VI. ATMOSPHERIC-ELECTRIC DIURNA L - VARI ATIO N RESULTS 

 EXPLANATORY NOTES AND COMMENTS 



Measurement of the several atmospheric-electric 

 elements through a twenty-four-hour period was sched- 

 uled for once each week while at sea, in order to obtain 

 information on the character of the diurnal variation of 

 the various elements. During fifty-three weeks at sea, 

 between May 11, 1928 and November 18, 1929, thirty- 

 two diurnal-variation series were undertaken, or about 

 two series in every three weeks. Actually the thirty- 

 two series were obtained in forty-six weeks, no series 

 being attempted in the first seven weeks, or until July 

 30, 1928, as the whole ship's program was being organ- 

 ized and systematized during this initial period. The 

 observational procedure in a diurnal-variation series 

 was to measure each element once each hour through a 

 period beginning approximately at noon, local mean 

 time, and ending at noon on the following day. Two ob- 

 servers worked in shifts of six hours observing and six 

 hours rest. 



Of the thirty-two series begun, several were not 

 completed because instrumental difficulties developed 

 or bad weather made observing impossible. The series 

 begun on October 8, 1928 was discontinued so early that 

 all data obtained on that occasion were placed in section 

 V rather than in the present table. There are, therefore, 

 only thirty-one complete and partial series tabulated 

 here. The nuclei measurements were completed through 

 the twenty-four-hour period on only sixteen occasions, 

 with one additional series lacking only four values; the 

 small-ion measurements on twenty occasions; and the 

 conductivity on twenty-seven. Recorder values of poten- 

 tial-gradient were available for twenty-four complete 

 series, and for all incomplete series except the first, 

 for which eye readings were used. 



Each diurnal-variation series is tabulated under lo- 

 cal dates, and latitude and longitude are given for the 

 average position for the twenty-four-hour period. Thus 

 the position for each series is approximately that for 

 twenty-four hours, LMT, of the first date given for the 

 series. The atmospheric-electric elements tabulated 

 are: potential-gradient, in volts per meter; conductiv- 

 ity, of the sign given at the head of the column; small- 

 ion concentration, of the sign at the head of the column; 

 computed small-ion mobility; nuclei concentration; com- 

 puted air -earth current density. 



Potential-Gradient. --Values of potential-gradient 

 vere taken from the photographic records for a period 

 ranging from ten to twenty minutes in each hour, coin- 

 ciding with the period during which the conductivity and 

 small-ion measurements were made. Frequently the 

 value of potential-gradient for this short interval differs 

 considerably from the mean hourly value which may be 

 found in section VII for the particular hour and date, ow- 

 ing to the fluctuations which occur in the potential-gradi- 

 ent even on comparatively quiet days. Comparison of the 

 twenty-four values in any series in the present table 

 with the corresponding twenty-four hourly mean values 

 in section Vn will give some Indication of the extent to 

 which the period is disturbed, as the more disturbed peri- 

 ods show larger and more frequent differences. The eye- 

 reading values of potential-gradient used for the series 

 of July 30-31, 1928 were obtained fifteen to twenty-five 

 minutes earlier than the values of the other elements 



with which they are associated in the series; some ques- 

 tion may therefore be raised as to the validity of the 

 computed air -earth current values which have been tab- 

 ulated for the group, and for the sake of homogeneity in 

 studies of the material in this table perhaps they should 

 be ignored. 



Conductivity and Small-Ion Concentration.- - Unlike 

 the procedure for the daily observations of conductivity 

 and small-ion concentration, under which the measure- 

 ments of positive and negative values of these elements 

 were alternated, the procedure for the diurnal -variation 

 measurements required that only one sign be measured, 

 and that this be done at hourly intervals through the 

 twenty-four-hour period. The time required for one 

 measurement in each hour ranged from ten to twenty 

 minutes, the conductivity and small-ion concentration 

 bieing obtained simultaneously. The small-ion measure- 

 ments were subject to more frequent interruption than 

 the conductivity, however, because the occurrence of 

 rain or spray on any occasion mr.de necessary the clos- 

 ing of the roof aperture through which the ion-counter 

 intake projected. 



Computed Small-Ion Mobility. - -From the simultan- 

 eous measurements of conductivity and small-ion con- 

 centration, values of small-ion mobility were computed 

 for the present table as in section V. For the last two 

 diurnal-variation series which are given, obtained Octo- 

 ber 21-22 and November 4-5, 1929, the mobility values 

 are conspicuously higher than for any of the other series 

 obtained on the cruise. These two series fall in the peri- 

 od October 16 to November 6, 1929, when high mobility 

 values were consistently obtained from the daily meas- 

 urements of conductivity and small ions, as stated in 

 connection with section V. No explanation has been found 

 for these high values. 



Nuclei Concentration. --The values of nuclei concen- 

 tration shown in the table, except for the last four series, 

 generally were not obtained at the particular times with 

 which they are associated. The times given are the mean 

 times for the conductivity and small-ion measurements, 

 and as the nuclei measurements were not begun until 

 after these had been completed, their mean times actually 

 are from fifteen to thirty minutes later than the times 

 tabulated. The magnitude of this time difference was as 

 indicated because the conductivity and ion measurements 

 required ten to twenty minutes for completion, while the 

 nuclei measurements required ten to fifteen, and five 

 minutes or more were consumed in changing from one 

 type of measurement to the other. For the last four se- 

 ries, when both potential gradient and conductivity were 

 being recorded, the eye-reading measurements of small 

 ions and nuclei were made simultaneously, and with po- 

 tential gradient and conductivity recorder values taken 

 to correspond in time with these eye-reading measure- 

 ments, the tabulated values for each hour in these series 

 represent simultaneous measurements. 



Air-Earth Current Densitv. --As in section V, each 

 value of air -earth current density was computed from 

 the sum of the positive and negative conductivities and 

 the corresponding value of potential gradient. For the 

 present table, however, since only one sign of conductiv- 

 ity was measured during any one diurnal -variation series, 



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