. The Bell System technical journal . conductance previously presented. It willbe recalled that these values of leakage are derived on the basis of 40pairs of insulators per mile, and are intended to represent, not averagevalues, but the highest values ordinarily obtained under conditions ofdry and wet weather. Systems are ordinarily engineered on the basisof the extreme wet weather attenuation values. When a line runsthrough the more arid parts of the country, however, advantage is i^See Decibel—The Name for the Transmission Unit, by \V. H. Martin,Bell System Tech. Jl., January 1929, pp. 1-2.
. The Bell System technical journal . conductance previously presented. It willbe recalled that these values of leakage are derived on the basis of 40pairs of insulators per mile, and are intended to represent, not averagevalues, but the highest values ordinarily obtained under conditions ofdry and wet weather. Systems are ordinarily engineered on the basisof the extreme wet weather attenuation values. When a line runsthrough the more arid parts of the country, however, advantage is i^See Decibel—The Name for the Transmission Unit, by \V. H. Martin,Bell System Tech. Jl., January 1929, pp. 1-2. i OPEN-WIRE TELEPHONE LINES 747 often taken of this fact by making the repeater spans longer thannormal. A comparison of the attenuation values for a 165-miI open-wirepair when equipped with different types of insulators is presented inFig. 11. For purposes of comparison with the normal values a curveof the attenuation due to resistance only, representing the ideal condi-tion of zero leakage conductance, also is given in Fig. 10 20 30FREQUENCY- KILOCYCLES 40 50 Fig. 11—Attenuation of 165-mil open-wire pair for various conditions of insulation. The values of line capacitance employed in determining the attenua-tion of values of Figs. 9, 10, and 11 include an allowance for the averagecapacitance increase due to the insulators. The attenuation curvesshown are strictly applicable to pairs having a wire separation of 12 in.,but they are approximately correct for spacings of 8 and in. When the number of pairs of insulators per mile differs greatly fromthe standard value of 40, a correction is applied to the attenuation val-ues. Special curves make it possible to obtain this correction conveni-ently. Curves are also available for correcting the standard attenua-tion values to take care of changes in temperature. 748 BELL SYSTEM TECHNICAL JOURNAL It should be understood that the attenuation of an open-wire pairvaries from time to time over a wide range of values, and th
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