986 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



to the inadequate readability of the capacitance standard of available 

 test, facilities. 



Since the theoretical values of resistance and inductance are specified 

 by design, the following methods of determining actual design parameters 

 are used: 



A. Determine mathematically, or by experiment, the niunber of turns 

 of a given resistance wire wound on the core tube to produce the required 

 resistance. 



B. Wind high Q inductors using copper wire of the same diameter as 

 the resistance wire and having the same number of turns as determined 

 in Step A, varying the winding pitch to produce the required inductance. 



C. Wind low Q inductors with the wire and number of turns of Step A 

 and the pitch as determined in Step B. 



D. Make Maxwell Bridge inductance measurements for the two 

 groups of low and high Q inductors. The two sets of readings are then 

 subjected to a statistical analysis to insure that the processes are in con- 

 trol so that the data may be used in selecting a nominal coil from the 

 low Q group to use as as a reference standard. While it is true that the 

 apparent value of the resistance-wound inductors will be appreciably 

 less than those of the copper wire variety, due to the CR correction, the 

 nominal intrinsic inductances of the two groups of coils will be the same, 

 due to their identical physical geometry. 



Thus, by a method of comparison of two sets of readings, each of which 

 is within process control, it is possible to select coils which can be used as 

 reference standards. Any measurement which is made on a bridge set up 

 from this reference standard will require correction of the inductance 

 reading to compensate for the resistance variation. Self -compensating 

 bridge methods have been developed which make it possible to read 

 with good accuracy, directly on the bridge, the low Q coils covered by 

 this family of inductors. 



During the initial production of the 1507 -type inductors, it was found 

 expedient, for easier maintenance of control of the resistance and in- 

 ductance variations, to use a nine-cell post-office method (three-cell by 

 three-cell) which permits selection of product in any one of five ways 

 using diametral axes, yet still maintaining the specification distribution 

 covered by the three-cell method. The nine-cell method has gradually 

 become less important, as experience has been gained with the use of the 

 winding machines and control of winding operator variations. 



Table III gives the over-all results of production of single-layer induc- 

 tors and retardation coils, over a period of approximately one year. It 

 can be seen that the design and production facilities are such that it is 

 possible to manufacture a product within the requirements imposed by 



