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BELL SYSTEM TECHNICAL JOURNAL 



phone systems In such systems for existing Hnes and cables, as 

 well as for projected new types of cables, those filters are of key im- 

 portance which separate individual message channels in the frequency 

 range from 3 to 108 kc. By using coils of powdered molybdenum- 

 permalloy insulated to permeabilities of 14 or 26, valuable economies 

 in space and cost of filters are realized. ^^-^^ Figure 14 shows a typical 

 coil employing a 14 permeability core designed for use in one of these 

 channel filters having its transmitted band in the vicinity of 108 kc, 



Fig. 12 — New and superseded cases containing six program loading coils. 



together with a shielded solenoidal air core coil which might be em- 

 ployed for the same purpose. The molybdenum-permalloy coil has a 

 Q at 100 kc about twice that of the air core coil, yet it occupies ap- 

 proximately 1/10 as much space. The third order modulation prod- 

 ucts are approximately 80 db below the level of the normal channel 

 currents. This is considered to be tolerable from the standpoint of 

 interchannel crosstalk on circuits used for one-way transmission. 

 An inductance-temperature coefficient of about —20 X 10"^ per de- 

 gree F. has been chosen to compensate for the positive capacity- 

 temperature coefficient of associated condensers. 



