7.— A PRELIMINARY NOTE ON SOME OBSERVATIONS 

 ON ATMOSPHERIC ELECTRICITY IN CAPE 

 TOWN AND BLOEMFONTEIN. 



By Dr. ]. C. Beattik, F.R.S.E., W. H. Logkmax, B.A., and 

 J. Lyi.e, M.A. 



Till recently, work on atmospheric electricity embraced obser- 

 vations on thunderstorms and allied phenomena and measurements 

 of the potential at a point in the air. In 1899. Elster and Geitel, 

 following up the work of Linss, proved the presence of gaseous 

 ions in ordinary atmospheric air. This discovery added new 

 interest to the subject, and when one of us came to reside in Bloem- 

 fontein, Dr. Beattie suggested that the study (jf atmospheric elec- 

 tricity might, with great advantage, be carried on simultaneously in 

 Cape Town and Hloemfontein. 



The stations are equipped with a Kelvin portable electrometer 

 and an Elster and Geitel " Zerstreuungs Ap^jaratus."' The former 

 requires no description ; the latter is simply a very sensitive aluminium 

 leaf electroscope, having, in place of the usual disc, a cylinder of 

 blackened brass, radius 2*5 cms., length to cms., which is insulated 

 inside of a larger earthed cylinder, radius 9 cms., length 14 cms. 

 The insulation is of such an excellent character that no allowance 

 need be made for loss of charge due to bad insulation. The method 

 of using the instrument is as follows: — The distributor, as the inner 

 cylinder is called, is charged to a potential which can be known from 

 the divergence of the leaves. It is usually 150 to 200 volts. The 

 instrument is then exposed freely in the open air. but out of the 

 direct rays of the sun. A reading is taken in about fifteen minutes, 

 and the instrument, charged oppositely now. is set to leak again. 



From the voltage difference it is easy to calculate a number 

 which is proportional to the quantity of electricitv which leaked in 

 one minute from the inner cxlinder to the outer. This number, 

 hereafter referred to as "n". mav be taken as a measure of the 

 conductivity of the air. 



Readings have been taken morning and afternoon, and in 

 Bloemfontein several all-day readings have been obtained. 



In Cape Town the instruments are in the charge of Mr. Loge- 

 man. The Elster and Geitel is exposed under the balconv in the 

 south front of the Physics Laboratory. 



The potential readings are made at a point seven feet above 

 the ground, in the centre of the College Quadrangle. 



In Table T. will be found the mean monthly values of ''n" for 

 this station. 



