Results of Earth‐current observations at Huancayo Magnetic Observatory, 1927–1929

The apparatus and equipment used and the natural features concerned with the measurement of earth‐current gradients at the Observatory maintained by the Carnegie Institution of Washington near Huancayo, Peru, are described. A dual system of electrodes is installed and the potential differences are registered by a recording potentiometer. Although the region embraced by the measuring system is fairly flat yet evidence is found which indicates that the general northwesterly direction of water‐course and mountain ridges here doubtless influences the direction of the electric currents. The character of the diurnal variation, as derived from records for all complete days, is exhibited by means of charts and tables and the Fourier coefficients of the diurnal variation are given for the three‐year means, for the means of individual years, and for the three‐year means grouped by months. The mean diurnal‐range of the northward component is 2.83 mv per km. The maximum occurs at 9h, the minimum at 15h (75° west meridian mean time) and the amplitudes during the hours of darkness are relatively small. The same description applies to the component at right‐angles to this (values being considered positive for currents flowing eastward) except that the mean range (3.26 mv per km) is somewhat greater. The direction of the current is northwestward from 2h.5 to 12h.5 and southeastward the remainder of the 24‐hour period. The times of the daily maximum and minimum are constant throughout the year, whi:h is contrary to the findings at Berlin, Ebro, and Watheroo. However, in general agreement with the results at these other places, the mean diurnal‐range changes with season, the principal minima (northward 1.89 mv per km, eastward 2.18 mv per km) occurring in June and maxima (northward 3.50 mv per km, eastward 4.00 mv per km) in March and October. Disturbances (variations of relatively short and irregular period with amplitudes considerably greater, in the average, than those of the diurnal variation) predominate during the daylight hours.