ADVANCES IN CARRIER TELEGRAPH TRANSMISSION 179 



Alternative arrangements have also been tried. Two of these, 

 which were used in actual installations, are shown in Figs. 9B and 9C. 

 In both cases, the loop arrangement is the same as in Fig. 9A. The 

 circuit of Fig. 9B consists of two parallel paths between generator and 

 sending filter, one of which impresses a steady a-c. voltage on trans- 

 former T2 while the second path serves to impress a second a-c. voltage 

 of the same magnitude at the same point, but this latter voltage may 

 be either in phase aiding or in phase opposing to the first, depending 

 on the polarity of the d-c. voltage impressed through the varistor 

 bridge. Thus it will readily be seen that if x is positive relative to y, 

 elements a and c are conducting, while b and d are not. Transmission 

 of the carrier then takes place around the path pi, qi, q^, p2, and the 

 voltages from the two parallel circuits are additive in T2. If, however, 

 X is negative relative to y, the conducting condition of the varistor 

 elements is reversed so that the carrier path becomes pi, q^, qi, pi, and 

 the net voltage impressed on the primary of T^ is zero. 



The direct-transmission branch contains a phase and magnitude 

 adjustment network to permit exact neutralization of the carrier 

 voltage for the spacing condition. 



Figure 9C is much like Fig. 9B except that the direct-transmission 

 branch is omitted and a "suppressor circuit" is added, which may be 

 thought of as changing the signals impressed on the varistor bridge from 

 polar to neutral. This is done by inserting element 5, which equalizes 

 the voltage between points x and y whenever y is positive with respect 

 to X. This effect is further enhanced by adding other series and shunt 

 elements as shown. The bridge conditions for marking are the same 

 as in Fig. 9B, while for spacing, all the elements are normal and alike 

 so that the bridge is balanced for a-c. as well as for d-c. ; thus no voltage 

 appears between points w and z, and the carrier is suppressed. 



While all these schemes involve balance between groups of varistors, 

 recent advances in design have made it possible to fulfill this require- 

 ment to the desired extent and to maintain it over long periods of time. 



The limited use to which varistor sending circuits have been put in 

 the telegraph plant of the Bell System is not due to unsatisfactory 

 operation in their present applications, but rather because they have 

 imposed certain operating limitations on recently developed arrange- 

 ments for interconnecting telegraph circuits. 



Grid Bias 



The fixed grid bias required by the level compensator exceeds the 

 filament battery voltage, hence the latter cannot be used as a bias 

 source and recourse is had to the negative 130-volt telegraph battery. 



