On Earth‐current observations at Watheroo Magnetic Observatory, 1924–1927

Abstract

The earth‐current measuring system at Watheroo Magnetic Observatory provides for a number of independent records of both components of potential gradient, a feature which has proven helpful in detecting and locating the source of disturbing effects. The results obtained during the four‐year period, 1924–1927, are discussed. The diurnal‐variation curve of the northward component has a mean range of 1.14 millivolts per kilometer with maxima at 17h.5 and 12h.5 and minima at 8h and 17h.5 (120th east meridian mean time). The records resemble those obtained in the northern hemisphere at Berlin and Ebro except that the curve is inverted and smaller in amplitude. The reversal in the direction of current‐flow points to a symmetrical distribution of earth‐currents with reference to the equator. The small amplitudes can be explained on the basis of the unusually high conductivity of the region about Watheroo. No regular change was observed in the mean diurnal‐variation for individual years. The range of the diurnal‐variation curve varies with season from 0.71 millivolts per kilometer in June (midwinter) to 2.06 millivolts per kilometer in October (late spring), and the morning minimum and midday maximum shift forward with increasing height of Sun. The amplitudes of the three principal harmonics vary with season in a similar manner, indicating a single predominating cause for seasonal variation in the normal activity. The eastward component is extremely small at Watheroo; its diurnal‐variation curve has, in general, a double period and a mean range of less than 0.15 millivolt per kilometer.