TELEPHONE SYSTEM OF THE UNITED STATES 



47 



factory as copper, taking into account transmission efficiency and 

 construction conditions. 



The wires are placed on 10 pin cross-arms and supported, in gen- 

 eral, by double-petticoated glass insulators. The grouping of wires 

 to form phantoms is indicated in Fig. 37. This arrangement has been 

 found desirable for conditions in the United States and transposition 

 systems have been designed by which are obtained satisfactory opera- 

 tion of the phantoms and side circuits, the mutual induction between 

 the various circuits being sufficiently neutralized to prevent mutual 

 interference. 



Fig. 38 — -Open wire pole line construction. Four 10-pin cross arms. 



On the longer circuits telephone repeaters are installed at an aver- 

 age distance of about 175 to 300 kilometers, providing in that way for 

 adequate transmission efficiency. 



The number of circuits which it is practicable to provide by means 

 of open wire lines has during the past decade been very greatly in- 

 creased by the extensive use of carrier telephone for superposing on 

 the telephone circuits additional channels of communication carried 

 by currents above the voice range of frequencies. These systems 

 now form a network covering the entire country and in some areas 

 a large proportion of the circuit growth on open wire lines is taken 

 care of by carrier systems. The systems range in length from a 

 minimum of 75 kilometers to a maximum of 3,800 kilometers. 



Two types of carrier telephone systems are standard for use in 



