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THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



to twisting. The ringer is provided with a manually adjustable volume con- 

 trol which permits the subscriber to change the loudness over a considerable 

 range. 



Less evident at first glance, but of greater importance both to user and 

 Telephone Company are some of the more technical aspects of the electrical 

 and mechanical design features. 



A schematic circuit of the set is shown in Fig. 3. This circuit is a varia- 

 tion of one of the commonly used Campbell anti-sidetone circuits^ long 



■p_ 





r^v 





Fig. 1 — External view of 500-set. 



Standard in the Bell System, with improvements added to meet tougher 

 requirements in all functional categories. 



The mechanical arrangement of components in the assembly is entirely 

 new, and is built around several concepts arising directly from service and 

 manufacturing experience. In general, controls and adjustments are reduced 

 or eliminated, and parts are enclosed and protected wherever possible against 

 effects of dirt, moisture or mechanical damage. 



Where field or repair shop replacement of components is to be anticipated, 

 as in the dial, ringer, handset, and cords, removal and replacement are de- 



