SOME OBSERVATIONS ON ATMOSPHERIC 

 ELECTRICITY TAKEN IN SOUTH AFRICA. 



By Prof. William Arthur Douglas Rudge, M.A. 



The author has published some observations on atmospheric 

 electricity, taken in 1910 at the Victoria Falls; and, as it was 

 found that the value of the potential gradient appears to vary 

 very considerably at different places in South Africa, he thinks 

 it may be of some interest to give here the results of some further 

 observations, taken over a rather extensive area. 



At the A'ictoria Falls, as is almost always the case in the 

 neighbourhood of a waterfall, the potential gradient reaches a 

 very high value, so high, in fact, that insulated wires stretched 

 across the gorge become so strongly electrified that sparks may 

 easily be obtained from them. The electrification is negative, 

 and reaches the value of probably over 25,000 volts per metre 

 close to the fall. The normal condition of the air, under a clear 

 sky, gives rise to a positive potential gradient between a point 

 in the air and the surface of the earth, the value of which varies 

 very considerably with the nature of the country, the altitude, 

 and the time of day when the observation is taken, and, of course, 

 with the presence of what may be called, for want of a better 

 term, " thunder " clouds. 



A thoroughly satisfactory theory as to the origin of the 

 enormous charges present in the atmosphere is wanting. The 

 older theories postulated that the condensation of water vapour 

 was accompanied by the production of an electric charge upon 

 the drops formed, and so also was the evaporation of water from 

 the surface of the earth supposed to be accompanied by electrifi- 

 cation of the earth itself — ideas probably incorrect. In the 

 present state of our knowledge, it is best to leave the origin an 

 open question. 



The natural charge present in the atmosphere is very large, 

 it having been estimated that a difference of potential of the 

 order of 150,000 ( !) volts exists between the confines of the 

 atmosphere and the surface of the earth. If we take_ the height 

 of the homogenous atmosphere as 10,000 metres, this gives an 

 average potential of 15 volts per metre, but the value near the 

 surface of the earth is much greater than this, so that the poten- 

 tial " gradient " diminishes as we go upwards. This normal 

 value may be largely exceeded — 10 to 100 times as great — and 

 may allso be reduced. The charge is usually positive, but occa- 

 sionally negative. The experimental facts of atmospheric elec- 

 tricity may be explained by supposing that the earth has always 

 a negative' charge, which gives rise to an electric field surrounding 

 the earth ; this would cause positive ions to move towards it, 

 thus producing the potential gradient between the earth and the 

 surrounding air. 



The observations were taken by means of an " Exner '*' 

 pattern electroscope furnished with a wire, to which was 



