IN NATURE of January 30, 1908, some results were quoted by me with reference to the effective inductance of two long parallel wires when the change of current distribution due to frequency is taken into account. These were extended later (Phil. Mag., February, 1909) to meet the case in which the wires are very close together. Pending more detailed publication, the following developments and extensions may be of interest from the practical point of view, as they do not require the construction of special tables, but may be used as they stand. The system of formulae determines the simple equivalent of the two wires, copper or iron, when their capacity is sufficiently small to be left out of account. One wire is the return of the other, and they are equal in all respects.