OBSERVATIONS OF ATMOSPHERIC ELECTRICITY AT 

 BLOEMFONTEIN. 



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



On pages 232 to 237 of this volume some account is given 

 of observations on atmospheric electricity at various places in 

 South Africa. The present communication gives some details 

 of daily observations carried on at Bloemfontein for a period of 

 nearly three months. 



The objects of the experiments were to ascertain (i) the 

 daily range of the potential gradient, (2) the time of the maxi- 

 mum potential, (3) the relation, if lany, between the variation in 

 the potential gradient and the barometrical pressure. 



The apparatus used in the couse of the investigation con- 

 sisted of an electroscope with a double wall, as devised by C. T. R. 

 Wilson. The gold leaf system was insulated by a quartz tube, 

 and to the end of the wire carrying the gold 'leaf there was 

 attached a brass plate 5 cms. in diameter coated with a radium 

 preparation. As is well known lan insulated metal plate exposed 

 in the open air tends to take the potential of the layer of air 

 immediately in contact with it, and this tendency is considerably 

 accentuated by attaching to the plate either (a) a small piece of 

 burning tinder, or (b) a vessel from which water is dropping, 

 or (c) covering the plate with a weak radioactive substance. 

 This latter method is the most convenient, but perhaps not the 

 ibest, unless Ionium is the radioactive material used, however, if 

 comparative values only are required, the radium coated plate 

 is in every way satisfactory, provided the radium preparation 

 is not very strong, as in that case the intense ionisation of the 

 surrounding air causes the acquired charge to be quickly lost. 



The observations were taken near the Grey University 

 College, 'but in such a position that the buildings had no influence. 

 The electroscope was placed so that the collecting plate was at 

 a height of 140 cms. froan the ground, and dbservations were 

 taken as times permitted, between 6 a.m. and 12 p.m. These of 

 course were not continuous, but when it was seen that the poten- 

 tial was dhlanging, a close watch was kept. 



As pointed out in a previous paper, there are two daily 

 maxima, one in the morning and the other in the evening. The 

 times of these maxima are not very regularly fixed, but this is 

 due proibably to winds and dust. If the atmospheric conditions 

 were always uniform it is likely that the times would be more 

 uniform. 



In genenal a day's observations show the following changes 

 in the potential gradient : From early morning until sunrise the 

 gradient is low. rising after sunrise, and steadily increasing 

 until the maximum is reached, then falling fairly rapidly to a value 

 which remains steady over many hours, and then rising to the 

 second maximum. A drop follows this, and 'before midnight, 

 as a rule, it has dropped to the low est value, and remains fairly 



