1889.] 



On Time-lag in the Magnetisation of Iron. 



285 



sequent creeping was 3. In steel and in hard iron the creeping 

 seemed to be completed in a few seconds after the institution of the 

 magnetising current. The steel specimen, like the iron, had a 

 diameter of rather more than 4 mm. Its susceptibility (annealed) 

 was considerably less than that of the iron in the hard state. 



It is scarcely necessary to observe that the protracted and exten- 

 sive creeping or magnetic " nachwirkung " in soft iron which these 

 experiments illustrate cannot be ascribed to the subsidence of 

 the circumferential currents which are generated by the imposition of 

 longitudinal magnetic force. The creeping is equally conspicuous 

 whether the magnetic force is suddenly or gradually imposed. Lord 

 Rayleigh has shown that circumferential currents started and left to 

 themselves will subside to e~ l of their initial magnitude in the 

 time 



7 ~ (2-404)V ' 



where a is the radius of the cylinder, fi its permeability, and p its 

 specific resistance.* In the present instance, taking the case of the 

 annealed iron rod, a = 0*202, fi = 125, p = 9827 (Everett), and t is 

 less than T oVo °^ a second. The subsidence would be practically 

 complete in a small fraction of a second : but the creeping persists 

 during many seconds and even minutes with no excessive change of 

 rate. Again, comparing soft iron with hard iron, in which /a is less 

 and p is greater, the values of t will differ, but not by any means so 

 much as to correspond with the very wide difference in magnetic lag. 



In view of this it is puzzling to find that the diameter of the rod 

 experimented upon has a most important influence on the magnetic lag. 



In testing various samples of soft iron wire, most of which were of 

 less diameter than the piece used in the above experiments, I 

 noticed that the phenomena of creeping were less marked in the 

 smaller rods. I then tried a bundle of nine very soft annealed iron 

 wires, which were bound together with fine copper wire, and formed 

 a core of about the same length and aggregate diameter as that of the 

 solid rod formerly used. With this bundle there was some creeping, 

 but very little in comparison with what was observed in the solid rod, 

 as the following notes show : — 



Bundle of nine soft Iron Wires. 



Magnetometer deflections. 



r~ . > 



Magnetic force H Immediate (balanced Subsequent 



suddenly applied by compensating creeping in Total 



(c.g.s.). coil). 1 min. in 1 min. 



0-052 17 2 19 



0147 52 9 61 



' * ' Brit. Assoc. Keport/ 1882, p. 446. 



