The effect of solar-induced earth potentials on long-haul communication systems has been observed for many years, particularly on transatlantic cable systems. Direct current excursions of over 3,000 volts, observed at the power stations of these cable systems, have led to a design goal for earth current potentials in the range of 2-3 V/mi. There are numerous observations of large dc currents on continental long-haul communication lines, particularly during the "great" geomagnetic storm of 1940.1 The shutdown of the Bell System L-4 long-haul communication system at Plano, Illinois, during the August 4, 1972 geomagnetic storm has renewed the interest of the communications community in the study of geomagnetic-induced earth currents and their effects on communication systems.

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