1889.] On Time-lag in the Magnetisation of Iron. 279 



Fig. 6. 



However small tlie step is it appears to be followed by a creeping* 

 up of magnetism. I have been able to discover nothing which would 

 correspond with the limit of perfect elasticity in straining a solid (if 

 there be any true limit of elasticity), either in the initial part of the 

 process of magnetisation, or after the prolonged application of a 

 constant magnetising force. 



But the prolonged application of a constant magnetising force 

 produces an effect which is a most interesting anologue of one effect 

 of prolonged loading in a stretched wire. It is well known that when 

 a load (sufficiently great to produce permanent set) is applied to a 

 stretched iron wire, there ensues, with the lapse of time, not only a 

 certain amount of supplementary viscous extension (the analogue of 

 the magnetic creep) but also a quasi-hardening of the metal which 

 becomes manifest when an addition is made to the load.* One effect 



* Of. * Roy. Soe. Proc.,' No. 205, 1880, or ' Encycl. Brit.,' art. " Strength of 

 Materials." 



