When a current is passed through earth materials not enclosing metallic minerals, the amount of current is related to the driving potential only by the electric resistance of the formations involved. When the formations contain metallic minerals, the currents give rise to an exchange of ions at the surface of contact between the minerals and the electrolytes dissolved in the fluid filling the inter-granular pore spaces. This electrochemical exchange creates a voltage which opposes the current flow though the material, and an added voltage is required to overcome the obstacle thus created. The extra voltage necessary to drive current though the barrier is sometimes referred to as the over voltage. When the outside applied current is turned off, the electrochemical voltages at the metallic grain surfaces are dissipated, but not instantaneously. The decaying voltages can be measured for a time after the current is switched off. The voltage is observed to differ with time.