OVER-VOLTAGE PROTECTION 621 



induction upon the two wires of the telephone line is practically 

 the same, with the result that the two wires will assume a certain 

 potential with regard to earth. With a well insulated and properly 

 transposed metallic line, the potentials of each wire against ground 

 will be nearly alike, and hence there will be no difference of poten- 

 tial between the two wires themselves. In telephone work, 

 however, even the smallest difference of potential between the 

 wires will create a flow of current through the telephone receiver. 

 This current, being alternating, produces a noise in the receiver 

 which may be loud enough to make talking impossible. The 

 higher the voltage of a transmission line and the closer the tele- 

 phone line is located to the same, the more prominent will be the 

 noise in the telephone, with slightly unbalanced telephone lines. 

 As this disturbing current is due to a difference of potential, it is 

 obvious that the noise in the receiver is in a measure independent 

 of the absolute value of the voltage on each line to ground, and 

 that it cannot be eliminated unless the voltage on both wires be 

 made exactly alike. This condition, which is termed " bal- 

 anced," is realized by properly insulating and transposing the 

 telephone lines. The larger the number of transpositions per 

 mile, the more will the potential on the wires be equalized and the 

 better the insulation of the lines, the less will there be a chance for 

 a leak to ground, causing a drop of potential on that particular 

 wire, with a subsequent result of unbalancing the line and ren- 

 dering it noisy. 



From the above, it will be seen that as far as the noise on the 

 line is concerned it can be kept down within any limits, provided 

 the telephone line is properly transposed and substantially insu- 

 lated. On the other hand, it will be seen that the existing poten- 

 tial between telephone lines and ground, by reaching high values 

 may not necessarily impair the transmission of speech, but will 

 seriously strain the insulation of the instruments and make the 

 use of the same by the operators dangerous. 



Various schemes and devices have been developed for the pro- 

 tection of telephone lines with more or less satisfactory results. 

 The proper protective equipment to be used depends entirely on 

 the arrangement of the lines and the abnormal conditions against 

 which it is required to protect. v 



For lightning disturbances only, the standard vacuum gap 

 gives the best and most reliable discharge path for these poten- 



