134 BELL SYSTEM TECHNICAL JOURNAL 



In d.-c. apparatus where high permeability and low coercivity are 

 of importance, and where high resistivity does not add to the usefulness 

 of the core material, 78.5 permalloy is suitable. It is used in certain 

 relay structures, usually of marginal type, in which the difference 

 between operating and releasing currents is small. 



For audio transformers, for retardation coils, and for other apparatus 

 in which high permeability and high specific resistance must be com- 

 bined, both 3.8-78.5 Cr-permalloy and 3.8-80 Mo-permalloy have 

 been used. The former has slightly higher resistivity, but the latter 

 has higher initial permeability and is more ductile. 



While the initial and maximum permeabilities of 45-permalloy are 

 not as high as those of 78.5-permalloy, the higher flux densities at- 

 tained by the former and its higher resistivity favor its use for certain 

 types of relays and transformers where high flux densities are required. 

 It is used also in some instances for cores of coils that require high a.-c. 

 permeability when d.-c. magnetizing forces are superposed. 



The magnetic characteristics of the perminvars make them espe- 

 cially suitable for use in circuit elements in which distortion and energy 

 loss must be a minimum; but their relatively high cost, and the ad- 

 visability of avoiding high magnetization throughout the life of the 

 apparatus, have prevented their extensive use in telephone plant. 

 One use for which perminvar is especially suitable is the loading of long 

 submarine telephone cables. Here a high resistivity is very desirable, 

 which has been shown to be obtainable in the 7-45-25 IVIo-perminvar. 

 The increase in resistivity resulting from the addition of molybdenum 

 more than offsets the accompanying increase in hysteresis loss, and 

 results in a continuous loading material satisfactory for certain types 

 of loaded cables. 



Permendur was developed for use in apparatus where very high flux 

 densities are desired. For a moderate magnetizing force flux densities 

 of 18,000 and 23,000 gausses readily are obtained. It is used for 

 cores and pole pieces in loud speakers, certain telephone receivers, 

 light valves, and similar apparatus. 



It may be seen from this survey that there is a great variety of 

 magnetic materials with widely different properties from which an 

 engineer may choose in designing magnetic elements in which magnetic 

 flux changes are essential. Already these alloys have an important 

 place in telephone plant. However, iron and silicon steel still are 

 used extensively, and will continue to hold their own on a cost basis for 

 some purposes. There is no doubt, however, that alloys of iron, 

 nickel, and cobalt will continue to supplant iron and silicon steel in 

 many places where circuits and apparatus are redesigned to take full 

 advantage of their magnetic properties. 



