DAILY VARIATION OF TOTENTIAL GRADIENT. 



317 



We thus see that during May, June, and July the potential gradient variation cannot 

 be ascribed to changes in the conductivity of the air caused by the variation of temperature 

 and humidity. It is therefore reasonable to assume that the potential changes in other seasons 

 of the year also are not due to this cause. 



The next step was to investigate whether the daily variation of potential gradient which 

 shows itself in the mean of a large number of broken periods exists when the weather re- 

 mains calm throughout twenty-four hours. There are not many periods during which the 

 wind did not rise above five miles an hour during twenty-four consecutive hours. There were 

 none during February, March, and April ; but six ware found during May, June, and July, 

 eight during August, Saptembsr, and Ocfcobsr, and four during November, December, and 

 January. Eich of thss3 periods generally extended over a few more than twenty-four hours, 

 but in each case the best series of twenty-four consecutive hours was chosen. From these few 

 observations the daily variation was obtained and the result has been shown by the thin curves 

 in figure 92. It will be seen at once that the character of the variation is the same and the 

 differences between the thick and thin curves are to be expected on account of the few 

 observations used to obtain the latter. This has confirmed the character of the variation 

 for calm weather, but there is still the possibility that meteorological conditions are the 

 determining factor. For example during Miy, June, and July, in spite of the absence of any 

 appreciable temperature variation there is a pronounced tendency for the air to be calmer 

 in the morning than iu the afternoon. This tendency for which no physical explanation 

 can be given probably acts during all periods of calm and may affect the potential gradient 

 as well as the air motion. If the potential gradient variation is not due to this meteoro- 

 logical condition, whatever it may be, the variation should be the same when the air is not 

 calm, i.e., it should be similar during winds as during calms. Unfortunately the presence of 

 drift so affects the potential gradient during appreciable air motion that its daily variation 

 is completely masked. The drift, however, does not become serious until the wind rises above 

 10 miles an hour. If therefore we investigate the potential gradient duiing i^eriods in which 

 the wind velocity was between 6 and 10 miles an hour the daily variation of the potential, 

 if it exits, should be recognisable. The hourly values of the potential gradient during winds 

 of 6 to 10 miles an hour were therefore tabulated and it was found that in all seasons 

 they showed the same general daily variation as during calms. The number of observations, 

 however, in each season is very small, so that the curves are irregular. On the mean of the 

 whole year, however, the observations are sufficient to give a rehable result ; numerical values 

 are contained in table 160 and the curve is shown at the bottom of figure 92. 



Table 160. 



Potential gradient during winds of 6 to 10 tniles an hour. 

 Volts per metre (Number of observations in brackets). 



The curve for the potential gradient during winds of 6 to 10 miles an hoar is practically 

 the same as that during calms; it is therefore quite clear that the variation of the potential 

 gradient is not caused by the peculiar meteorological conditions existing during calm weather 



