STUDIES IN ATMOSPHERIC ELECTRICITY 



139 



similarity on the two cruises, since it shows a gradual 

 downward trend from Greenwich midnight luitil 16h for 

 cruise VI, and to 18h for cruise Vn. The "hump" seen 

 in figure 7 between 16h and 20h GMT, not only on the 

 graph for negative conductivity but also on the graphs 

 for positive and total conductivity, is the result of "non- 

 cyclic change." For nearly all the nine series of ob- 

 servations, begun between 16h and 20hGMT and finished 

 twenty-four hours later, the magnitude of the conductivity 

 value differed appreciably at the beginning and end of the 

 observing period, thus causing a discontinuity evidenced 

 by the hump. Had the data been obtained on nine succes- 

 sive days instead of on nine days scattered over a period 

 of several months, the discontinuity probably would not 

 have appeared. With the discontinuity contributing to the 

 form of the graph for negative conductivity in figure 7, 

 the inverse relation between this conductivity and the 

 potential-gradient is not so closely adhered to here as 

 was the case in figure 5 and the suggestion of an elec- 

 trode effect not as well supported. The field changes, 

 however, and the average value of the gradient both are 

 much smaller for cruise VI than for cruise VII and so 

 for the former cruise the electrode effect would be ex- 

 pected to be less. 



Turning to the graph for the ratio of positive to 

 negative conductivity in figure 7, the general character 

 of the variation consists of a gradual increase in the 

 ratio through the Greenwich day up to 20h, with a rapid 

 decrease thereafter, much like the variation in potential- 

 gradient. In both figures 5 and 7, therefore, the graphs 

 for the ratio of the two conductivities seem to be in gen- 

 eral agreement as to trend through the Greenwich day, 

 with the maximum values of the ratio at the times of 

 highest potential-gradient. The results for both cruises 

 thus agree in supporting the idea that the electrode ef- 

 fect is present, even though local or transitory factors 

 are operating to obscure the effect as far as the separate 

 diurnal variations of the positive and negative conductiv- 

 ities are concerned. 



From figures 5 and 7 the mean values for positive 

 and negative conductivity and for potential gradient may 

 be taken and the value of earth-current density computed. 

 The results are shown in table 2. 



Table 2. Computed air-earth current density for the 



central Pacific Ocean from measurements on 



cruise VI and cruise vn, 1921 and 1929 



Cruise 



Potential- 

 gradient 

 V/m 



Conductivity 



Positive 

 10-4 esu 



Negative 

 10-4 esu 



Air -earth 

 current 

 density 

 10-7 esu 



VI 



vn 



94 

 149 



1.65 

 1.14 



1.44 

 0.96 



9.7 

 10.4 



Although the method here used of computing the air- 

 earth current density from mean values of large groups 

 of data does not provide an accurate value of that quan- 

 tity, it suffices nevertheless to show that the two groups 



of data support the view that in fair weather the air-earth 

 current density has an essentially constant average value 

 approximating 10 x 10"'' esu. The concordant results 

 engender confidence in the reliability of the techniques of 

 measurement and the instrumental constants, for two 

 periods several years apart. In particular, the reduction 

 factors used in converting volts measured with the po- 

 tential-gradient apparatus to volts per meter appear to 

 have been satisfactorily determined. 



To accoxmt for the lower conductivity and higher 

 potential-gradient values in the central Pacific Ocean 

 for cruise VH as compared with the values for cruise VI, 

 one may surmise that the condensation nuclei were more 

 numerous in that region on cruise VII than they were on 

 cruise VI. Evidence to support this conclusion is lack- 

 ing, however, since nuclei measurements were not made 

 until cruise VH. The nuclei results for cruise Vn (see 

 pages 65 to 112) showed large concentrations of nuclei 

 in the western Pacific Ocean, probably produced by vol- 

 canic activity in the islands of that region. Possibly 

 volcanic activity in the years between 1921 and 1929 may 

 have caused a higher level of nuclei content over the 

 ocean in 1929 than existed in 1921. 



The nuclei content of the atmosphere over the oceans 

 generally is much less than that over land or near land, 

 because in the vicinity of land or over land smoke parti- 

 cles and other condensation nuclei are much more nu- 

 merous. The "land effect," so called, is strikingly 

 illustrated by conductivity records obtained when ap- 

 proaching land and while in harbor. Sample records are 

 reproduced in figure 8. The upper record was obtained 

 when the Carnegie was approaching the Hawaiian Islands 

 and for the first eighteen hours is typical of an undis- 

 turbed day at sea. Baseline spots may be seen at hourly 

 intervals on the record, and a set of calibration spots is 

 shown between 5h and 6h. At IBh the ship had just 

 reached the leeward side of Oahu Island, and the conduc- 

 tivity thereafter shows large fluctuations and a general 

 trend to lower value. In the middle record, with the 

 ship in Honolulu harbor, the value of conductivity is, on 

 the average, only about one-third or one-fourth as great 

 as it was at sea just before arriving, and very large 

 fluctuations are constantly taking place. These two rec- 

 ords, obtained on successive days, show the striking 

 change from sea to land conditions. 



The lower record in figure 8 shows the effect of bad 

 weather. After 8h, llh, and 21h, the positive conductiv- 

 ity is decreased almost to zero, remaining so for twenty 

 to thirty minutes or more. Such decreases in the posi- 

 tive conductivity have been shown by other observations 

 to be associated with very large negative values of po- 

 tential-gradient, with the negative conductivity in the 

 meantime remaining .unaffected. With large positive 

 values of potential- gradient, on the other hand, the nega- 

 tive conductivity is much decreased and the positive 

 conductivity shows no effect. Records obtained at sea 

 which were disturbed in the manner illustrated, were 

 omitted from the investigation into the character of di- 

 urnal variation of conductivity. 



LITERATURE CITED 



1. Mauchly, S. J. 1926. Studies in atmospheric electricity 

 based on observations made on the Carnegie . 1915- 



1921. Researches of the Dept. Terr. Mag., Carnegie 

 Inst. Wash. Pub. No. 175, vol. 5, pp. 385-424. 



