464 BELL SYSTEM TECHNICAL JOURNAL 



Another use of permalloy in the telephone plant has been found in 

 the case of relay cores. Relays are used to connect and disconnect 

 mechanically the telephone circuits at the central office and at other 

 points. These relays are magnetically operated and their efficiency 

 depends largely on the magnetic quality of the core material. Certain 

 groups of these relays are required to operate under very severe 

 circuit conditions where the operating currents are small and where 



Fig. 23 — Cast-iron cases each containing 200 loading coils: left, permalloy core coils; 



right, iron core coils. 



the relays are required to distinguish between very small changes in 

 current values. For such circuits, relays with permalloy cores are 

 now used extensively. 



The permalloy alloys are also used extensively for cores in high 

 quality audio transformers, in telephone receivers and earphones, and 

 in electrical measuring instruments. 



The permalloy series was first developed, and is also the one which 



