ADVANCES IN CARRIER TELEGRAPH TRANSMISSION 177 



Experience has shown that owing to the small a-c. voltages involved, 

 there is a tendency for the contacts of the sending relay to increase in 

 resistance, sometimes reaching a value as high as 1,000 ohms or more. 

 In view of the low impedance of the circuit originally used, this caused 

 considerable residual current to flow during spacing intervals. In 

 order to remedy this condition, the sending relay circuit was modified 

 to the form shown in Fig. 8, in which an auto-transformer is so con- 



-t^^W 



AUTO 

 TRANS- 

 FORMER 



POLAR 

 RELAY 



AW 

 Rs 



SENDING 

 FILTER 



Fig. 8 — Relay sending-circuit. 



nected as to give a high impedance looking towards the generator, 

 while Rs, which is of the order of 50,000 ohms, provides a correspond- 

 ingly high resistance towards the output. The sending filter input is 

 suitably padded to insure a satisfactory termination. It will be 

 evident that with this arrangement the contact resistances in both the 

 spacing and marking positions may vary considerably without seri- 

 ously affecting the transmitting efficiency. Another advantage of R, 

 is that it eliminates the bias due to the transit time of the sending-relay 

 armature, which may therefore be increased, and need not be kept 

 within such precise limits : a matter of considerable convenience where 

 demountable relays are used. 



A trial has also been made of various schemes using varistors 

 (copper-oxide rectifier-elements) to control the flow of carrier current 

 by means of the changes in voltage in the loop circuit, thus dispensing 

 with sending relays of the electromagnetic type. Figure 9A shows an 

 arrangement which has been in actual operation for a number of years 

 at several central offices and has given satisfaction. The loop circuit 

 is provided with two equal apex resistances RR ; hence when the key is 

 closed the point x is positive relative to y regardless of the position of 

 the receiving relay. This follows from the fact that the current 

 through the loop is twice that through the loop-balancing resistance. 

 On the other hand, if the receiving relay is on its marking contact, 

 opening the loop key reverses the relative voltage between points x 

 and y so that x becomes negative relative to y. In other words, polar 

 signals are impressed between points x and 3; as a result of the trans- 

 mission of signals in the loop. The a-c. part of the circuit contains a 



