20 



BELL SYSTEM TECHNICAL JOURNAL 



hook signal is received over a switching trunk, the sleeve resistance of 

 the hne or trunk is changed from 1,800 ohms to 80 ohms, which in- 

 creases the current in the sleeve of the cord sufficiently to light the 

 lamp. 



Line Relay Functions in Twofold Capacity 



The majority of toll lines in the plant today are of the ringdown 

 type and the operator at one end calls the operator at the distant end 

 by ringing over the line. To receive this ringing signal in the No. 1 

 board, the lines are equipped with relays which respond to the ringing 

 current received from the distant end of the line and give a line signal. 

 After the operator answers this signal by connecting a cord to the line, 

 the line relay is disconnected and replaced by the ringing relay in the 

 cord which responds to further rering signals over the line. This 

 arrangement of the line and cord, as well as the switching trunk for the 

 No. 1 board, is shown schematically in Fig. 3. 



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Ul Kn Kn^::^ 



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to to 



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Switching Trunk 



Fig. 3 — Schematic: Toll switchboard No. 1 circuits 



By transferring the ringing relay from the cord to the line in the No. 

 3 toll board, this relay is made to function in a twofold capacity, that is, 

 to give the line signal as well as the cord rering signal. When a call is 

 received from a distant point, the apparatus in the line functions to 

 light the line signal and this remains lighted until a toll cord is inserted 

 in the line jack. Further signals over the line cause the apparatus in 

 the line to light the lamp in the cord. This is obtained by changing 

 the sleeve resistance of the line from 1,800 ohms to 80 ohms and is 

 illustrated in schematic form in Fig. 4. As in the past, the line signal 

 is multipled before several operators and appears as a steady illumi- 

 nated lamp which is extinguished by an operator answering the call. 

 The cord signal appears before one operator and has been changed 

 from a steady lamp signal to a flashing signal for the purpose of ob- 

 taining prompt attention on the part of the operator. The cord 

 signal is extinguished when the operator connects to the circuit by the 



