U.S. Patent 3٬361٬957, D. L. Hings, Telluric current responsive device having spaced conductors for positioning adjacent the Earth's surface
U.S. Patent 4٬686٬475, C. L. Kober, Passive geophysical prospection system based upon the detection of the vertical electric field component of telluric currents and method therefore
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Chisholm, Hugh, ed. (1911). "Earth Currents" . Encyclopædia Britannica (بالإنجليزية) (11th ed.). Cambridge University Press. Vol. 8. pp. 813–816. This has a detailed history of observations as understood at the time.