| http://www.w3.org/ns/prov#value | - coupled thereto; first AC source means connected to said toroidal winding to energize said source inductor with alternating current, said winding acting as the primary of a transformer to induce a secondary current in a transformer secondary including said drill stem, the earth surrounding said borehole, and said conductive body; a second source inductor mounted on said drill stem, said second ind
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