C.IRKir.R TELEPHONY ON HIGH lOl.TAC.E LINUS IM 



1)\- tin.' ()|M'rati()ii of the ri-l.i\' slmwiinii l-ii;. II, ili.it is to s.i\-, wlu-n 

 the transmitting circuit is oporatini^ at a fri.(incnc\- lower than 80 K.C. 

 the low pass filter is connected to it and when the transnuttinj; circuit 

 is operating at a frequenc\- higher th.m 1(H) K.C. the high pass filter 

 imist l)e connecleil to it. 



SlUNAI.INd SVSTKM • ' 



Signaling or rinj^in.;; is acconiplisiuMl at tlie Iransniitling end l)y 

 changing the fre(|uenc>- of the oscillator frttiu a fre(|uenc\- helow 80 K.C. 

 to a fre(iuency ahove 100 K.C. without changing the fiUors. This is 



Fig. 10 — Front \'ie\v of Low Pass Filter with Cover Removed 



Fig. 11 — Rear View of Low Pass Filter with Cover Removed 



accomplished by operating and releasing the relay in the oscillator 

 circuit. Since the filter connected to the transmitting circuit will 

 pass only one of these frequencies, pulses of the carrier frequency are 

 sent out on the line. At the receiving end these pulses are ampli- 

 fied and rectified and the change in the space current of the detector 

 operates a marginal relay. The number and arrangement of these 

 pulses is controlled by a spring-operated selector key of the type com- 

 monly employed for telephone dispatching on railroad lines. At the 

 receiving end these pulses operate a train dispatching selector relay 



