OX THE TIME-LAG IX THE MAGNETISATION OF IRON. 319 



menon of time-lag, the above may be taken provisionally as account- 

 ing for something. I am quite ready to give it up when any 

 theoretical advance supersedes its purpose. 



Before closing this paper, I must refer to that of Holborn, 

 on the same subject, to which I had access while writing this. 

 He used the method of Helmboltz's " Pendelunterbrecher," and 

 the ballistic galvanometer. In one of his experiments, the specimen 

 was a bundle of fine iron wires, each -20 mm. thick. With a mag- 

 netising field of 48 C.G.S., the magnetising current reached its full 

 strength after .00508 second, and with it the magnetisation also. 

 With a magnetising field of 2 C.G.S., the magnetisation remained 

 constant after .005 second. With a thicker wire, of diameter 4.5 

 mm., under a magnetising field of 50 C.G.S., the magnetising current 

 reached its full strength after .01 second, and with it the magnetisa- 

 tion also. He concludes that as soon as the magnetising current 

 reaches its full strength the magnetisation does so also ; that is to say, 

 there is no time effect in the magnetisation. I think that this con- 

 clusion can only be true within a certain degree of approximation. It 

 is clear that in such a comparatively strong field as 50 C.G.S., the 

 time-lag will be very small, especially in thin wires. But the case is 

 different when the field is low and the wire is thicker. I should 

 like very much to know whether he tried with the wire 4.5 mm. thick 

 in a magnetising field of 2 G.G. IS. and. if he did, what result he got. 



1 Sitzungsberichte, h. />. A. W. (Berlin), XI. p. 173, 27, Feb. 1896. 



