Earth- Air Electric Currents. 721 



latter charge was subtracted from Q/A, and the remainder 

 gave the mean value of x during the hour. 



The following gives a typical example: — The smoothed 

 curve for the hour 20 to 21 Indian standard time on the 

 19th November, 1909, was measured at five equidistant 

 points, giving a mean value of 30'8 mm. As the sensitive- 

 ness of the electrometer was 5*9 volts per mm. deflexion, the 

 mean voltage recorded at the end of each two minutes during 

 the hour was 30*8 x 5*9 =a 181*7 volts, i. e. -606 els. unit, 

 The combined capacity of the receiver and electrometer was 

 35 cm. ; hence, during this hour the average quantity of 

 electricity removed from the plate during each two minute 

 interval was *606 X 35 = 21*21 els. unit, or during each 

 second 21*21/120=: "17 7 els. unit. The area of the plate was 

 17xl0 4 cm. 2 , hence Q/A = • 17 7/17 x 10 l = 104 x 10~ 8 els.' 

 unit. 



At the commencement of the interval the potential gradient 

 over the plate was 20'8 volt/metre and at the end 49 3 

 volt/metre, hence 



,---., l~ * = — .y-^Tr "', 1- 1 . - - 2 x 10" 8 els. unit. 

 t . 3*77 x 10 5 3b00 x 3* / < x 10' 



Hence ,r, the mean charge received from the air each 

 second by each cm. 2 of the exposed plate, was 



(104 -2)10- s = 102 x lO" 8 els. unit, 



The mean potential gradient over the plate during the 

 hour under consideration was 34 r 8 volt, metre, hence 



X=102vlO-»/^=8-7xlO- els. unit. 



The values obtained during ten days in November are 

 shown in Table I. Only those days on which the weather 

 was fine, without a cloud in the sky, and on which an unin- 

 terrupted record was obtained from midnight to midnight, 

 have been utilized. 



The lines marked x give the mean charge received from 

 the air by each cm. 2 of the exposed plate in a second during 

 the hour stated at the head of the column. 



The lines marked P.G. give the mean potential gradient 

 over the exposed plate during the hour. 



The lines marked X give the mean conductivity of the air 

 during the hour. 



The time entered at the head of the table is Indian Standard 

 Time, which is 21 minutes ahead of mean local time. 



Phil. Mag. S. G. Vol. 19. No. 113. May 1910. 3 A 



