36 



THE MAGNETIC CIRCUIT 



[ART. 15 



rium until the energy is dissipated by being converted into heat. 

 This heat represents the loss of energy due to hysteresis. 



This theory of saturation and hysteresis is due originally 

 to Weber, and has been improved by Ewing, who has shown 

 experimentally the possibility of various stable groupings of 

 a large number of small magnets in a magnetic field. By varvinu; 

 the applied m.m.f. he obtained a curve similar to the hysteresis 

 loop of a sample of iron. For further details of this theory see 

 Ewing, Magnetic Induction in Iron and other Metals (1892), 

 Chapter XI. 



The following analogy is also useful. Let a body Q (Fig. 8), 

 rest on a support and be held in its central position by two springs 



FIG. 8. A mechanical analogue to hysteresis. 



S, S, which can work both under tension and under compression. 

 Let this body be made to move periodically to the right and to 

 the left of its central position, under the influence of an alterna- 

 ting external force H. Call B the deflections of the body from 

 its middle position. The relation between B and H is then similar 

 to the hysteresis loop in Fig. 7, provided that there is some 

 friction between the body Q and its support, and provided that 

 the springs offer in proportion more resistance when distorted 

 greatly than when distorted slightly. 



Starting with the neutral position of the body let a gradually 

 increasing force H be applied which moves the body to the right. 

 This corresponds to the virgin curve in Fig. 7, except that this 

 simple analogy does not account for the inflection in the virgin 

 curve near the origin. Let then the force H be gradually reduced, 

 allowing the springs to bring Q nearer the center. When the 



