142 DE. C. CHKEE: ATMOSPHEEIC ELECTEICITY POTENTIAL GEADIENT 



Even fair weather days in the same month vary immensely amongst themselves 

 electrically, and there is a good deal that is "accidental" in means derived from 10 

 days only. When the rainless days available are largely in excess of 10, as is 

 frequently the case, the natural tendency, at least in summer, is to prefer days in which 

 the potential is high to those in which it is low. The former usually show the diurnal 

 variation more clearly, and there is in their case a greater presumption against bad 

 insulation, a not infrequent occurrence in damp weather, or at seasons when spiders 

 are most busy. Thus the mean potential in a month when the choice of days is small 

 is apt to come low. There are, however, individual winter months in which the 

 contrary tendency prevails. During fog the potential at Kew is often much above the 

 average. If only part of a day is foggy one prefers to omit it, because the diurnal 

 variation shown is largely dependent on the accident of what hours the fog was 

 incident. If, however, the choice of days is very restricted, one is obliged to include 

 days of intermittent fog. Again the number of days available depends considerably on 

 how the apparatus has been working. At times there are somewhat numerous 

 defects, whether from poor insulation, freezing of the jet, accident or lack of 

 attention, which restrict the choice, leading to the same consequences as frequent rain. 

 Finally sensible changes sometimes take place between two successive scale deter- 

 minations, leading to uncertainty in the scale values to be applied. Thus " accident " 

 plays some part in the individual monthly values in Table 1. 



Judging by the fluctuations in the values for months of the same name, a longer 

 period would be desirable. The smoothness in the annual variation derived from the 

 15 years is, however, truly remarkable in view of the irregularities visible in the 

 corresponding variations deduced from either of the sub-periods 1898 to 1904 or 

 1905 to 1912. It can hardly be doubted that it is a fair approximation to normal 

 conditions. 



The results are shown graphically in the top curve of fig. 4, p. 142, unity representing 

 304 volts, the mean value for the whole year. The maximum and minimum appear 

 respectively at about midwinter and midsummer, and so somewhat in advance of 

 the times of minimum and maximum of temperature at ground level. They are 

 naturally still more in advance of the times of minimum and maximum temperature 

 at any depth underground. This suggests that direct solar radiation has more to do 

 with the annual variation than earth temperature or meteorological conditions in the 

 lower strata of the atmosphere. 



The extent of the annual variation in potential gradient is pretty similar to that in 

 vapour pressure or density. But the minima of these elements at Kew occur in 

 February and. the maxima in July, while June and September values are closely alike. 

 Thus a formula of the type once proposed by ELSTER and GEITEL dV/dn = A/(l + kq ), 

 where A and Jc are positive constants and q vapour density, cannot be made to fit the 

 Kew data very satisfactorily. 



8. In E! conspicuous differences were pointed out between the diurnal inequalities 



