THE ELECTRICAL CONDUCTIVITY AT SEA 



Information regarding the distribution and variation 

 of conductivity at sea has been derived chiefly from ob- 

 servations made on board the Carnegie (1). The data 

 thus obtained show no definite evidence of a simple de- 

 pendence of conductivity on position, but near land the 

 values are more variable than at sea both with position 

 and time, and apparently tend to be somewhat smaller 

 than well out over the open sea. The data seem to justi- 

 fy the conclusion that there is no dependence on latitude 

 amounting to more than a few per cent. This result is 

 consistent with the assumption (a) that the cosmic radia- 

 tion is the sole ionizer at sea and (b) that the content of 

 condensation nuclei in the atmosphere is independent of 

 latitude. Calculations which involve these assumptions 

 show that although the cosmic radiation is less at the 

 magnetic equator than in higher magnetic latitudes- -the 

 difference amounting to about 13 per cent in the eastern 

 Pacific and about 21 per cent in the Indian Ocean--yet 

 when the difference in temperature is taken into account, 

 the average conductivity in low latitudes should not dif- 

 fer from that in high latitudes by more than a few per 

 cent. Such considerations lead one to expect that the 

 conductivity of air over the open oceans should vary only 

 when the concentration of nuclei varies. Observations of 

 nuclei are not yet adequate to disclose a definite depend- 

 ence on position or to test this surmise in other ways. 



Variations in total conductivity from year to year 

 apparently are indicated by the data in table 1 . 



Table 1. Variations from year to year in conductivity 

 of air over open oceans 



Value 



Carnegie cruise number 



IV 



VI 



vn 



Mean epoch 1916.2 1920.8 1929.1 



Mean of total conductivity 

 (in 10-4 esu) 2.2 2.7 2.0 



This feature merits further investigation. It implies 

 that over a great part of the earth either the rate of ion- 

 formation was less or the content of nuclei was greater 

 during the first and last epochs than during the interme- 

 diate one. Either alternative presents a challenge to the 

 investigator. 



An annual variation in the conductivity over the 

 oceans has escaped detection since scarcely any data 

 are available for winter, the season of rough weather at 

 sea. 



Variations from day to day occur in an irregular se- 

 quence. In the course of a few days or weeks the highest 

 mean value for a day will be found to be severalfold as 

 great as the lowest mean value. This is shown in figure 

 2, where daily mean values of conductivity are shown for 

 sixty-five days in the period September 3 to November 

 18, 1929. The daily mean values were obtained from 

 continuous registrations of conductivity in the central 

 Pacific Ocean during the last months of cruise Vn. Reg- 

 istrations of positive and negative conductivity were 

 made on alternate days, resulting in thirty complete days 

 of record of positive conductivity, twenty-eight complete 



days of negative, and seven days of partial record. In 

 figure 2 the two graphs have been drawn on the basis that 

 the negative conductivity is approximately 15 to 25 per 

 cent lower than the positive, and means for alternately 

 mirsing values of each sign of conductivity have been 

 interpolated accordingly. For the days of partial rec- 

 ord, mean daily values have been placed in parentheses 

 in figure 2. 



Bad weather was responsible for large variations in 

 the conductivity on a number of days between September 

 3 and November 18, 1929. The days most affected were 

 October 10 and 11, October 24 to November 2, inclusive, 

 and November 15 and 16, as may be seen from inspection 

 of the tabulated conductivity data on pages 124 and 125. 

 Most of the bad weather occurred in the late morning or 

 in the afternoon, local time, and as the ship's position 

 varied in longitude only between 235° east and 190° east 

 during the three months, the effect when viewing the data 

 on the basis of Greenwich rather than local time is to 

 place abnormally low values of conductivity in the hours 

 just preceding and following 24h GMT. Thirteen days 

 affected by bad weather accordingly were omitted from 

 studies of the diurnal variation in conductivity, as well 

 as two additional days on which large changes were noted, 

 leaving forty-three complete days. 



A diurnal variation in both positive and negative con- 

 ductivity of small amplitude--about 4 or 5 per cent of the 

 mean--is indicated by the data for the forty-three days 

 selected as least disturbed. The character of the diurnal 

 variation of both positive and negative conductivity may 

 be described with the aid of figures 3 and 4. The ordi- 

 nates there represent conductivity for the three months 

 of September, October, and November and, in the lower- 

 most graph, for all three months. The abscissas repre- 

 sent the hours of the day counted from Greenwich mid- 

 night on the bottom scale and, since the registrations 

 were obtained in the relatively narrow range of 45 de- 

 grees of longitude, on the top scale they are made ap- 

 proximately to represent the hours of the local day by 

 shifting the scale ten hours to the right so that local 

 midnight comes at lOh Greenwich time. For figure 3, 

 twenty-three days of record of positive conductivity were 

 available and for figure 4 twenty days of record of nega- 

 tive. 



It will be noted that the diurnal variation of positive 

 conductivity proceeds in a manner opposite to that of the 

 negative, although the two are not exactly in opposite 

 phase, the minimum in negative conductivity occurring 

 later than the maximum in the positive. Viewed on the 

 basis of local time, the positive conductivity may be 

 described as varying gradually during the day from a 

 minimum in the afternoon to a maximum in the morning. 

 This variation appears not only in the average for all 

 three months, but is prominently shown in each of the 

 monthly graphs. Hence a diurnal variation of this type 

 appears to be characteristic of positive conductivity over 

 considerable areas of the ocean. For the negative con- 

 ductivity, the graph for all three months shows a mini- 

 mum in the morning hours, on the basis of local time, 

 and a maximum just after local noon. The separate 

 graphs for October and November show considerable 

 similarity, but the September graph is irregular, and 



137 



