JOINT DEVELOPMENT AND LOW-FREQUENCY INDUCTION 223 



to achieve the end sought without serious sacrifice in transmission, 

 work along this line is continuing. 



Improved Insidation. — A slight reduction of susceptiveness to inter- 

 ference by low-frequency induction could be secured by providing 

 increased dielectric strength to ground in communication circuits and 

 their associated apparatus. Another method would be to insulate or 

 isolate all conducting parts of the communication system so as to pre- 

 vent contact by employees or others with wires or apparatus which 

 may carry a dangerous voltage. Neither of these appear practicable 

 at this time. 



Drainage. — Drainage is a method for controlling the parts of the 

 circuit in which the induced voltages appear and causing these voltages 

 to be consumed in those parts where they are least harmful. This is 

 accomplished by connecting the telephone conductors to ground, 

 preferably through balanced impedance coils, at certain points through- 

 out the exposure. Assuming low resistance grounds at the drainage 

 points, the resulting voltage to ground at such a point after drainage 

 is established is limited to a value corresponding to the voltage drop 

 over the impedance of the coil and ground connection. If this im- 

 pedance is small compared to the other impedances in the drainage 

 section, the voltage to ground at the drainage point is a small part of 

 the total voltage induced in that section. 



Under present conditions, the application of drainage is limited to 

 special situations where interference with circuit testing and main- 

 tenance is of relatively minor importance and where superposed d-c. 

 telegraph and carrier telephone are not used. 



Neutralizing Transformers. — The neutralizing transformer is a device 

 for introducing into an exposed communication wire a voltage in op- 

 position to the voltage induced by the disturbing circuit, thereby to a 

 certain extent neutralizing the latter. The neutralization is effected by 

 means of transformer action, the primary coils of the neutralizing 

 transformer being connected to conductors which are grounded at the 

 terminals of the exposure (or section of exposure), so that the voltage 

 induced in these conductors will send currents through the transformer 

 primaries. These primary currents induce in the secondaries of the 

 transformers voltages substantially in opposite phase to the voltages 

 induced in the telephone wires by the power circuit. The secondaries 

 being connected in series with the exposed communication wires, the 

 neutralizing action is obtained. 



On account of introducing crosstalk and adversely affecting tele- 

 phone transmission and carrier, application of neutralizing transformers 

 has been confined chiefly to telegraph circuits. No applications of 



