"Earth Current" means electric currents flowing in the Earth. It is measured as the potential difference between two electrodes both buried in the Earth, and thus the unit is electric field. The components is usually earthward and northward.
Natural "Earth Current" of shorter periods is induced by the geomagnetic field variation, and therefore useful in estimating the conductivity distribution in the Earth combined with the geomagnetic data, and it also can be used the subsitution of geomagnetic field variation data.
Analog data of "Earth Current" are referred to as "Tellurigram". and they are subclassified into "Normal run Tellurigram" and "Rapid run Tellurigram", depending on their recording speed. This classification is same as for "Magnetogram".