When direct current introduced into the ground at two points is interrupted, a small voltage, which may take several minutes to decay, appears between another pair of electrodes. This effect, called by C. Schlumberger 'provoked polarization,' is found useful in prospecting for ground water. Laboratory experiments indicate that induced polarization (I.P.) depends on cation exchange in the clay minerals contaminating the aquifers. It is suggested that the effect is due to electrodialysis of the clay within the aquifer, which acts as a distributed electronegative membrane. The magnitude of the I.P. depends on the kind of clay and the kind of positive ions in the water. It is inversely proportional to the conductivity of the water and independent of the kind of negative ion dissolved therein. The rapidity with which the I.P. decays appears to depend only on the grain size of the aquifer. Field trials and experiments on laboratory models give promise of useful application of the method to depths of about 350 feet.