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BELL SYSTEM TECHNICAL JOURNAL 



Coils 

 Inductance coils are used in some crystal filters, particularly the 

 channel filter for the newer types of carrier telephone systems. Since 

 it may be necessary to connect as many as ten such filters in tandem 

 in a long-distance circuit without appreciable impairment of the quality 

 of transmission, the filters must meet exacting requirements not only 



Fig. 7 — Toroidal inductance coil used in crystal channel filter, shown before potting. 



at the time of their manufacture but throughout service life. Con- 

 sequently inductance coils used in the filters must exhibit little aging 

 or shift with temperature, either in inductance or in effective resistance. 

 Losses must be kept low in order to meet a Q requirement of approxi- 

 mately 200. The types employed in channel filters range from 25 to 

 50 millihenries in inductance and from 60 to 120 kilocycles in operating 

 frequency. 



Unusual features of design and manufacture are employed to meet 

 these requirements. The coil is essentially a toroidal winding with 

 low distributed capacitance, applied to a permalloy dust core, im- 



