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THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



With the pawl in this position both springs follow the motion of the cam 

 so that the bifurcated contact does not open. During "rundown" when im- 

 pulses are sent to the central office the pawl moves to Position 2 with the 

 first revolution of the cam shaft and remains there during successive revo- 

 lutions. Only the bottom pulsing spring then follows the cam, the top 

 spring being restricted by the pawl. The contacts are therefore opened and 

 closed D times for every D -h 1 revolutions of the cam shaft. In effect, this 

 action of the pawl during "rundown" eliminates what would be the first 

 pulse of each sequence, thus providing a minimum interval of one-pulse 



TOP PULSING 

 SPRING --*. 



POSITION 2 



BOTTOM OR 



FOLLOWER 



PULSING SPRING 



!5UN_0OWN_ 



Fig. 13 — Pulsing mechanism schematic. 



cycle between successive pulse sequences. The per cent break or portion of 

 the pulse cycle during which the contacts are open is controlled by the 

 shape of the cam itself and by adjusting the height of the shepherd's crook 

 portion of the cam follower spring. 



An important improvement in the pulsing mechanism of the new dial 

 over its predecessor is in its pulse to pulse uniformity. This is accomplished 

 by closer regulation of speed and by the use of a single-lobe cam in contrast 

 to the ten-lobe cam in the former dial. With the ten-lobe cam unavoidable 

 variations in dimensions between lobes result in variations between pulses. 



Closely associated with the action of the pulsing mechanism is the opera- 

 tion of the off-normal or receiver shorting contacts. In the new dial a pair 



