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BELL SYSTEM TECLINICAL JOURNAL 



of normal speech or music to make the accidental operation of the 

 signaling system a very remote possibility. 



The normal plate current in the rectifier tube is a small fraction of 

 a milliampere. Upon a signal being received, this current increases 

 sufificiently to operate a relay connected in the plate circuit. The 

 operation of this relay closes a circuit through the winding of another 

 relay, which is the reversing relay for the operation of the selector 

 controlling the red signal lamp which indicates a call. 



The selector is the heart of the signaling system. This selector 

 is shown in Fig. 8. It consists of a mechanism unit mounted upon a 

 magnet unit, the whole being enclosed in a glass case for protection. 



Fig. 8 — Selector 



The magnet windings are connected to a source of direct current of 

 suitable voltage in series with a large fixed condenser and the contacts 

 of the reversing relay mentioned above. When this relay is in either 

 end position the circuit is completed through the condenser and the 

 windings of the selector. As the power source is direct current it serves 

 only to charge the condenser to its own potential. When the reversing 

 relay is alternately operated and released, however, the repeated 

 charging of this condenser in opposite directions sends pulses of current 

 through the windings of the selector. This gives a rocking motion 

 to the armature of the selector which motion is transmitted through a 

 ratchet to the selector code wheel, advancing it against the tension of 

 a spring one step for each movement of the armature. If the code 

 wheel is kept from flying back during the pauses, two in number, which 

 occur between groups of impulses, as is the case when the signal 



