6o2' MR JAMES RUSSELL ON 



during this demagnetising process the induction were measured we would obtain a 

 series of decreasing hysteresis loops, the curves of which would not be closed upon 

 themselves. Their extreme positive and negative values would continuously decrease, 

 until they vanished in the zeros of H and B. At the first positive value of B, the 

 molecular magnets may be supposed for the most part to be well in line with the 

 magnetising force H. On this being reversed to a somewhat less negative value, the 

 measured induction would be less. To what does this correspond in terms of the 

 molecular magnets ? The force being again reversed to a still less positive value, the 

 measured induction would again prove to be less, and the same question arises, and so 

 on until the reversals of H and B are as small as you choose to make them. 



In unmagnetised iron the resultant axes in all stable molecular groups are on the 

 average equally distributed in all directions, and on the average the stability is the 

 same in all directions. The turning moment tending to turn such a group when the 

 axis makes an angle with the force H is 



m — a H sin 6. 



During the process of demagnetisation by reversals, and as the decreasing, repeatedly 

 reversed force dies down, the force which is just able to turn a group into a stable 

 position 9 which is just retained when the force is reversed, is smaller and smaller as the 

 angle increases. Assume that it is proportional to cos 6, then 



m = <zK cos 6 sin 6 = \ aK sin 2 



, . , dm . , 



and m is a maximum when ^ is equated to zero 



din ^ „ n 

 -77, =aKcos2 = O 

 dO 



or when 6 = -7 ■ 



The turning moment is greatest when the angle 6 = 45°, and it vanishes when = 0° and 

 when 0-90°. 



§ 35. Now repeat the demagnetising process, the magnetic axis of the molecules 

 coinciding on the whole with the direction of H. On reversing to a somewhat less 

 negative value of H, the majority of the molecules will topple over, but some will be 

 left. On reversing again, the lower positive value of H will again fail to topple over 

 all the molecules, but still the majority will topple over under the influence of the 

 deflecting force H = K cos 0. And so on the process will go, leaving molecules or groups 

 of molecules distributed at all angles between = and = 90°. But the turning 

 moment, m, being a maximum when = 45°, it follows that at each reversal the molecule 

 or group of molecules left stranded which the force H has failed to topple over will be 

 wider spaced the nearer the angle in which they are left approximates to 45°, and closer 

 spaced the nearer the angle in which they are left approximates to = 90° or to = 0°, 

 because as these angles are approached the turning moment is getting less and less. 



