The assumptions on which the so-called magneto-telluric method to determine the subsurface conductivity of the earth is based are examined and it is shown how the method can be revised to get rid of those assumptions which are not necessarily legitimate. The principle of this revised or generalized magneto-telluric method is that the magnetic and telluric field components which observation can provide over the entire surface of the earth are more than sufficient viewed as boundary conditions to determine the electromagnetic field inside the earth with a prescribed conductivity distribution and, therefore, the extra boundary conditions can be consistent with each other only by the correctly prescribed (or chosen) distribution of electrical conductivity. The purely magnetic method to determine the conductivity, which relies on the assumption that the magnetic field in space above the surface of the earth, is a potential field, is also revised to free it of the assumption which, does not hold true unconditionally.

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