254 CARNEGIE INSTITUTION OF WASHINGTON. 



TERRESTRIAL ELECTRICITY. 



Investigational work in atmospheric electricity and earth currents, 

 preparatory to the inauguration of continuous observational work at 

 the future magnetic observatories of the Department, was continued 

 at the laboratory of the Department by Doctors W. F. G. Swann and 

 S. J. Mauchly. 



Dr. Mauchly's experiments on apparent vertical earth-currents, 

 mentioned in last year's annual report, had concerned themselves with 

 the investigation of the apparent changes in the electromotive force, 

 produced when a long vertical tube containing earth, or salt solution, 

 was turned through 180°. Since then Dr. Mauchly buried two pairs 

 of electrodes in the ground, one pair being inclosed in an insulating 

 tube and the other being free with corresponding electrodes, however, 

 at the same level. Arrangements were made to record continuously 

 the changes in electromotive force of the two pairs of electrodes and 

 the temperature difference between the two levels concerned. As 

 other duties permitted, the investigation was continued throughout 

 the time that Dr. Mauchly was a member of the Physical Division. 

 Among other things, correspondences between the indications of the 

 two pairs of electrodes were shown to be of such a kind as to point out 

 that the apparent diurnal variation of the ''vertical earth-current" 

 was, at any rate in part, a spurious effect resulting from diurnal varia- 

 tions in temperature between the electrodes. 



Dr. Swaim, assisted by Mr. Kotterman, had brought the design of 

 the methods and instruments for the continuous registration of the 

 potential gradient and the electric conductivity of the atmosphere to 

 such a stage that on June 30, 1917, the further development and adap- 

 tation of the instruments to observatory use could be turned over to 

 the Observatory Division, in charge of Chief Magnetician J. A. Fleming. 

 Under the latter's direction and with the assistance of Dr. Mauchly, 

 assistant chief of the Observatory Division since June 1, 1917, further 

 good progress has been made and additional continuous records have 

 been obtained. 



The problem of the origin and maintenance of the Eai'th's electric 

 charge has been given further consideration by Dr. Swann (see abstract, 

 pp. 276-278). As time has permitted, he has also continued the inves- 

 tigation on the detection of single a particles shot out from the radium 

 emanation in the air, and in connection with this investigation he has 

 made certain experiments on a new form of plate for the unifilar- 

 electrometer plates, the idea being to increase the stability and sen- 

 sitivity of the instrument (see abstract, pp. 279-281). 



INSTRUMENT WORK. 



The following instruments were designed and constructed in the 

 Department's instrument shop, which is a section of the Observatory 

 Division under the charge of Mr. Fleming: 



