134 



OCEAN ATMOSPHERIC -ELECTRIC RESULTS 



Table 8. Comparison of Apia land values (reduced to 



V/M) with Carnegie values for period immediately 



following departure from Apia 



Date 



165th w 

 MT 



Apia 

 land 



R.-F. 

 = 1.09 



Ship 

 re- 

 cord- 

 er 



Sail 

 posi- 

 tion 



Ship's 

 R.-F. 



aShip values somewhat disturbed. ^Omitted from 

 mean; sail changed during hour. '^Standardization 

 value of April 10 to 13, 1929 = 2.87 (MUBP). ^gtand- 

 ardization value of April 10 to 13, 1929 = 3.28 (MUBS). 



tained after July 28, 1929, were included because only 

 one sign of conductivity was measured after that date. 

 The results of the grouping are shown in table 10. 



Had the Iceland reduction factor for eye -reading ap- 

 paratus no. 2 been used to obtain the potential-gradient 

 values from which seven of the ten air-earth current 

 values in the first group were computed, the average of 

 the ten observations in that group would have been 6.9 

 rather than 9.0. The average of 6.9 would have given 

 much less satisfactory agreement than the average ac- 

 tually obtained. 



In connection with potential-gradient measurements 

 on previous cruises of the Carnegie , study of reduction 

 factor determinations indicated that the factors should 

 be adjusted for differences in the ship's draft at the dif- 

 ferent ports, although such corrections in general would 

 be small. In tables 3, 5, and 6 in this discussion, re- 

 marks are made concerning the ship's draft. Similar 

 remarks should have accompanied tables 1 and 7, but 

 they apparently never were transcribed from the ship's 

 log, and are not available now as the log was destroyed 

 with the ship.. For lack of these data and because the 

 series of reduction factor observations are few in num- 

 ber, it has not been possible to investigate the effect of 

 changes in draft of tha ship on the reduction factors for 

 cruise VII. 



In view of the comparatively limited number of re- 

 duction factor determinations and of the several changes 

 in potential-gradient apparatus during cruise VH, it is 



Table 9. Accepted reduction factors for potential-gradient measurements, Carnegie cruise VII 



Apparatus on stern rail 



Applicable period 



Reduction factors 



MUBP-MDBP 



MUBS-MDBS MDBPC 



Eye-reading no. 2 

 Recorder (bent rod) 

 Recorder (straight rod) 



May 11 - Sep. 16, 1928 3.3 



July 7 -Nov. 4, 1928 0.7 

 Nov. 5, 1928 - 



Nov. 18, 1929 2.9 



3.3 

 0.7 



3.2 



4.0 



3.9 



From the preceding considerations, the reduction 

 factors finally adopted and their periods of applicability 

 were as given in table 9. 



That these factors may be accepted as substantially 

 correct, may be seen from examination of the computed 

 air-earth current values tabulated in Section V of the 

 present volume. There is little reason for believing that 

 the air-earth current density differs greatly from one 

 part of the ocean to another in regions free from steam- 

 ship routes and well away from large inhabited land 

 masses. If, therefore, the air-earth current values in 

 Section V are grouped in accordance with the three ar- 

 rangements of apparatus shown in table 9, the average 

 air-earth current values for the three groups might be 

 expected to be similar. Such groupings were made, but 

 the values for the period August 10 to 25, inclusive, 

 were omitted as being obtained from a region not free 

 from disturbing factors and, in addition, no values ob- 



believed that the factors given in table 9 are the most 

 satisfactory that can be secured. 



Table 10. Comparison of average values of air-earth 



current density derived from potential-gradient 



measurements made with three different 



instrumental arrangements 



Apparatus on 

 stern rail 



Number of 

 observations 



Average computed 



air-earth current 



density in 10"'' esu 



9.0 



8.8 



10.4 



