264 



Professor Fleming 



[June 5, 



as seen from the curve (see Fig. 17) — that cooling the iron to — 186° C. 

 slightly diminishes the permeability. In other words, it requires 

 a greater magnetic force to produce a given amount of magnet- 

 isation when the iron is at — 186° C. than when it is at the ordinary 

 temperature. 



When, however, we began to study the behaviour of hardened 

 iron in this respect, we found ourselves in the presence of very 



Fig. 17. 



Magnetisation and permeability curves of soft iron at 20° C. and - 186° C. 



curious effects. If pure iron, which has been carefully annealed, is 

 twisted, knocked, bent, stretched, or compressed, it passes into a state 

 known as hard iron, and hard iron has very different magnetic 

 qualities from soft iron. A very extended series of experiments 

 with rings of hard iron have shown that hard iron, at least in certain 

 cases, has its permeability greatly increased by cooling, and this 

 change takes place with great suddenness. We can show you by a 

 simple experiment that this is the case. If we take this hard iron 



