DEPARTMENT OF TERRESTRIAL MAGNETISM. 291 



at the same station. Accordingly it has been possible not only to confirm 

 and extend certain results previously reached by others, but also to draw 

 important new conclusions. 



It is hoped that the present investigation, which had to be confined to a 

 discussion of the observational data on magneticalfy-calm, or on electrically- 

 calm days, may be supplemented later by a discussion of earth-current data 

 on disturbed days. 



The chief conclusions may be stated as follows: 



(a) The resultant horizontal earth-currents, as observed at the Ebro Observatory, flow* 

 on the average for the year, in the direction from about 29° west of north to 29° east of 

 south, or, approximately, in the direction from the Magnetic North Pole towards south- 

 southeast. The average value, for the magnetically-calm days during 1914-1918, of the 

 potential gradient of the component of the current flowing from true north to south was 0.20 

 volt per kilometer, and that of the component towards geographic east was 0.11 volt per 

 kilometer, or about one-half of the north-south component. The resultant horizontal 

 potential-gradient was 0.23 volt per kilometer, which during electric or magnetic storms 

 may reach a value 0.8 to 1.0 volt per kilometer. 



(b) The annual variations of the earth-current potential-gradients and of the components 

 of the Earth's magnetism, as observed at the Ebro Observatory, may be related to one 

 another as cause and effect only to a very minor extent; both sets of variations may have 

 to be referred, more or less, to common causes. The range of the annual variation of the 

 north-south electric component is about 2.5 times that of the west-east component. 



(c) The diurnal variation of earth-currents as observed at the Ebro Observatory along 

 lines somewhat over 1 km. long is remarkably similar to that observed at Berlin along 

 telegraph lines, 120 and 262 km. in length, from 1884-1887. In both cases the diurnal varia- 

 tions for the component of the current along the meridian is considerably more pronounced 

 (2 to 3 times) than that along the parallel of latitude. The diurnal variation in the north 

 component of the Earth's magnetism is not such as to correspond to the direct magnetic 

 effect of the diurnal variation of the west-east component of the earth-currents. A similar 

 conclusion had to be reached with regard to the east component of the Earth's magnetism 

 and the north-south component of the earth-currents. The general conclusion was that the 

 north-south earth-current might be the result of electromagnetic induction, caused by 

 the fluctuation during the day of the west-east component of the Earth's magnetism. If it 

 be recalled that all analyses of the diurnal variation field of the Earth's magnetism have 

 shown that the magnetic diurnal variation is in part to be ascribed to electric currents 

 circulating in the regions overhead and in part to currents circulating within the Earth's 

 crust, exact agreements between magnetic variations and earth-current variations are not 

 to be expected. It further remains to point out that until we have some knowledge of the 

 actual course or distribution of the earth-currents in the Earth's crust and as to how the 

 conductivity of the crust may vary with temperature and other meteorological causes dur- 

 ing the day and at the actual place of observation, attempts to find a quantitative relation- 

 ship between terrestrial-magnetic and earth-electric effects may be futile. 



(d) The horizontal vector-diagrams both for the magnetic and earth-electric components 

 vary during the sun-spot cycle in about the same proportion. The earth-current vector- 

 diagram is symmetrical about a line approximately in the direction of the Magnetic North 

 Pole. 



(e) The extreme diurnal range of the Ebro earth-currents reaches its highest values near 

 the equinoctial months, and lowest near the solstitial months. Earth-currents, atmospheric 

 electricity, the aurora borealis, and the Earth's magnetic disturbances all show similar 

 annual variations in the ranges of their fluctuations. 



(/) The potential gradients of earth-currents and of atmospheric electricity apparently 

 vary during the sun-spot cycle, the former decreasing in the direction of normal flow of 

 current, and the latter increasing with increased sun-spot activity. The diurnal ranges of 

 the potential gradients of earth-currents, as well as of atmospheric electricity, just as is the 

 case for the diurnal variation of terrestrial magnetism, increase with increased sun-spot 

 activity. 



(g) There is evidence of a similar six-hour wave in atmospheric electricity, earth-currents, 

 and terrestrial magnetism. 



