against the attenuation ol the telephone current in the various line 

 sections of the circuit. As a result, with increase in length such cir- 

 cuits become increasingly susceptible to the effect of variations in 

 circuit efficiency caused by changes in weather conditions. In the 

 case of toll cable circuits this variation is primarily due to changes in 

 temperature and in a long cable circuit such changes may in a single 

 day make a 10,000 fold difference in the overall efficiency of the cable. 

 In the case of open wire circuits the change is mostly due to rain and is 

 particularly prominent in the highest frequency carrier systems. 



In order to offset these variations and provide circuits of approxi- 

 mately constant overall net efficiency these longer circuits are equipped 

 with regulating systems which make it possible to offset the variations 

 in the efficiency of the circuits by either manually or automatically 

 changing the gain of certain of the repeaters. 



In the case of the cable circuits the regulation system, known as a 

 "pilot wire regulator," is automatic. It makes use of a direct-current 

 channel over a metallic composite circuit, variations in the resistance 

 of this pilot wire which result from the variations in temperature 

 causing automatic adjustment of the regulating repeaters. A single 

 pilot wire with its associated regulating equipment can be used for 

 controlling all of the circuits in a cable. Sometimes as many as 300 

 circuits or more in number are regulated with a single pilot wire. In 

 order to get sufficiently accurate regulation and because of practical 

 layout considerations the cable is generally regulated separately in 

 sections of 100 to 150 miles in length. 



In the case of a type C carrier system on open wire the regulating 

 system depends upon attenuations of carrier currents transmitted over 

 the same pair as the carrier system. On most of the type "C" 

 systems the regulators give an indication of the net efficiency, which is 

 kept within a prescribed limit by manual adjustment of the repeaters. 

 In some cases apparatus providing this adjustment automatically is in 

 use. 



Equalizers 



The transmission efficiency of telephone lines is generally different 

 for the different single frequencies comprising the voice-frequency 

 band. This variation is particularly great on some loaded cable 

 circuits where the maximum frequencies transmitted approach the 

 maximum frequencies which the circuit is capable of transmitting. 

 If such variation in efficiency were permitted, resulting in the higher 

 frequencies having much greater losses than the lower frequencies, the 

 normal relative proportion of different frequencies would in some 

 cases be so distorted that the transmitted speech would not be clear or 



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