. The Bell System technical journal . lephone circuits, sit as to secure more telephone channels on agiven pair; (^v^i for applicatii>n to new t\-pes of conductors capable oftransmitting a \-er> wide frequency band, such as the coaxialciMiductor. nmv being tried experimentally. 1 need hardly point out to you that as the frequency of transmissionis raised, the attenuation or line loss is greatly increased. This is duemore particulaiK- to two factors: an increase in series resistance dueto skin ettect. and an increase in shunt conductance due to increaseddielectric losses. As the frequenc&
. The Bell System technical journal . lephone circuits, sit as to secure more telephone channels on agiven pair; (^v^i for applicatii>n to new t\-pes of conductors capable oftransmitting a \-er> wide frequency band, such as the coaxialciMiductor. nmv being tried experimentally. 1 need hardly point out to you that as the frequency of transmissionis raised, the attenuation or line loss is greatly increased. This is duemore particulaiK- to two factors: an increase in series resistance dueto skin ettect. and an increase in shunt conductance due to increaseddielectric losses. As the frequenc>- increases, the currents transmittedtend moie and nune to a\oid tlie inner paits of the conductor and lo TOLL TRANSMLSSION IN TIIK UNITED STATES 127 concentrate on ihe surface, so increasing the effective series losses in the insulalion between the conduclors also increasewith the frequency, and so increase the effective shunt 6 shows graphically the increase in attenuation with frequency,. 100 200 BOO 1000 300 400 500 600 700 FREQUENCY IN KILOCYCLES PER SECOND Vi^. 6- At I ciuiat ion-frequency characteristics of various types of tclcijlione circuits. and also shows the relative attenuations of various types of con-struction. Until ihe vaciuim tube ami)lifier became available, the only practicalmethod of overcoming high attenuation in a given type oi line con- 128 BELL SYSTEM TECHNICAL JOURNAL struction was to provide larger conductors. With the development ofthe vacuum tube, high amplification became available as an alternativemethod. Since increasing the size of the conductor in order to decreaseattentuation involves large expense, we are naturally led to considerthe use of as much amplification as possible. Before referring further to the utilization of high amplification,I wish to point out that at the present time for distances greater thanabout 150 miles (240 km.) in cable, we utilize the so-called 4-wiremethod to obtain two-wa
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