982 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



3.2 APPLICATION TO COILS 



3.21 Single Layer Type Inductors 



The problems encountered in the application of quality control tech- 

 niques to the various codes of the single layer type coils follow a similar 

 pattern so that a discussion of any one code covers many of the condi- 

 tions common to the entire group. The early application of distribution 

 requirements to L3 carrier coil characteristics was associated with the 

 development of the 1500-type inductor commonly referred to as the 

 "Splitting Coil". This coil derives its name from its primary circuit 

 function of separating or splitting the transformer winding capacitance 

 from the input capacitance of the amplifier vacuum tube. The circuit in 

 which this coil is used permits an estimated variation in inductance from 

 a desired value of approximately dbj^ per cent, which includes manu- 

 facturing deviation, aging and temperature effects. A coil of this type 

 having such close tolerances could not be produced economically with 

 normal manufacturing methods. At this point the use of control chart 

 methods played an important part in the development of tolerances and 

 measuring techniques. The control chart techniques combined with the 

 simultaneous development of manufacturing methods and sources of 

 critical raw material have permitted the widening of limits of coils under 

 the general distribution specification using "A" limits of ±1 per cent in 

 place of the originally estimated limits of dbj^ per cent without a con- 

 trolled distribution. 



The splitting coil consists of a single layer winding wound on a ceramic 

 form under a tension of 50 to 75 per cent of the wire breaking strength. 

 The winding is terminated in lead wires which have been secured in holes 

 located in the ceramic core transverse to the axis of the coil. The use of 

 the ceramic core on this coil has reduced the temperature and aging 

 effects to the point of becoming negligible, due to the nature of the core 

 material and winding conditions. Major factors affecting the inductance 

 variation are as follows: 



a. Diameter of wire — standard commercial tolerances on wire of the 

 sizes used, equalling ±0.0003" will produce a variation in coil inductance 

 of approximately ±0.15 per cent. 



b. Diameter of core — a variation of core diameter of 0.0005'' will 

 produce a variation of approximately ±0.5 per cent in the coil induct- 

 ance. 



c. I^ength of winding — a variation in the nominal length of winding 

 of ±0.001'' will result in a variation in inductance of about ±0.13 per 

 cent. 



