158 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



were developed to take advantage of the availability of (new) superior 

 materials, improved design techniques, and fabrication methods, for cost 

 reduction or service improvements. In other important instances, new types 

 of loading coils were necessary to provide for new types of facilities; for 

 example, (1) to permit the commercial exploitation of phantom working, 

 (2) for a network of coarse-gauge, long-distance cables, (3) for facilities 



(a) Initial permeability. 



to transmit programs for radio broadcasting stations, and (4) for incidental 

 cables in open-wire carrier systems. 



Certain economic concepts have dominated the design work on the indi- 

 vidual loading coils. In the introductory section of the review, the general 

 need for having the loading coil resistance low relative to that of the circuit 

 resistance \yas mentioned. In applying this design rule, the principle of 

 cost-equilibrium^*^ has been a basic criterion. It has resulted in the de- 



^') This is a condition of cost-balance in which a small transmission improvement can 

 be made by improving the coils at al)out the same cost as would be involved in improving 

 the circuit in other ways — for example, by using a slightly larger size of conductor. 



