190 THE BELL SYSTEM TECHNICAL JOUBNAL, JANUARY 1951 



cussion was developed primarily for voice-frequency uses. At much higher 

 frequencies, lower permeabiUties are necessary to prevent the core losses from 

 becoming too high. Accordingly, other grades of compressed molybdenum- 

 permalloy powder having lower permeability values are available. These are 

 obtained by diluting the molybdenum-permalloy powder with inert material 

 before pressing. Also, smaller-size particles are used. A new grade not de- 

 scribed in the Legg-Given paper,^^ previously referred to, which has an 

 effective permeability of 60, was used in small carrier loading coils for the 

 Army spiral-four field cable during the war,^^ and is now being used in 

 the cores of cable loading coils for 15-kc program transmission circuits 

 which are described in Subdivision 13.3. 



11.2 M-type Molybdenum-Permalloy Core Loading Units ^ 



The initial standard, molybdenum-permalloy core, phantom loading units 

 which were coded in the M-series became available for commercial use 

 during 1938. The individual coils were about 60% smaller than the standard, 

 75-permeability, permalloy-core coils which they superseded for use in new 

 plant. In the headpiece, these size-relations are typified by coils D and C, 

 respectively. 



By design, the new coils had about the same d-c resistance and hysteresis 

 loss as the superseded designs. Their eddy-current losses were considerably 

 lower than those of the 75-permeability permalloy designs, and in conse- 

 quence the total effective resistance was lower at the upper frequencies, 

 thereby improving the steady-state frequency-distortion characteristics of 

 the circuits in which they were used. In plant-design engineering, the new 

 coils were accepted as being equivalent to the older coils. The residual 

 inductance stability was a little better, and the telegraph-flutter distortion 

 characteristics were considerably better. The susceptibility to superposed 

 d-c magnetization, however, was worse. This minor impairment was the 

 only adverse effect of the substantial increase in permeability, and the 

 substantial reduction in coil size. 



The development was timely in that the new coils were available for use 

 in meeting the accelerating demand for toll cable loading that started in 

 1939 and continued for several years. In the five-year period 1938-1942, a 

 total of about 800,000 side and phantom toll cable loading coils were manu- 

 factured for Bell System use notwithstanding the large installation of Type 

 K carrier systems on non-loaded and unloaded cables that occurred in this 

 period, thereby reducing the demand for additional, repeatered, loaded 

 cable voice-frequency facilities. 



The economic advantage of the broad-band carrier system is largely due 

 to the fact that the cost of the conductors and the repeaters and of the 

 distortion corrective-networks and regulating devices which are used to 



