ON THE TIME-LAG IN THE MAGNETISATION OF IRON. 297 



about 125 per second, the magnetic cycle is unaffected by the frequency 

 so far as the maximum induction for a given magnetising force is 

 concerned ; but that there is a sensible difference between the curves 

 as determined by a slow cycle and a r.-ipid one, most apparent in that 

 part of the curve preceding the maximum induction; and that this 

 difference possibly arises from something peculiar to experiments with 

 the ballistic galvanometer. 



Dr. J. Hopkinson with his two co-workers continued the investi- 

 gation, 1 and examined hard steel particularly, fully subjecting it to 

 reversing fields of frequencies, 5, 72, 125 times per second. Their 

 conclusions are: — " 1) As Prof. Ewing has already observed, after 

 sudden change of magnetising force, the induction does not at 

 once attain to its full value, but there is a slight increase going; 

 on for some seconds. 2) The small difference between the ballistic 

 curve of magnetisation with complete cycles and the curve determined 

 with a considerable frequency, which has already been observed, is a 

 true time effect, the difference being greater between a frequency of 72 

 per second and 5 per second than between 5 per second and the 

 ballistic curve." 2 



As the subject seemed to be interesting, I undertook to examine 

 the phenomenon myself and sought to obtain some information about 

 its nature. 



Arrangement of Apparatus. 



My plan was to follow Prof. Ewing's method of usin^ an ordinary 

 magnetometer, and observing the creeping, the only difference being 

 that the magnetising forces were greater than were tried by him, and 



1 Proc. Royal Society, 53 } p. 352 (1893.) 



2 Loc. cit., p. 358. 



