MAGNETIC MATERIALS IN TELEPHONE SYSTEM 449 



$2.80/lb. increase the cost of an alloy very considerably in comparison 

 with iron or steel at less than 7(z5/lb. High priced magnetic alloys can 

 only be justified in general when their extraordinary characteristics 

 permit ofifsetting apparatus performance or economies. Of course, 

 they may be absolutely necessary for certain types of apparatus. 



The cost data for Tables I and II have been calculated from recent 

 prices ^^ for raw materials of quality suitable for magnetic alloys. 

 The cost of raw materials is given in Fig. 3 for selected alloys. The 

 low raw materials costs of iron, silicon-iron, and chromium steel are 

 notable, as well as that of the powder core material "Sendust." 

 45 permalloy is the cheapest of the high permeability materials, while 

 Mishima steel is the cheapest of the high quality permanent magnet 

 materials. 



Comparisons based on raw materials costs are not entirely satis- 

 factory. The cost of alloying and reducing to finished form may 

 overshadow the cost of raw materials, particularly when high purity, 

 exact tolerances, and small rates of production are involved. 



Applications 



Almost all magnetic properties are utilized in some type of telephone 

 apparatus. They are generally linked inseparably with electrical and 

 mechanical properties. The proper design of any apparatus strikes 

 a compromise between the various technical features and cost. The 

 technical features of materials used in present day apparatus are 

 listed below. Acceptable common properties, such as mechanical 

 soundness and workability, are assumed for all materials unless spe- 

 cifically mentioned. Listing is made on the basis of the magnetic 

 effect utilized. 



1. Simple Tractive Force {Relays) 



The force of attraction between two neighboring surfaces of area A, 

 between which the flux density Is B, is 



F = kABK 



The primary telephone application of this effect is to relays and 

 switches. For greatest tractive force, materials capable of attaining 

 high flux densities are desirable. However, the air gap in the magnetic 

 circuit absorbs such a large proportion of the available magnetomotive 

 force that higher or lower permeabilities in the core material are 

 frequently less important than efficiency of design. A typical relay 

 structure is shown in Fig. 4. 



2' Steel, Oct. 3, 1938. 



