44 Effects of Retentiveness in Magnetisation of Iron. [May 25, 



zero. The magnetic rigidity, as the reciprocal of this differential co- 

 efficient might be called, is infinite at the beginning of any change 

 from loading to nnloading, or from nnloading to loading, provided 

 that the change takes place without^ agitation of the wire. In the 

 thermoelectric experiments described in my former paper, the beginning 

 of the new curve generally continued to show the same kind of change 

 as had been going on before. The same peculiarity can be reproduced 

 here if the loading or unloading occurs with a slight vibration of the 

 wire, and I now think it almost certain that its presence in the thermo- 

 electric curves was due to a very small amount of mechanical dis- 

 turbance which accompanied the changes of load. 



When the wire is vigorously tapped during the loading and un- 

 loading, the " on " and " off " curves so nearly approach coincidence as- 

 to lead to the conclusion that a sufficient amount of vibration would 

 destroy the hysteresis altogether. As an instance of this, the full 

 lines in fig. 2 show the changes undergone by another wire (which 



