DEPARTMENT OF TERRESTRIAL MAGNETISM. 293 



TERRESTRIAL ELECTRICITY. 



The investigational work under this head during the past year has 

 been in the subjects of atmospheric electricity and earth-currents. 

 The first special report of Volume III, referred to above, by L. A. 

 Bauer and W. F. G. Swann, concerns itself with the compilation and 

 discussion of the atmospheric-electric observations made on the 

 Galilee, 1907-1908, and on the Carnegie, 1909-1916. (For abstract, see 

 pages 326-329.) 



Progress has been made by Doctors W. F. G. Swann and S. J. 

 Mauchly with newly designed instruments for automatic registration 

 of the atmospheric-electric elements, to be used at Washington and at 

 the observatories of the Department. It is hoped that the automatic 

 registration of the potential gradient and the electric conductivity of 

 the atmosphere with the new instruments can be begun at Washington 

 by the end of 1916. Abstracts of various related investigations are 

 to be found on pages 331-336. 



The great advantage of being able to combine laboratory investiga- 

 tions with observational work is again being demonstrated, this time 

 in the subject relating to the Earth's electric currents. It is the 

 intention to include in the work of the Department's proposed observ- 

 atories the automatic registration of earth-currents. However, in 

 view of the conflicting results obtained by past observers, it has been 

 decided to undertake first some theoretical and experimental studies 

 at Washington. The problem has been assigned to Doctors Swann 

 and Mauchly. 



The first point under investigation concerns itself wdth the origin of 

 vertical earth-currents. It appears that many investigators have 

 observed vertical earth-current densities of an order of magnitude 

 much larger than the vertical atmospheric-electric current density. 

 The existence of a general distribution of such vertical earth-currents 

 appeared inconsistent with the principle of continuity of electric flow. 

 Laboratory experiments show that the currents can largely be attrib- 

 uted, both as regards their nature and order of magnitude, to spurious 

 effects introduced by the apparatus through differential effects of such 

 agencies as temperature or hydrostatic pressure, for example, at the 

 electrodes. The investigation is now being continued in the grounds 

 around the Laboratory. 



Further studies and experiments have been made in connection with 

 the atmospheric-electric work of the Carnegie. Dr. Swann also has in 

 progress various investigations regarding the maintenance of the Earth's 

 electric charge and related topics. (See abstracts of papers, pages 

 331-336.) 



