CONTEMPORARY ADVANCES IN PHYSICS 



317 



sharp maximum, and so also does the more-commonly-plotted ratio 

 of B to H, the permeability /i. 



^ = BIB = (// + 4.ttI)IH = 1 + ^T^mii). 



Pure iron attains much higher values of permeability than does 

 either of the other metals which can be ferromagnetic when pure. 

 By careful purifying and long annealing, T. D. Yensen elevated Mmax. 



25000 



20 000 



H 



Fig. 2 — Initial curves for annealed iron, nickel, and cobalt. 

 (After L. W. McKeehan.) 



for iron to 19000; the best recorded values for nickel and for cobalt are 

 very considerably lower. Certain alloys of iron, however, leave the 

 pure metal far in the rear; by slight admixtures of silicon (between 

 0.15 per cent and 4 per cent) Yensen produced materials which, after 

 being melted in vacuo, annealed at a high temperature and very 

 slowly cooled, developed a value of /Zmax. as high as 66500. These in 

 their turn were surpassed by the permalloys of G. W. Elmen for 

 which Mmax. ascended past 100000. 



Other alloys of iron, and in fact nearly all of them, are much inferior 

 to the pure metal in respect of /^max.- Carbon in particular is 



