It is well known that the actual measurements of coefficients characterizing the electrostatic induction from power lines to parallel or transverse telephone lines do not always agree with the values obtained using the generally accepted theory. The paper analyses the possible causes of the observed discrepancies. By taking into account the effect of harmonics existing in the power system, the conclusion is reached that these discrepancies arise mainly through ignoring the effects of pylons or other earthed structures and the unevenness of the ground.The second part of the paper examines the relations between the values given by the simplified formulae contained in the Directives of the C.C.I.F. and those given by the more rigorous formulae. It is found that the C.C.I.F. formulae are generally adequate.