720 Dr. Gr. C. Simpson on 



the air Axt els. units. If the potential gradient imme- 

 diately over the plate changes from P T to P 2 volts/meire 

 during the interval, there will be also a quantity of electricity 



A(P P ) 



4tt30Q00 els * Units Set free ° n the plate ' " Both these 



charges are transferred to the receiver ; hence, if the mean 

 charge per second recorded by the electrometer is Q els. units, 



we have 



(V-- Art A CP.-PQ 

 W~ Axt 4tt30000 



, Q. Pi-P 2 , 



• t= A+O-77xl0 5els - unlts - 



If it is assumed that the whole charge x reaches the plate 

 by means of the ordinary conductivity of the air (i. <?., 

 neglecting the charge carried by dust, <fec. which settled on 

 the plate against the electrostatic field), the conductivity can 

 be calculated from the relationship 



x= ''7 mm eh - m,its ' 



in which P is the mean potential gradient expressed in 

 volts/metres during the interval t. This mean potential 

 gradient will in general be different from ^Pi + Pg). 



This treatment applies to a plate in a uniform electrical 

 field, and therefore will not hold strictly for the plate exposed 

 in the disturbed field described above. It has, however, been 

 applied, and in discussing the results it must be remembered 

 that while the absolute values obtained are likely to be 

 affected^ the daily range of the different factors are no doubt 

 near approximations to the truth. 



The data obtained were treated thus : — The trace printed 

 by the electrometer consisted of a series of dots, each repre- 

 senting the deflexion at the end of a two minute interval. 

 These dots were first joined by a line, and then a freehand 

 curve was drawn to take out the smaller irregularities. 



The approximately smooth curve thus obtained was measured 

 at five equidistant points in each hour, and from these five 

 measurements the mean deflexion of the electrometer during 

 the hour determined. The mean deflexion was then con- 

 verted into volts and the quantity Q/A determined. The 

 part of this value due to the change in the potential gradient 

 was then obtained from the actual value of the potential 

 gradient at the beginning and the end of the hour. The 



