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BELL SYSTEM TECHNICAL JOURNAL 



Variable air condensers are used in adjusting the assembled filter. 

 For the channel filter four such condensers are manufactured on a 

 single ceramic base, as shown in Fig. 9, to eliminate unnecessary parts 



Fig. 9 — Four-section variable air condenser. 



and reduce assembly cost. The posts supporting the stator plates are 

 extended both upward and downward to serve as convenient terminals 

 for leads from adjacent pieces of apparatus. Freedom from binding 

 is important, since condensers in crystal filters must be adjusted 

 through angles as small as 2 minutes. To insure smooth adjustment 

 the rotor shafts and their bearings are held to close dimensional 

 tolerances and lubricated with petrolatum. Stability is secured by the 

 use of thrust springs providing a substantial holding torque. 



Assembly 

 The foregoing components — crystals, coils and condensers — are as- 

 sembled into complete filters by methods somewhat similar to those 

 employed in the manufacture of radio receivers, the principal differ- 

 ences arising from smaller volume requirements. The channel filter 

 alone is produced in sufficient quantities (1500 to 5000 per year) to 

 warrant a substantial degree of tooling. Figure 10 illustrates the 



