262 



THE BELL SYSTEM TECHNICAL JOUENAL, APRIL 1951 



previous design, which results in a more pleasing and effective tone. The 

 two gongs differ in their fundamental frequencies by a major third to pro- 

 duce a harmonious sound. 



An outstanding feature of the new ringer is that the sound output is 

 adjustable by the subscriber. A notched wheel that projects through the base 

 of the set can be shifted to four different positions for four levels of sound 

 output. This wheel simultaneously controls the armature stroke and clear- 

 ance between gong and clapper ball so that the force of the clapper striking 

 the gongs changes with each position of the wheel. The armature stroke is 



20 



6 

 5 



4 



S 3 



I 



o 



-. 2 



U 



2 



< 1.0 



Q 



UJ 0.8 

 Q. 



1 0.6 



0.5 

 0.4 



0.31- 



XtO* 



0.2 



0.1 



20 



40 



60 100 200 400 600 



FREQUENCY IN CYCLES PER 



1000 2000 

 SECOND 



4000 



10,000 



Fig. 17 — Ringer impedance characteristics. 



controlled by the position of a cam which moves in relation to the armature 

 stop rod as the notched wheel is shifted from one position to another. In 

 this manner, fairly uniform steps in volume control are obtained for the 4 

 positions of the notched wheel. A range of approximately 14 db may be ob- 

 tained by means of the volume control feature. A mechanical stop on the 

 volume control is provided so that the customer cannot adjust the level below 

 a certain minimum. 



The higher static forces required in the new ringer impose problems of 

 adjustment much more critical than in any previous design. The strong 

 permanent magnet flux tends to keep the armature in the operated position 

 in spite of the restoring force of the biasing spring. To counteract this 



