中文摘要：

一对平行的圆截面导线所组成的传输线是双导体传输线的典型代表，其单位长度电感参数包括内电感和外电感两部分。在高频情况下，由于集肤效应，内电感通常可以忽略。然而，在低频情况下，特别是导线相距较近时，需要考虑内电感的影响。目前，一般简单认为内电感是单根导线内电感（μ0／8π）的两倍。本文基于磁场能量得出了该传输线单位长度电感在静态情况下的精确表达式，结果表明，内电感并不直接等于单根导线内电感的2倍，而是比之略大。

英文摘要：

A pair of parallel conductor wires with circular cross section denotes a classic case of two-conductor transmission lines. Its per unit length inductance consists of inner inductance and external inductance. At high frequencies when the skin effect becomes obvious, the inner inductance is very small and may be neglected. However, at low frequencies, especially when the two wires are close to each other, the influence of inner induct- ance increases compared with the external inductance and should be included in the total inductance. At present, the inner inductance is usually calculated simply as the two times of the inner inductance of a single wire （ μ0/ 8π）. In this paper, the accurate expression for the static per unit length inductance of the transmission line is derived by using the magnetic field energy method. The result shows that it is a little larger than the common value.