264 ATxMOSPHERIC ELECTRICITY AT BLOEMFONTEIN. 



Dust and rain thus have the effect of completely reversing 

 the normal electrical charge in the atmosphere. The reversal due 

 to the rain is undou'btedly caused by the breaking up of larger 

 drops into smaller ones, either by collision or 'by internal stresses 

 set up iby the drop falling through the air. The breaking up of 

 water iby mechanical means into fine spray is always accompanied 

 by a strong negative electrification of the air, las was shown by 

 a series of observations taken at the Victoria Falls (see Phil. 

 Mag., May, 1911). 



The effect of a " dust devil " may be perceived at a consider- 

 able distance from it, as the following extract from the observa- 

 tion shows : — 



August 20th.— Windy ; atmosphere misty until 8.50; poten- 

 tial gradient rose to maximmn at 9.25, fell rather rapidly to 

 15 at 10.30; varied between 10 and 15, until 12, when it sud- 

 denly fell to zero. A " dust devil " was seen at Tempe, albout 

 two miles away. It circled round and finally worked itself out, 

 but its influence was recorded by the electroscope, as the table 

 shoW'S : — 



Time. Pot. Charge. 



11.50 18 + 



12.0 o o 



12.3 150 — 



12.5 300 — 



12.10 120 — 



12.12 o o 



12.14 . 20 4- 



12.30 50 -f 



i-o 50 + 



1-35 50 + 



2.15 50 + 



The rest of the day was normal. 



The cause of the electrical field surrounding the earth under 

 normal fine weather conditions is somew'hat difficult to trace, and 

 various theories have been advanced to account for it. It is quite 

 definite that during fine weather a current of electricity is passing 

 from the air into the earth, and the origin of the current may be 

 due either to a positive charge in the atmosphere or a negative 

 charge upon the earth. The value of this air-earth current has 

 been determined by C. T. R. Wilson, Gerdien, Siimpson, Elster 

 and Geitel, and many others, and has a value of about 

 5 -}- 10"^ E.S. imits per square cm. It varies with the altitude 

 baqause the current is detemiined from the potential gradient and 

 the conductivity of the air. The potential gradient diminishes 

 with the altitude, whilst the conductivity increases. 



Arrangements are being made fo-r carrying on a series of 

 observations on the conductivity of tlie air and the earth air 

 current. 



The following table shows the manner in which the maxi- 

 mum value of the potential gradient varies from day to day. 



