DEPARTMENT OF TERRESTRIAL MAGNETISM. 307 



To furnish exceedingly constant speeds and frequencies for all these 

 investigations, the system developed by Wenner has been adopted, with 

 an improvement in the synchronism indicating device. The indicating 

 commutators are so placed that the current in one of the two lamp lines 

 is either a minimum or a maximum when the proper phase relation 

 is estabhshed, and an ammeter in one of these lines indicates by its 

 reading whether synchronism has been attained or is maintained. The 

 commutator is placed on the D. C. motor or A. C. generator if fre- 

 quencies are to be controlled; on the A. C. motor driven by the A. C. 

 generator, itself operated by the D. C. motor, if the motor speeds are 

 to be controlled. 



Sine galvanometer. — Plans for the construction of this instrument 

 were completed and much of the work of the instrument makers, 

 Messrs. Jung and Huff, is now also complete. There remain the mirror 

 and suspension, the scale, the supports for the magnetometer and the 

 coils, and the winding of the coils. The circle has been investigated 

 for errors of graduation and for eccentricity, and satisfactory magnetic 

 tests have been made on the essential parts of the apparatus, so far 

 as the construction has proceeded. 



For papers published, see Abstracts, pages 313-315. 



Throughout the observational work a great deal of assistance has 

 been received from Mrs. Barnett. 



Mr. Kotterma.n has performed the duties of laboratory assistant and 

 has been engaged in work on instruments, photographic work, and 

 clerical work. 



SECTION OF TERRESTRIAL ELECTRICITY.^ 



The principal activities during the year under review ha,ve been as 

 follows : 



(1) Reduction of eclipse atmospheric-electric observations. — The atmo- 

 spheric-electric data secured during the total solar eclipse of May 29, 

 1919, by the Department's expedition to Sobral, Brazil, were reduced 

 and studied in detail. A preliminary report thereon was presented 

 before the meeting of the American Physical Society at Washington, 

 April 23, 1920 (see abstract, p. 320). A more detailed report on the 

 results for this eclipse, together with a summary and discussion of the 

 results of similar observations during solar ecUpses from 1887 to 1919, 

 inclusive, was given before the Philosophical Society of Washington 

 on October 9, 1920. The full report and summary will appear in 

 Volume IV of the ''Researches of the Department of Terrestrial 

 Magnetism." 



(2) Reduction of ocean atmospheric-electric observations. — Owing to 

 the interruptions in the Department's work occasioned by the war, 

 no detailed publication of the atmospheric-electric observations made 



'From the report of the chief of the section, S. J. Mauchly. 



