78 BELL SYSTEM TECHNICAL JOURNAL 



sender. As the district selector moves upward, it produces pulses by 

 means of the brushes which slide over the commutator at the top of 

 the selector. These pulses are transmitted back to the sender, and are 

 there counted. When the sender has counted the number of pulses 

 which indicates to it that the district selector has proceeded to the 

 proper position, the sender opens the fundamental ciicuit to the 

 selector and causes it to stop. This method of controlling the move- 

 ment of the selector is termed the reverse control method. 



The first selection made chooses the set of brushes to be tripped 

 into engagement with the terminals. Assume, as shown in Fig. 18, 

 the desired trunk appears on the second panel from the bottom. 

 Therefore, the district selector is allowed to make two pulses and 

 is then stopped by the sender. The brush-tripping device is thus set 

 in position to trip the second brush, and the selector is started again 

 by a signal from the sender, which operation completes the process 

 of tripping the brush. 



The selector now continues upward, making a pulse for every group 

 of trunks which it passes over, until, having reached the desired group, 

 as indicated by the number of pulses counted by the sender, it is again 

 stopped by the sender at the beginning of this group. The selector 

 is now started again, and this time under its own control, hunts for 

 an idle trunk in the group. Busy trunks are grounded on the third 

 or signaling terminals, whereas idle trunks are open. A testing relay, 

 associated with the selector, keeps the selector moving upward until 

 a trunk with an open third wire is found, whereupon the selector stops, 

 makes connection with this trunk, and renders it busy to other selectors 

 by grounding the signaling strip. 



This trunk, as indicated in Fig. 18, leads to an office selector. The 

 same process is repeated by the office selector, under control of the 

 sender, to trip first the proper brush, then choose the proper group, 

 and finally to choose an idle trunk in the group. The connection 

 is now extended to an outgoing trunk. The sender still remains 

 attached to the connection, since it must still control the further 

 setting up of the connection. 



The sizes of the working trunk groups on district and office selectors 

 can vary from 5 to 90, depending upon the traffic to be handled. 



Calls Between Machine Switching Offices. If the call is for a sub- 

 scriber in a machine switching office it is completed as shown in 

 Fig. 20. This figure shows a diagram of the apparatus used to con- 

 nect an incoming full mechanical trunk to a subscriber's line, whether 

 this line is in the originating machine switching office or in another 

 which must be reached over interoffice trunks. 



