DEPARTMENT OF TERRESTRIAL MAGNETISM. 305 



(a) The mean diurnal-variation curve for the entire year, derived from 136 

 selected days, shows a strong maximum about 2 hours before noon and a weak 

 secondary maximum about 7 hours after noon. This is in rather marked con- 

 trast with the corresponding curves for Kew and Potsdam, which show very 

 little difference between the forenoon and late afternoon maxima. In this 

 respect the Washington curve more nearly resembles the 1918 potential 

 gradient curve for Tortosa, except that the times of occurrence of the primary 

 and secondary maxima are approximately interchanged. 



(6) During the summer months the mean diurnal variation from the Wash- 

 ington records for 1918 is similar to that at Kew, Potsdam, and Tortosa. 



(c) The chief difference between the diurnal variation of the potential 

 gradient at Washington as compared with the European stations already 

 mentioned occurs in the winter months. Here the secondary maximum 

 (about 7 p. m.) is almost entirely suppressed, while the principal maximum 

 continues to appear about 2 hours before noon, and the principal minimum 

 shortly after midnight. Thus, in winter the predominating maximum not 

 only is more pronounced than at Kew and Tortosa, closely resembling in this 

 respect the winter curve for Potsdam, but also it occurs some 8 or 9 hours 

 earlier in the day at Washington than at the other stations named. 



(d) The annual variation of the recorded potentials during 1918 followed 

 very closely the annual variation of the potential gradient at Kew and Pots- 

 dam for the same year, in being both more pronounced and more regular than 

 the annual variation at Tortosa. The maximum occurred in January and 

 the minimum in August. On the basis of an approximate reduction-factor, it 

 is estimated that the mean value of the potential gradient for the year 1918 

 was about 200 volts per meter. The estimated values for January and August 

 were about 250 and 125 volts per meter, respectively. 



Results from a single year are, of course, not conclusive, and it is quite 

 possible that the results for the entire 6-year period (1917-1922, inclusive) 

 may show considerable variations in detail from those for 1918. The depend- 

 ence of the results upon the immediate surroundings of the observatory and 

 the constancy of the reduction-factor are also matters which require further 

 investigation. However, in view of the results obtained from observations 

 aboard the Carnegie (see Year Book for 1921, p. 354), no small interest will 

 attach to the comparison of the ultimate diurnal-variation results at Wash- 

 ington with those obtained at other stations for the same period. 



Progress report of the Committee on Earth-Currents and Polar Lights.^ S. J. Mauchly. 



The report deals primarily with plans considered by the committee for 

 stimulating interest in earth-current and polar-light investigations and in- 

 creasing the number of reliable observations. 



On account of the rather extensive equipment required for suitably con- 

 trolled earth-current observations, it seems desirable to urge the importance 

 of such work upon institutions and organizations having adequate facilities 

 rather than to encourage a large amount of work on the part of individual 

 observers whose equipment is likely to be inadequate. 



Since no systematic earth-current data are available for the American con- 

 tinent, it is suggested that information of some value could probably be 

 obtained by a study and coordination of data regarding earth-currents ob- 

 served on the lines of telegraph, telephone, and cable companies. 



> Presented at the annual meeting of the Section of Terrestrial Magnetism and Electricity 

 of the American Geophysical Union, Washington, March 7, 1922. Members of the committee: 

 J. H. Delhnger, A. J. Henry, A. G. McAdie, S. J. Mauchly, chairman, A. G. Mayor, and W. E, 

 Parker. 



