280 



BELL SYSTEM TECHNICAL JOURNAL 



The adjusting shield as finally constructed, and as now used 

 both by the Bell Laboratories and by the Western Electric Company, 

 consists of two of the open shields previously described, mounted on 

 an angle iron frame with the open ends facing each other, as illustrated 

 in Fig. 10. The two shields, which are separated by a distance great 



(T'ffflD^^^ 



Fig. 10 — Photograph of apparatus adjusting shield. 



enough to allow the filter assembly to be inserted between them, are 

 equipped with a number of adjusting tools sufThcient to adjust coils in 

 any of the standard positions. The right hand shield was arranged 

 to accommodate only the coils mounted on the right of the sub- 

 panel center line, and the left hand shield was arranged to care for 

 the coils mounted on the other half of the panel. Resonant frequency 

 adjustments made in the shield have been checked to within a 

 maximum of about ± 0.05 per cent against the frequency of the same 

 combination in a standard soldered shield. 



The shield is used with a special bridge and associated oscillator 

 and detector, designed for the purpose. The bridge is very similar 

 to the standard current bridges employed for measuring impedances 

 at carrier frequencies, and the complete circuit is shown in Fig. 11. 



Fig. 1 1 — L-C circuit arrangement. 



