142 



SCIENCE PROGRESS. 



the necessity of having such curves constructed for his iron : 

 fig-. 1 shows two recent curves, the one for mild steel magnet 

 forgings and the other for steel castings ; the vertical ordi- 

 nates represent magnetic induction and the horizontal ab- 

 scissae the corresponding magnetising" force required to pro- 

 duce it. We are thus led to note the fact that improved 

 metallurgical processes have brought about an apparent revo- 

 lution in the nature of the material used for the magnets of 

 dynamos. A few years ago, annealed wrought iron was the 

 favourite material, and this was made of hand-picked scrap 



Fig. 1. — Curves of exciting power and induction for mild steel. 

 18000 



16000 



14000 



12000 



PQ 



C IOOOO l 



o 



C 



80OO 



6000 



4OOO 



2000 



„L^ 



Magnetising force H in C.G.S. units. 



iron forged by hammering into large slabs. While hard 

 steel was used for permanent magnets owing to its power of 

 retaining its magnetism, the presence of steel in the forgings 

 was regarded as well-niodi fatal to their magnetic value 

 owing to its low permeability, and it was claimed that any 

 admixture of this substance was best avoided by the 

 process of hand-picking. Now, mild steel is freely used, 

 and the question may be asked : How is it possible to 

 reconcile present and past practice ? The answer turns 

 upon the vagueness of the term "steel " ; in former days it 



