466 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



mercially available for use on small copper wires. Studies of its application 

 to telephone apparatus indicated that further, worth-while size-reductions 

 in loading coils could be achieved by virtue of the greatly superior space- 

 factor of this insulation, relative to that of the combination of cotton and 

 enamel insulation that had been used for more than a decade in small 

 loading coils. Another advantageous possibility was the reduction of the 

 coil resistance by employing a larger size of conductor to utiHze the winding 

 space saved by the thinner conductor-insulation. 



Although the better space efficiency of the Formex conductor insulation 

 was a contributory factor in the further size reduction of the smallest load- 

 ing coils, the size-reduction achievement under discussion was mainly de- 

 pendent upon the development and use of a new type of winding machine. 



The new non-phantom type coils that resulted from the redesign work are 

 described below under appropriate headings. They all use compressed mo- 

 lybdenum-permalloy powder cores. Additional information regarding them 

 is given in an A.I.E.E. paper,^° previously referred to. In Table XIV (page 

 471) electrical and dimensional data are given on the individual coils, along 

 with corresponding data on the designs which they superseded. 



22.2 632 {88 mh), 638 (44 mh), and 639 {22 mh) Formex Insulated Coils 



The large current and expected future demand for the low-inductance 

 exchange area coils, relative to that for all other types of loading coils, 

 resulted in the concentration of the initial redesign efforts on these types 

 of coils. 



In the redesign of the low-inductance coils, it was decided to reduce the 

 coil size as far as possible without degrading transmission performance. 

 An important secondary requirement was that the new design should not 

 be more susceptible to magnetization by superposed signaUng currents than 

 the current standard coils, previously described. 



Before these transmission requirements were finally set, the experimental 

 design studies had shown that worth-while cost-reductions could probably 

 be secured by using improved winding machines capable of winding the 

 coils to a new size-limit of 0.35-inch finished inside diameter. In due course, 

 the very difficult winding-machine design problem was solved by the factory 

 engineers. The above stated transmission requirements made it necessary to 

 use the same amount of core material (molybdenum-permalloy) as that used 

 in the 622, 628, and 629 coils, previously described. The coil design problem 

 was solved by a redesign of the core to obtain a shorter magnetic circuit 

 having a larger cross-section, keeping the same volume. (The inside and 

 outside diameters were reduced and the axial height increased.) This per- 

 mitted about a 20% reduction in the over-all volume and weight of the 

 wound coils, without appreciable degradation in transmission performance. 



