TRANSCONTINENTAL TELEPHONE LINES 239 



large number of circuits through the application of the twelve-channel 

 carrier-current telephone system then under development. Among the 

 conditions favoring this particular route is freedom from winter storm 

 hazards throughout most of the distance, which, looking ahead, is 

 particularly important to the future application of twelve-channel 

 carrier telephone systems. The work done in 1937 consisted of build- 

 ing a length of nearly three hundred miles of new pole line and stringing 

 four pairs of wires throughout most of the section from Oklahoma 

 City to Whitewater, California, a distance of 1,200 miles. Initially 

 the voice channel and three-channel carrier telephone systems have 

 been developed on these four pairs, providing a total of sixteen tele- 

 phone circuits. 



Wire Spacing and Transpositions 



Open-wire telephone lines designed to carry frequencies up to 140 

 kilocycles per second, as used in the operation of the twelve-channel 

 carrier telephone systems, have structural requirements substantially 

 more stringent than those designed to carry only three-channel sys- 

 tems, which use frequencies up to 28 kilocycles. The usual type 

 of open-wire toll telephone line has ten wires on each crossarm, 

 spaced at about one-foot intervals, five on each side of the pole 

 and with the crossarm spaced twenty-four inches apart. In the case 

 of the line designed to conduct high carrier telephone frequencies, this 

 configuration has been changed and is illustrated by Fig. 2. Eight 

 wires are strung on each arm, grouped as four pairs, two on each side of 

 the pole. The wires of the pair are spaced eight inches apart, and 

 the nearest wires of the two pairs on each side of the pole are spaced 

 twenty-six inches, while the spacing at the pole is thirty inches. Cross- 

 arms are spaced thirty-six inches apart. 



These new wire spacings reduce the coupling between pairs on the 

 same line or between pairs on this line and pairs on other lines which 

 may parallel it. New transposition systems are used further to reduce 

 this coupling. Transpositions are closer together and a transposition 

 bracket of the type shown in Fig. 3 is used to turn the wires completely 

 over at as nearly a given point as possible. Transpositions in one or 

 more pairs are installed on every pole with an occasional exception, 

 and certain pairs are transposed at every other pole. The wires of a 

 pair must be adjusted to the same sag within close limits. These sag 

 variations are held to a fraction of an inch, and a check of the completed 

 work indicated that fifty per cent of the spans had been adjusted to 

 within one-quarter inch. Telescopes are used to help obtain these 

 close sag adjustments, and a final check is made by oscillating the 

 wires in a span and observing the periods at which they oscillate. 



