1889.] On Time-lag in the Magnetisation of Iron. 



269 



II. " On Time-lag in the Magnetisation of Iron." By J. A. 

 EwiNG, B.Sc., F.R.S., Professor of Engineering in Univer- 

 sity College, Dundee. Received June 18, 1889. 



When any change is made to take place in the magnetic force 

 acting on a piece of soft (annealed) iron, a considerable time elapses 

 before the resulting change in the magnetism of the piece is complete. 

 The sluggishness which soft iron exhibits in assuming its full mag- 

 netism when a magnetic force is imposed upon it was referred to as 

 follows in the account which I wrote, some years ago, of experiments 

 on the magnetic qualities of iron : — * 



" Some evidence was given that, in addition to much static 

 hysteresis, there is a small amount of viscous lagging in the changes 

 of magnetism which follow changes of magnetising force. I re- 

 peatedly observed that when the magnetising current was applied to 

 long wires of soft iron, either gradually or with more or less sudden- 

 ness, there was a distinct creeping up of the magnetometer deflection 

 after the current had attained a steady value, as measured by the de- 

 flection of the galvanometer through which it passed. This action 

 was sometimes so considerable as to oblige me to wait for some 

 minutes before taking the magnetometer reading." 



In his paper " On the Behaviour of Iron and Steel under the Opera- 

 tion of Feeble Magnetic Forces, "t Lord Rayleigh has remarked on 

 the same phenomenon in soft iron. In his experiments the relation 

 of the magnetic force to the resulting magnetisation of the specimen 

 was studied by means of a magnetometer furnished with a "compen- 

 sating coil," through which the magnetising current passed, and 

 which was so placed that its action on the needle of the magnetometer 

 balanced the action of the iron, giving no deflection. When very 

 feeble magnetic forces were applied to hard iron or to steel, he found 

 that a perfect balance might be obtained by adjusting the position of 

 the compensating coil, and so established the fact that the suscepti- 

 bility to small magnetic forces, or to small changes of force, is a defi- 

 nite quantity, which is independent of the amount of the small change 

 of force. He observes that with hard iron and steel the compensat- 

 ing coil might be set so that neither at the moment of closing the 

 circuit of the magnetising current nor afterwards was there any deflec- 

 tion of the magnetometer, which means that (so far as the magneto- 

 meter can decide) the metal assumes its magnetic state instanta- 

 neously. He goes on to say that soft iron shows much more 

 complicated effects : " When the coil was so placed as to reduce as 

 much as possible the instantaneous effect, there ensued a drift of the 



* " Exp. Eesearches in Magnetism," 'Phil. Trans.,' 1885, p. 569, § 52. 

 f ' Phil. Mag.,' March, 1887, p. 230. 



