LOADING FOR ri'J.liPlJONIi CIRCUITS 243 



the new requirements were met by the design of an air-gap type of 

 wire-core coil on which data are given in Table VIII. They were 

 somewhat smaller and not quite so expensive as the improved open- 

 wire coils. 



Compressed Powdered Iron Core Loading Coils. It soon became 

 evident that the economical extension of the toll plant would involve 

 the general introduction of telephone repeaters in cable as well as 

 open-wire circuits. The use of telephone repeaters made it possible 

 to supersede the coarse gage conductors by 16 and 19-A.w.g. con- 

 ductors for toll connections, and this greatly increased the need for 

 an efficient and stable loading coil of lower cost than the air-gap 

 wire core coil. 



As a result of investigations carried on over a period of several 

 years, there was developed for commercial use early in 1916 a new 

 magnetic material, compressed powdered iron, which has been of the 

 utmost value in loading coil design. ^^ This improved magnetic 

 material is described in a paper presented before the Institute by 

 B. Speed and G. W. Elmen ^^ which also discusses the electrical and 

 magnetic properties of the material. 



Briefly, the method of production consists of grinding electrolytically 

 deposited iron to the desired fineness, insulating the particles of iron, 

 and finally compressing these insulated particles in steel dies at such 

 very high pressures as to consolidate the mass into a ring, the specific 

 gravity of which is substantially equal to that of solid iron. The 

 rings are then stacked in a manner similar to laminations of sheet 

 material to form a core of the desired dimensions. Though the 

 separate rings are approximately 0.2 in. thick, the insulation between 

 the individual particles is so effective that despite the use of molding 

 pressures of 200,000 lb. per sq. in., the eddy current loss in a powdered 

 iron core is less than that obtainable with 0.004 in. iron wire. De- 

 pending on the heat treatment and the amount of insulation, the 

 initial permeability can be varied from approximately 25 to about 

 75. The specific resistance is about 20,000 times that of ordinary 

 iron. The permeability can be controlled within comparatively 

 narrow limits by the manufacturing processes, thus making for greater 

 uniformity. The great advantage of this material for loading coils, 

 however, lies in its self-demagnetizing property. The powdered iron 

 core by virtue of its very numerous, though extremely small dis- 



16 U. S. Patents No. 1,274,952, B. Speed; 1,286,965, G. W. Elmen; 1,292,206, 

 J. C. Woodruff. 



""Magnetic Properties of Compressed Powdered Iron," B. Speed and G. W. 

 Elmen, Trans. A. I. E. E., Vol. 40, 1921, p. 1321. 



