The paper discusses interference between power circuits and telegraph and telephone circuits. A distinction is made between “radiation” and “induction.” The former cause produces both electric and magnetic effects and is used in radio-telegraphy. The conductivity of the earth, which is not a homogeneous body, should be taken into account. Experiment, however, shows that in radio work we can get approximate solutions by assuming that the earth is a non-conductor and that its inductivity is unity. Making this assumption, it is shown that the intensity of the “radiation” field in simple cases falls off inversely as the distance and inversely as the wave-length. On this assumption also the intensity of the induction field, whether electrostatic or electromagnetic, obeys this law in a few cases. It is pointed out that a balanced three-phase system produces both electric and magnetic rotary fields in its neighbourhood, the amplitudes of which fall off according to the inverse square of the distance. A three-phase four-wire system, however, may produce serious interference with telephone systems even when the phases are balanced. In an appendix an easy method of finding the numerical value of the mutual capacity coefficient between two spheres is given.