168 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



the manufacturing standpoint it was desirable to have a better control of 

 the magnetic properties of loading coil core material, and to be free from 

 limitations on quantity such as were occasionally experienced in obtaining 

 iron core wire from outside manufacturers. These particular difficulties 

 were very serious during the First World War in consequence of the greatly 

 increased demand for loading coils and the impossibility of securing an 

 adequate supply of diamond dies for drawing the 4-mil core wire. (In 

 normal times, all dies were imported from Europe.) Incidentally, the supply 

 limitations on diamond dies made it necessary to permit the use of over- size 

 core wire even though this resulted in an impairment in transmission per- 

 formance due to the abnormal (eddy current) core losses. Fortunately, the 

 compressed powdered-iron core material became commercially available in 

 time to be of great value in helping the Western Electric Co. to increase 

 the output of loading coils. ^ 



The success achieved in this development subsequently led to the applica- 

 tion of the compressed, insulated, magnetic-powder technology to magnetic 

 alloys, for use in the cores of loading coils and of other types of coils used 

 in various types of transmission networks, including electric wave filters. 

 Initially worked out for voice frequency applications, the new technology 

 expanded to become an important factor in the design economy of carrier 

 and radio transmission systems. The low eddy current losses made possible 

 in large part by the use of very small, insulated, magnetic particles, were 

 inherently important elements in the high frequency applications. Following 

 its development in the United States, the compressed, magnetic powder 

 core, in one form or another, spread to Europe, and became important in 

 world wide communications. 



The prior art is of historical interest, in that experiments with finely 

 divided magnetic particles had extended over a period of several decades. 

 As an early example, Oliver Heaviside described in his ''Electrical Papers" 

 some work on magnet cores with magnetic powder embedded in wax. It 

 is also of interest that during the Bell System pioneering efforts to obtain 

 satisfactory loading coil cores, considerable experimental work was done 

 (1901) on magnetic oxide cores involving high temperature heat treatments 

 of loosely formed iron wire or iron tape core structures in an oxygen atmos- 

 phere. (U. S. Patents Nos. 705,935, and 705,936, July 29, 1902.) 



^ The total output, however, could not be increased sufficiently fast to meet the high 

 1917-1918 demand for loaded facilities. This resulted in a temporary practice of what 

 came to be known as "omitted-coil loading" on a substantial mileage of toll cable facilities. 

 In the initial installation of loading on these particular facilities, the coils were placed at 

 alternate load points along the line; — for example, at 12,000 ft. spacing instead of the 

 standard 6,000 ft. spacing. The resulting transmission impairments were accepted as being 

 tolerable under war emergency conditions. As soon as practicable, however, the "omitted 

 loads" were "filled in," so that shortly after the end of the war the coil spacing conformed 

 to the established standard practices. 



