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BELL SYSTEM TECHNICAL JOURNAL 



forces. The manner in which the initial permeabiUty of these alloys 

 varies with composition, when heat-treated in a particular way, is 

 shown in Fig. 2, which is taken from the Arnold and Elmen paper. 

 The magnetic properties of the alloys of this series depend in an extra- 

 ordinary degree on their previous mechanical and thermal history. 

 In general, high initial permeability is obtained by rapid cooling after 

 a thorough softening of the metal by heating at a high temperature, 



40 60 



PER CENT NICKEL 



Fig. 2 — Variation of initial permeability with composition of permalloy 



this effect of rapid cooling being particularly marked on the com- 

 positions in the region of 80 per cent nickel. By control of the 

 composition and heat treatment an initial permeability of more than 

 12,000 has been obtained with an alloy of 78| per cent nickel and 

 1\\ per cent iron, whereas iron or nickel alone ordinarily have 

 initial permeabilities of only about 200 or 300. It is the high 

 initial permeability of permalloy that is most important in its use on 

 cables, though such an initial permeability as 12,000 would be even 

 higher than is generally desired for a telegraph cable. For use on 

 cable conductors permeabilities of the order of from 2000 to 5000 

 have been desired and obtained in practice. 



Another important property of permalloy with regard to its use on 

 cables is its resistivity, since high resistivity prevents excessive eddy- 



