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BELL SYSTEM TECIISICAL JOURNAL 



ing methods but they have been ineffectual as yet, for the equi%-alent 

 combination of sensitivity and rcliabihty and a delicate means of ad- 

 justment is ditificult to attain. In order to satisfy the severe circuit 

 conditions the stepping relay is adjusted to operate on 10 mil-amperes 

 and not to operate on 9 mil-amperes, a difference of onl>' 10 per cent, 

 in the operate and non-operate adjustments. 



The stepping relay must reproduce the pulsations of current orig- 

 inated by the commutator brush with sufficient accuracy to insure 



Fig. 17 — Stepping relay 



the p()siti\e operation of the counting relays, lor any lailure of the 

 latter will result in false selection. The stepping relay must there- 

 fore maintain a circuit through its make contact for a sufficient time 

 to enable the lower relay of any counting pair to operate and must 

 open the circuit through the same contact long enough to permit the 

 upper relay of the pair to lock up in series with the lower relay. 

 Since the stepping relay does not always reproduce the commutator 

 pulses perfectly and since any pulse distortion must necessarily 

 reduce the operating time margin for one of the relays of a counting 

 pair, it is evident that rapid operation and reliability of operation 

 are essential characteristics for the counting relays. A punciied 

 type relay similar to the line relay cannot operate with sufticient 

 speed. The stepping relay would qualify for speed, but a complete 

 set would rcciuire considiTabli' sjiace and would be iiicon\ iniinl 

 to mount. 



