When wires are close to each other as they are in cable, the mutual inductance coupling between pairs is not a simple number, constant at all frequencies. Because of the non-uniform and non-symmetrical distribution of current over the cross-sections of the conductors the “effective mutual inductance” is of the form M = Ma + jMb where both Ma and Mb vary with frequency. This paper discusses the results of certain measurements which have been made of the effective mutual inductance between straight wires and between cable pairs over a wide range of frequencies extending up to a million cycles. This is of interest in connection with crosstalk problems in cable carrier telephone systems.