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Prof. J. A. Ewing. 



the change of stress. The same thing is true in the present case. 

 After the circular magnetisation has been reduced from 39 to 32 by 

 successive twistings between +60° and —60° under constant magne- 

 tising force, it can be at once raised again to 39 by tapping the wire 

 vigorously. Again, if in making the steps +60° to 0° and 0° to 

 — 60° under constant magnetising force, we permit the wire to spring 

 back freely to the zero of torsion and oscillate there (instead of 

 making the twisting arm come against a stop which arrests vibration), 

 we find that the two steps give nearly equal transient currents — in 

 other words, that at the zero of torsion there is then almost no 

 circular magnetisation. 



§ 13. A still more effective way of restoring the circular magneti- 

 sation to its normal value after it has been affected by hysteresis is to 

 change suddenly the longitudinal magnetisation of the wire. The 

 molecular agitation so produced acts like mechanical vibration. 

 Almost all trace of the effects of hysteresis vanishes if we simply 

 break and remake the magnetising current in the solenoid. This is 

 especially true of soft iron; with steel the effects of hysteresis are 

 only partially removed by this means. 



For example, let the circular magnetism be reduced to 32 by 

 successive twistings between +60° and —60° under the A current,, 

 then if we break A and remake it, each of these operations gives a 

 small positive transient current, the two together showing that the 

 circular magnetism has risen to very nearly its full value of 39. Or, 

 again, if after applying A at +60° we untwist to zero, keeping A on, 

 there is a residual circular magnetism of -h 15 : then let A be broken 

 and remade, and each of these operations will give a negative tran- 

 sient current, the two together amounting as nearly as possible to 

 — 15, after which subsequent makes and breaks of the magnetising 

 current will give no effect. If on the other hand we reach the zero 

 of torsion from —60° with A on, the residual circular magnetism is 

 negative, and in that case positive transient currents are produced 

 when A is broken and remade. Generally, to remove and reapply, 

 or better, to reverse the longitudinal magnetising force while there 

 is no torsion, has the effect of removing any circular magnetisation 

 which msy have remained from previous operations. 



§ 14. To determine the normal value of the circular magnetisation 

 corresponding to different angles of twist, with the same longitudinal 

 magnetising force, we have simply to bring the twisting arm to any 

 angle, and, holding it there, reverse the magnetising current more 

 than once. The first reversal wipes out the effect of previous opera- 

 tions and sets up the normal state of circular magnetisation; the 

 second reversal changes that from + to — , and so gives a transient 

 current half of which is to be taken as the measure of the circular 

 magnetisation. The following observations were made at different 



