284 



Mr. E. H. Nichols on the Dh 



Therefore the current 



= 5'$9v{( j g 2 -go')/(gi + g 3 -ffo-g "')} X lO" 16 amp./cm. 2 , 

 and the conductivity 



= 5'S9i(92-go')l(9i+9s-9o-9o")} x 10" 25 e.in.u. 



The mobility, or the ionic velocity under the influence of 

 unit gradient of 1 volt per cm., can be calculated from the 

 conductivity (X) and the positive charge (E + ). It is con- 

 sidered that the conductivity measured by the Wilson 

 electrometer is that due to the positive ions *, so that 

 A. + = E + /x + , where /jl + is the mobility of the positive ions. 



Observations of the diurnal variation of the electrical 

 elements were commenced at Kew Observatory in May 

 1914, and continued during June and July on four days 

 of each month (Table L), with an additional day in August 

 (Table III.). During May and June hourly observations 

 were taken, from 7 a.m. to 8 p.m., each extending for about 

 15 minutes, while during July and August the times were 

 from 4 a.m. to 8 p.m. The weather conditions were excep- 

 tionally favourable, only a very few days being affected by 

 rain. As the instruments must be suitably mounted in the 

 open, any rain beyond a slight drizzle prevents continued 

 observation. Synchronous values of the potential were 

 obtained from the Kelvin water-dropper, a factor being 

 applied to convert the values to the potential gradient in 

 the open. 



A few hourly results are missing for the different ele- 

 ments. In these cases the mean value for the preceding 

 and following hour has been used, while in one or two 

 instances hourly values have been extrapolated by the 

 usual method. 



The mean summer diurnal variation for Kew obtained from 

 twelve days in May, June, and July is shown in fig. 1. There 

 is a maximum for the positive charge at 11 a.m. and for the 

 conductivity at 2 p.m. The air-earth current shows no im- 

 portant variation, but the mobility gives a decided increase 

 after noon. The potential curve indicates a definite minimum 

 in the afternoon, and agrees closely with the normal for 1898- 

 1912 t for those months, thus proving that the» variation of 

 the elements obtained should be generally applicable to 

 electrically quiet days. 



* Lutz, LvfteleMrisehe Messungen am Miinchen, 1911. 

 f 0. Chree, Phil. Trans. A.ccxv. p. 141. 



