We report the observation of ~40 min quasi-periodic variations during local daytime conditions in the differences between the voltages measured on two parallel (separation distance ~100 km) unpowered telecommunications cables between California and Hawaii (length of each ~4000 km). The events are inferred to be produced by small changes in magnitude and/or position of the Sq ionosphere current system, the focus of which passes on average daily over the cables. The changes in the current system are probably produced by medium-scale traveling ionospheric disturbances (TID's). This dual cable system appears to provide a very sensitive technique for measuring small amplitude, medium-scale mid-latitude changes in ionospheric currents, especially those produced by TID's. ¿ American Geophysical Union 1992