134 



BELL SYSTEM TECHNICAL JOURNAL 



about twenty-five parts per million per degree centigrade, which is not 

 objectionable. 



The obtaining of inductance coils and condensers that were adequate 

 in stability for use in conjunction with the crystals required consider- 

 able development effort. The inductance coils were required to have 

 not only a high degree of stability with respect to temperature and 

 time but also a high ratio of reactance to effective resistance, low modu- 

 lation, and at the same time be small in size. Air core coils might have 

 been designed for the purpose but they would have been quite large. 

 The coils used are of the toroidal type wound on about one and three- 

 fourths inch rings of molybdenum-permalloy. To reduce eddy current 

 losses the cores are made of very fine powder and then annealed to 

 reduce hysteresis losses. The particles are mixed with insulating 



1 o 



^m^ 



CWT\ 



HH 



Fig. 9 — A two-terminal reactance network is connected in shunt across the filters 

 at their paralleling end to improve the characteristic of the highest and lowest fre- 

 quency filters. 



material and formed into rings by extremely high pressure. The 

 inductance of the coils has a temperature coefficient of less than 40 

 parts per million per degree centigrade. The cores of the coils for the 

 higher-frequency filters are wound with finely stranded wire to help 

 secure good Q's (about 225). Because of the high impedance of the 

 coils called for by the filter design, care is taken to make the capacitance 

 between the windings and the core and between the windings and the 

 case as low as practicable and also to make stable all such small 

 capacitances as must be present. 



The two extra condensers used at one end of each filter for paralleling 

 purposes are of a high grade mica type. The other condensers are all 

 quite special. The fixed ones, ranging in magnitude from about 7 

 mmf to 100 mmf, are made by plating short lengths of high grade glass 

 tubing inside and outside with silver. Because of the intimate associ- 

 ation of electrodes with the surfaces of the tubes and the low expansion 

 coefficient of the glass used, a condenser is obtained that has a tem- 

 perature coefiicient comparable with that of the coils and crystals. 

 No aging effect has been observed. It will be noticed that four small 



