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Induced Polarization and Earth-current
At the first and final times of introducing the direct current into the earth through two electrodes，a transient decaying potential difference is found sometimes between another pair of electrodes (Fig. 1). This induced polarization was noticed by H. Note at 1930. and has been applied to electrical prospecting for ground water. (M. Iwasa. 1942).
Dielectric absorption of the layered earth for interpreting the phenomena became meaningless because the relaxation time by the theory was the order of ε/4πσ， that was less than 10・-3 sec (T. Kiyono, 1949). An alternative is proposed in this paper. That is the concentration cell at the effective membrane formed by the capillary of clay particles.
Distribution of these concentration cells in the ground determines the magnitude of the induced polarization voltage of which observed value reaches sometimes to some tenths of the voltage measured between the electrodes at the stationary state. (Figs 5-8). These large observed values were not discussed in the paper by Vacquier et al (1957) who proposed the concentration cell earlier. But their cells are formed at the capillary between sand particles to which clay adheres， and clay plays the role in the source of ions.
The relaxation time of growth or decay of concentration cells at the capillary between clay particles in our theory is accounted as several seconds to some minutes: that is observed decay times.
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