192 BELL SYSTEM TECHNICAL JOURNAL 



reduction in permissible repeater amplification as we change from two 



4-vu to two 8-vu talkers is evident. This figure also shows that by 



the use of the volume limiters mentioned at the beginning of the paper, 



the repeater gains with the higher volume talkers may be made at least 



as great as for the lower volume. This makes it possible to use the 



present telephone circuits for carrier telegraph purposes without 



change. 



Drainage 



The occurrence of atmospheric disturbances, and particularly 

 lightning, constitutes a potential hazard to the operation on open-wire 

 circuits of a service as exacting as carrier telegraph. Where such 

 circuits have been transposed for the operation of carrier telephone, the 

 transverse or metallic-circuit effects due to lightning discharges in the 

 neighborhood of the line are in general not serious, but the voltages 

 generated to ground are very often of sufficient magnitude to cause a 

 breakdown of the protectors. Since it has been found impracticable to 

 devise protectors with such precise limits that they will operate at the 

 same voltage and possess the same discharge characteristics, a transient 

 transverse current is set up in such cases which may cause telegraph 

 errors. 



The remedy adopted consists in bridging drainage coils ^^ across the 

 line at all points where protectors are required, and in so connecting 

 them that they will either prevent a breakdown of the protectors or 

 assure simultaneous operation with equal discharge currents from 

 either wire to ground. 



In Fig. 18^1 the drainage coil is shown bridged directly across each 

 end of the open-wire line between two sections of entrance cable. 

 These coils consist of two carefully balanced windings with the mid- 

 point grounded. They present a high impedance to voice or carrier 

 currents transversely, but offer only a small resistance to ground for 

 longitudinal currents compared with that across the adjacent protector 

 blocks. The chief disadvantage of this method, which is called 

 "direct drainage," is that it prevents the use of grounded telegraph 

 and interferes with the testing of the line by means of direct current. 

 To obviate this, the scheme shown in Fig. 18-5 has been devised, which 

 is termed "protector drainage." In this case, the drainage coil is 

 connected to the line wires through protectors having a low breakdown- 

 voltage. This combination is backed by high-voltage protectors to 

 insure unimpeded discharge in case of large disturbances. With this 

 arrangement the drainage coil does not come into operation unless 

 there is a severe disturbance, and furthermore owing to the mutual 

 inductance between the two halves of its windings it tends to cause 



