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chargeability (M) One of the units of measurement of induced polarization in the time domain. True chargeability is the ratio of the over- or secondary voltage, Vs, to the observed voltage, Vo, applied by way of an electrode array so that M = Vs/Vo, expressed as a percentage or as millivolts per volt; this quantity is independent of topographic effects and of electrode geometry and is thus a good measure of induced polarization. In reality, what is measured is the apparent chargeability (Ma) which is the area (A) beneath the voltage-time decay curve over a defined time interval (t1 to t2) and normalized by the supposed steady-state primary voltage, Vp, such that Ma = A/Vp = (1/Vp) × t2t1 of V(t)dt, in units of mVs/V.