LO.iniXG FOR ITJ.r.I'lfONlL CIRCUITS 269 



addition to improved flutter characteristics, the replacing loading 

 coils had somewhat lower iron losses, and their cost was slightly 

 higher. 



A substantial reduction in llutter distortion effects was later ob- 

 tained in superposed telegraphy over loaded open wire lines and long 

 coarse gage cable circuits, with the adoption of the air-gap type of 

 loading coils already described (Table Mil) as the latter had con- 

 siderably better hysteresis characteristics than the corresponding 

 types of continuous wire core coils which they superseded. 



The operating requirements for the grounded telegraph systems 

 referred to above, necessitated the use of telegraph currents of very 

 large amplitude relative to the telephone currents; conseciuently 

 on such circuits it was impracticable to realize the benefits of reduced 

 flutter distortion which would have resulted from the use of small 

 amplitude telegraph currents. These possibilities, however, have l)een 

 fully realized by the development of a metallic polar duplex telegraph 

 system -^ to meet the special requirements imposed by superposed 

 telegraph operation over long small gage telephone circuits. In this 

 system, the superposed telegraph current is of the same order of 

 magnitude as the telephone current. Under these favorable operating 

 conditions, the flutter distortion effects caused by modulation in the 

 cores of the present standard 35-permeability compressed iron powder 

 core loading coils, are within satisfactory limits on the longest circuits 

 which are used simultaneously for telegraph and telephone service. 



The recent development of a voice frequency carrier telegraph 

 system ^^ providing 10 or more independent channels over a loaded 

 four-wire cable circuit has made it economical to concentrate a large 

 part of the telegraph service over the long repeatered cables on a 

 special group of wires which are not used simultaneously for telephone 

 purposes. This method of operation obviously eliminates all possi- 

 bility of modulation effects between the carrier telegraph circuits 

 and the speech transmission circuits. However, the possibility of 

 intermodulation effects between the different superposed carrier 

 telegraph channels involves the same fundamental requirements in 

 the loading coils as when the telegraph circuits are superposed on 

 telephone circuits. The requirements of these systems are satis- 

 factorily met by the 35-permeability compressed iron powder core 

 loading coils now standard for use in toll cable loading. 



2* "Metallic Polar-Duplex Telegraph System for Cal)les," Messrs. Bell, Shaiuk and 

 Branson, Trans. A. I. E. E., 44:337, 1925. 



2' "Voice-Frequency Carrier Telegraph Systems for Cables," Messrs. Hamilton, 

 Nyquist, Long and Phelps, Trans. A. I. E. E., Vol. 44, 1925, p. 327. 



