Abstract
The tidal movement of sea water relative to the Earth's magnetic field induces electromotive forces of a few millivolts per kilometre. Recent measurements off Plymouth show that the potential gradient is at right angles to the streams in that part of the English Channel. Observations on cross-channel telephone cables indicate that a considerable flow of electric current takes place, which can be accounted for b assuming the mean conductivity of the sea-bed to be of the order of 6 × 10−5 (ohms-cm.)−1. Tidally generated earth-currents spread into the land on either side of the Channel and have been measured near Lulworth. It may be possible to use them for the measurement of tidal stream velocities. In the second part of the paper the potential gradients generated by water flow in shallow channels of rectangular or elliptical section are examined. The horizontal gradient in the water is almost independent of vertical variations in water velocity but is affected critically by the depth of the channel and the conductivity of the channel-bed. The induced electric currents can be expected to extend to depths comparable with the width of the channel.
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