164 



ni:i.L sysriiM ii:ci ixic.il joirwil 



(see Fig. 12) which responds to 17 impulses. This selector relay will 

 respond to only two arrangements of these 17 pulses. The first 

 arrangement is 17 consecutive pulses in which case these pulses must 

 follow one another at the correct speed and must be of the correct 

 duration. This makes it possible to ring all stations at the same time 

 as may be desirable in issuing general orders. The selector relay 

 will also respond to 17 pulses broken up into three groups in which 

 case the correct number of pulses must occur in each group and the 

 total of the three groujjs must be 17. This makes it possible to 



Fig. 12— Ki-.ir \ iew ol Sij;iKilinK anil Low Frcfjiicncy Pant'l Sliowiii^ llii- Sigiialiiii; 

 .-\pparatus 



select one station from a grouj) of more than 50 stations without dis- 

 turbing the others. In addition to these desirable characteristics a 

 single selector relay will provide selective ringing on four low fre- 

 quency extensions from the carrier terminal. 



The carrier ecjiiipment may be operated with complete control and 

 talking facilities from either a telephone located at the carrier terminal 

 or a telephone some distance from the carrier terminal but connected 

 to it by a ph%sical telephone circuit. In any event the control is 

 aMlt)maiic, the transmitting circuit operating only when the receiver 

 is off the switchhook, while the rccei\ing circuit operates continuously 



Designating the carrier fre(|uenc>' which is below 80 K.C as Fi 

 and the carrier freciuenc\' which is above 100 K.C. as Fn, the opera- 

 tion of a carrier system com|)rising three carrier terminals designated 

 as /I, /J and C with a remote control station designated as .-1 1 located 

 at the load dispatcher's office and separated from the carrier terminal 

 by several miles of physical telephone circuit is as follows. Each of 

 these stations may communicate with any of the other stations. 

 Communication between A, B and C is carried on over carrier cir- 

 cuits; communication between ,1 and .1 1 is carried on oxer the jihysical 



