THE DIURNAL VARIATION OF THE ELECTRIC POTENTIAL OF THE ATMOSPHERE OVER THE OCEAN 



Successful continuous recording on the seventh 

 cruise of the Carnegie (May 1, 1928 to November 18, 

 1929) of the electric potential of the atmosphere was be- 

 gun August 7, 1928, after a three-month period of exper- 

 imentation and adjustment at sea. 



In the early part of the latter period the recording 

 apparatus had been mounted at the top of the mainmast. 

 After a three-week trial of the instrument in that po- 

 sition, it became apparent that the whipping of the mast- 

 head would be a constant source of lossof record because 

 the motion was imparted to the fibers of the electrome- 

 ter to such an extent that no photographic record of the 

 fibers could be obtained even under the more favorable 

 sea conditions. The instrument later was mounted on 

 the roof of the atmospheric-electric laboratory for a 

 short period, but this location was soon found to be so 

 greatly shielded that diurnal variations did not appear. 



While at Hamburg, Germany, the apparatus was 

 mounted on the stern rail of the ship, slightly to star- 

 board of the center line. It was retained in this position 

 throughout the remainder of the cruise (until November 

 18, 1929) without modification except in the shape and 

 length of the rod supporting the four ionium discs of the 

 collecting system. Between August 10 and November 5, 

 1928, a long rod, bent in a rough arc so as to project 

 astern approximately one meter, was used. Between 

 November 5, 1928, and November 29, 1929, a short ver- 

 tical rod was used to support the collecting discs. 



Between July 8, 1928, the date of departure from 

 Hamburg, and August 7, 1928, when the first of the suc- 

 cessful records was obtained, the apparatus was under 

 constant testing and adjustment. 



Control of the records was very effectively main- 

 tained throughout the cruise. Tests of the adequacy of 

 the insulation of the recorder were made regularly; 

 usually twice each day, frequently several times a day 

 when conditions demanded it, but always at least once 

 each day. Calibration of the electrometer was done 

 regularly once every week or ten days. Observations 

 for the reduction of the observed potentials to volts per 

 meter were made at five ports where conditions on shore 

 were such as to provide a suitable location for the stand- 

 ard comparison station. The results from the five ports 

 did not agree quite so satisfactorily as had been hoped 

 for, but it is believed that the subsequent adjustment of 

 the data has given reduction factors of reasonable ac- 

 curacy. 



Notes describing weather conditions were entered 

 in the ship's log by the sailing officers and were ab- 

 stracted into a general description of each day for the 

 entire cruise. These notes included direction and veloc- 

 ity of the wind, condition of the sea, amount of cloud, the 

 time of occurrence of distant lightning and thunder and 

 their direction, and the beginning and ending time of rain 

 squalls, showers, and thunderstorms in the vicinity of 

 the ship. 



In addition to the weather notes, record was kept of 

 the various changes in the position of the mainsail and 

 the beginning and ending times of the periods of opera- 

 tion of the auxiliary engine used in calm or nearly calm 

 weather for propulsion. It was necessary to note the 

 mainsail positions because the mainsail boom projected 



out into the region where the potentials were being meas- 

 ured and changes in the boom position affected the re- 

 corded values quite considerably. It was necessary to 

 record the periods of operation of the engine because the 

 exhaust was located directly below the potential record- 

 er and the charge of the exhaust gas generally affected 

 the records. 



Notes of instrumental difficulties show that practi- 

 cally all the trouble with apparatus was owing either to 

 breaking down of insulation by rain or sea spray, or to 

 loosening or wearing of parts of the apparatus due to the 

 motion of the ship. 



Between August 7, 1928, and November 18, 1929, the 

 ship was on the open sea 317 days. On 181 days complete 

 twenty-four-hour records of potential-gradient were ob- 

 tained, a few of these days being completed by interpo- 

 lation over periods of one or two hours. On 86 addition- 

 al days several hours of record were obtained each day. 

 Of the 50 days at sea when no record was obtained, in- 

 strumental defects were responsible for the loss of 46, 

 and running of the main engine for the remaining 4 days. 

 Of the 267 days when record was obtained, 128 were af- 

 fected by bad weather, and on 59 of these days some 

 negative potential-gradient was recorded. Table 1 shows 

 the distribution by months of the number of days at sea, 

 the days of complete or partial recording, and the days 

 when various disturbing factors existed which reduced to 

 82 the days acceptable as typical of fair-weather condi- 

 tions over the oceans. 



The 82 days in the last column of table 1 represent 

 critical choosing of data for the study of diurnal varia- 

 tion of the electric potential under least disturbed con- 

 ditions. These days were selected, however, without 

 giving consideration to the type or amount of clouds or 

 of the conditions of temperature, humidity, barometric 

 pressure, or atmospheric pollution. Undoubtedly some 

 of these factors did affect the values of the recorded po- 

 tentials on some of the selected days. 



The 82 selected days have been grouped into three- 

 month periods and mean diurnal-variation curves pre- 

 pared for each group as shown in figure 1. Comparable 

 curves for 47 days of potential-gradient results from 

 cruises IV, V, and VI are shown also. The curves for 

 cruise VTI are in very satisfactory agreement with those 

 for previous cruises, and support the conclusion drawn 

 from the earlier data that the atmospheric potential, 

 when freed from local disturbing effects, varies in the 

 same way and at the same time over the whole earth. 



It will be noted that the 82 selected days of cruise 

 Vn exceed by 74 per cent the combined number of days 

 from cruises IV, V, and VI. The data of cruise Vn 

 represent observations over the north and central At- 

 lantic and the northcentral, southeastern, and southwest- 

 ern Pacific oceans, whereas the data of the previous 

 cruises were obtained from even more widely distributed 

 areas; so the results may be accepted as representative 

 of world-wide conditions. 



Table 2 shows the results of harmonic analysis of 

 the diurnal variation of the potential-gradient as previ- 

 ously reported for cruises IV, V, and VI, and as deter- 

 mined from the observations obtained on cruise VII. It 

 will be noted from the last column of table 2 that the 



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