184 



MAGNETISM 



magnetisation retained, represented by O M in the figure, and this 

 retained magnetisation is the residual magnetism. It measures the 

 retentivity of the specimen. If the current is now gradually 

 increased from zero in the opposite direction the magnetisation 

 decreases and will be zero for a certain magnetising force O N in the 

 direction opposite to that of the original force. This is termed the 

 coercive force. On still further increasing the reversed current the 

 magnetisation is reversed. If we cease to increase it at Q and then 

 begin to diminish again, hysteresis is shown, and when the current 

 is again zero there will be residual magnetisation O M' now in the 

 opposite direction to O M. Increasing the current from zero in 

 the original direction, we shall travel to a point near P, but a little 

 below it, when the original maximum current is reached, and 

 repeating the cycle we shall get another curve. After a number of 

 cycles the curves obtained for each practically coincide, as we have 

 supposed in the figure. The area P M Q M ' P is termed ///< 

 hysteresis loop. 



The hysteresis curve varies for different specimens of iron and 

 steel. Thus for magnet steel O M is not very great, but O N is 

 considerable; while for soft iron O M may be very great and 

 ON may be small. 



Susceptibility and permeability. The greater the poles 

 developed in a bar of iron or steel under a given magnetising force, 

 the greater is the snwrplibilitii of the bar to magnetisation. When 



Fit;. 182, 



the magnetising force and the magnetisation are Miitably measured 

 the ratio magnetisation /magnetising force is termed the nm^m'tic 

 susceptibility (see p. 240). It is usually denoted bv K. Without at- 

 tending to the scale of measurement we can see that if O A B C, 

 Fig. 152, represents the curve of magnetisation, then the eurve repre- 

 senting the susceptibility will have a form somewhat like that of the 

 dotted line. In the first straight line stage O A, the susceptibility 

 will be constant. Then in the early part of A B as the curve 

 beads round it will rise rapidly, increasing to somewhere near the 

 second bend about B and then falling off' again. 



