, or magnetic field strength H, caused by the current forces some or all of the atomic magnets in the material to align with the field. The net effect of this alignment is to increase the total magnetic field, or magnetic flux density B. The aligning process does not occur simultaneously or in step with the magnetizing field but lags behind it. Magnetic hysteresis loop If the intensity of the magnetizing field is gradually increased, the magnetic flux density B rises to a maximum, or saturation, value at which all of the atomic magnets are aligned in the same direction. When the magnetizing field is diminished, the magnetic flux density decreases, again lagging behind the change in field strength H. In fact, when H has decreased to zero, B still has a positive value called the remanence, residual induction, or retentivity, which has a high value for permanent magnets. B itself does not become zero until H has reached a negative value. The value of H for which B is zero is called the coercive force. A further increase in H (in the negative direction) causes the flux density to reverse and finally to reach saturation again, when all the atomic magnets are completely aligned in the opposite direction. The cycle may be continued so that the graph of the flux density lagging behind the field strength appears as a complete loop, known as a hysteresis loop. The energy lost as heat, which is known as the hysteresis loss, in reversing the magnetization of the material is proportional to the area of the hysteresis loop. Therefore, cores of transformers are made of materials with narrow hysteresis loops so that little energy will be wasted in the form of heat. Figure 9: General magnetic hysteresis curve, showing magnetization (J) as a function of the external field (Hex). Js is the saturation (or spontaneous) magnetization; Jr,sat is the remanent magnetization that remains after a saturating applied field is removed; Jr is the residual magnetization left by some magnetization process other than IRM saturation; Hc is the coercive field; and Hc,r is the field necessary to reduce Jr to zero. hysteresis loop hysteresis loop Magnetic hysteresis loop hysteria a type of psychiatric disorder in which a wide variety of sensory, motor, or psychic disturbances may occur. It is traditionally classified as one of the psychoneuroses and is not dependent upon any known organic or structural pathology. The term is derived from the Greek hystera, meaning uterus, and reflects the ancient notion that hysteria was a specifically female disorder resulting from disturbances in uterine functions. Actually, hysterical symptoms may develop in either sex and may occur in children and elderly people, although they are observed most commonly in early adult life. Hysteria, in its clinically pure form, seems to occur more often among the psychologically naive than among sophisticated persons. Hysterias tend to be more common among persons in the lower ranges of intelligence than among those in the higher ranges. The incidence of hysteria appears to have been diminishing over the years in many areas of the world, probably because of cultural factors such as increasing psychological sophistication, diminishing sexual prudery and inhibition, and a less authoritarian family structure. Cases of classical hysteria, such as those frequently described by 19th-century clinicians, have become rare. Most psychoneuroses encountered in actual clinical practice are apt to be mixed forms in which hysterical symptoms may be found interspersed with other varieties of neurotic disturbances. Isolated hysterical symptoms may also occur in conjunction with psychotic disorders. The sensory and motor manifestations of hysteria take many forms and are designated conversion reactions because the underlying anxiety is assumed to have been converted into a physical symptom. Sensory disturbances may range from paresthesias (peculiar sensations) through hyperesthesias (hypersensitivity) to complete anesthesias (loss of sensation). They may involve the total skin area or any fraction of it, but the disturbances generally do not follow any anatomic distribution of the nervous system. In medieval times in Europe and as late as the end of the 17th century, the finding of such discrete areas of anesthesia on the body of a person was considered proof that the person was a witch. Other hysterical sensory disturbances may encompass the special senses of vision, hearing, taste, or smell; or they may involve the experiencing of severe pain for which no organic cause can be determined. Motor symptoms vary from complete paralysis to tremors, tics, contractures, or convulsions (hystero-epilepsy). In each instance neurological examination of the affected part of the body reveals an intact neuromuscular apparatus with normal reflexes and normal electrical activity and responses to electrical stimulation. Other motor disturbances that are at times hysterical in origin are loss of speech (aphonia), coughing, nausea, vomiting, or hiccuping. Psychic symptoms may be equally varied and are usually classified under the broad heading of dissociative reactions. Attacks of amnesia, in which the person is unable to remember who he is or anything about himself, are among the more striking of these. Sleepwalking (somnambulism) is also considered to be a hysterical dissociative reaction, as are also the occasional dramatic cases of multiple personality.