DEPARTMENT OF TERRESTRIAL MAGNETISM. 233 



atmospheric-electric observations of the Department during the total solar 

 eclipse of September 10, 1923, care was taken to incorporate, so far as prac- 

 ticable, improvements resulting from the earlier eclipse observations of the 

 Department at Lakin, Kansas, in 1918, and at Sobral, Brazil, in 1919. A 

 large part of the preparatory work in connection with both the MacMillan 

 North Greenland expedition and the California Eclipse expedition was 

 carried out by Mr. H. F. Johnston, who also cooperated with the chief of 

 the section in preparing "General directions for absolute potential-gradient 

 observations." 



The experimental observatory on the deck of the Laboratory was contin- 

 ued in operation, although the proportion of usable records has temporarily 

 greatly decreased owing to the frequent operation of steam shovels in the 

 vicinity of the Laboratory. Observations were made at stations near by in 

 November 1922 and July 1923 to determine the reduction factor for the 

 potential-gradient electrograph in the deck observatory; the field observa- 

 tions for this purpose have been under the direct charge of Mr. H. F. Johnston, 

 who was assisted at various times by Messrs. O. W. Torreson, R. H. Goddard, 

 W. C. Parkinson, and 0. H. Gish. 



The detailed design of the recording potential-gradient apparatus for 

 installation at the observatories of the Department was partly completed, 

 and various forms were devised for recording the daily journals and instru- 

 ment controls required at the observatories in connection with conductivity 

 and potential-gradient observations. The ionium collectors and radioactive 

 resistance cells required for observatory equipment and for the needs of 

 special expeditions and field observations were prepared and tested in the 

 Laboratory. Conducting quartz fibers for the string electrometers so 

 largely used on field expeditions were prepared by Mr. O. H, Gish. In the 

 course of this work the method of preparing the fibers was modified and a 

 mounting device designed, so that now completed fiber systems may be 

 prepared with greater facility and certainty (see abstract, pp. 259-260). 



Reduction of atmospheric-electric observations. — A thorough and detailed 

 study of the observations made at various times and places for the standard- 

 ization of the potential-gradient apparatus aboard the Carnegie was com- 

 pleted, thus making it possible to reduce to absolute values the observations 

 which have been regularly made during Cruises IV, V, and VI. Valuable 

 assistance in this work was received from Captain J. P. Ault, who com- 

 manded the Carnegie during the fourth and sixth cruises. 



The observations on the diurnal variation of the potential gradient made 

 aboard the Carnegie during the last j^ear of Cruise VI (November 1920 to 

 November 1921) were reduced and the results compared with those obtained 

 from the earlier Carnegie observations. The investigation of the diurnal 

 variation was extended to include practically all the available data from land 

 observations (see abstract, pp. 262-263). 



Among other reductions carried out during the year were the following: 

 (a) The diurnal-variation observations on the conductivity of the air over 

 the oceans from observations aboard the Carnegrze during 1921 (see abstract, pp. 

 264-265) ; (6) continuous potential-gradient records from the deck observatory 

 in Washington for the years 1917 to 1922 (see abstract, p. 265), and (c) pre- 

 liminary reductions of the potential gradient at Apia Observatory from May 

 1922 to April 1923 (see abstract, pp. 263-264). 



