148 BELL SYSTEM TECHNICAL JOURNAL 



Magnetic Materials 



Telephone apparatus presents a great diversity of applications for 

 magnetic materials. Both soft * and permanent magnet materials are 

 extensively used. The soft magnetic materials are employed both as 

 sheet and rod and in a finely divided form for compressing into cores 

 for inductance coils. Previous to 1920 the primary soft magnetic 

 material was iron; small quantities of silicon steel also were used. 

 Since that date a large number of new soft magnetic materials have 

 been developed with superior properties for particular applications. 

 The discovery of permanent magnet characteristics in dispersion- 

 hardening iron alloys containing no intentional carbon has resulted in 

 a number of new permanent magnet materials of superior properties. 



At this time, in the field of soft magnetic materials, iron and silicon 

 steel find by far the most extensive application. The iron is a high 

 grade commercial iron. The silicon steel used is the grade normally 

 containing about 4 per cent silicon. For applications requiring higher 

 permeabilities and lower losses, alloys of iron and nickel, known as the 

 permalloys, are used. There are two principal permalloys, one con- 

 taining about 80 per cent nickel and another 45 per cent. The higher 

 nickel composition is also modified by molybdenum or chromium 

 additions to increase electrical resistivity and improve magnetic 

 properties. Sheet and rod stock are used in relays, transformers, 

 miscellaneous coils, and ringers, 



, In investigating magnetic materials in the laboratory it is desirable 

 frequently to fabricate the alloy into extremely thin sheet. The 

 twenty roll cold-reduction mill shown in Fig. 6 is of value for this 

 purpose. It is equipped with small diameter working rolls, each 

 backed by a cluster of nine supporting rolls. With this arrangement 

 high unit pressures are obtained and sheet a fraction of a mil thick can 

 be produced readily. 



In the form of 120-mesh powder and even in finer sizes certain of the 

 permalloys find application in loading coils, filter coils and associated 

 equipment. To secure low losses the powder particles are each insu- 

 lated with a high resistivity, heat resistant material prior to pressing 

 into cores. Manufacture of this fine alloy powder is a unique metal- 

 lurgical process taking advantage of the effects of small amounts of 

 added elements to achieve a desired result. The presence of a few 

 thousandths per cent of sulphur in the iron-nickel alloys in the range 

 of 80 per cent nickel results in a structure which can be rolled to small 



* The term soft is used to designate materials of relatively high permeability and 

 low magnetic loss. Likewise, permanent magnet materials are frequently referred to 

 as "hard." 



