The analysis and interpretation of the temporal variations of the geomagnetic field observed at forty-nine stations in the British Isles are presented. The variations in the horizontal components are very similar over the whole area, while the variations of the vertical force may show large differences between stations less than 80 km apart. At nearly all stations there is a correlation between the variations of the vertical component and some component of the horizontal field. For variations of period exceeding 120 min, this correlation is mainly caused by electric currents induced in the Atlantic Ocean. The results are compatible with the theoretical response derived for induction in a thin strip model of the Ocean. For variations of period less than 90 min, concentrations of current also flow in the sea water surrounding the British Isles. It is postulated that these currents too are produced by induction in the Ocean, being frequency dependent branch currents flowing from the Ocean into the shallow seas. A current concentration flowing NE-SW through southern Scotland has been delineated. This current, which has frequency characteristics similar to the currents in the shallow seas, could be driven by an electric potential between the North and Irish Seas. The presence of a long narrow structure in the crust with only a small conductivity contrast would then be sufficient to channel the current and produce the observed anomaly in the geomagnetic field.