Variation in Value of Atmospheric Electrical Potential. 403 
this for half an hour. The wind was blowing strongly from Norwood to 
Johannesburg. 
From the Observatory to the centre of the town the gradient 
diminished, reaching a zero value in Joubert Park. On the roof of the 
School of Mines it was + 85 at 1 p.m., but, as this place is near to the 
railway station, every passing train caused the value to change rapidly. 
Passing to the south side of the town, an open space is reached just 
before the Village Deep Mine, and observations taken in this neighbour- 
hood showed surprising changes in the magnitude and sign of the 
electrification. A strong wind was blowing from the north at the time, 
and standing to the north of the mine the potential gradient was about 
+ 40. From the mine heap at the south end of the mine a large cloud of 
dust was being blown, and taking an observation at this end the magnitude 
of the potential gradient was 300 to 400 and negative. This abnormal 
electrification persisted for a considerable distance and was more than 
150 volts per metre at a mile and a half aw^ay and still negative. 
Inside the mine enclosure the electrification was positive close to the 
engines, zero in some places, negative at the beginning of the mine heap, 
the magnitude of the negative charge increasing as the heap was ascended. 
The same thing was seen near to other mines. 
At the School of Mines the potential gradient was +120 at 4.30 and 
at the Observatory +160 at 6.30. 
The general electrical conditions about Johannesburg are thus very 
abnormal as compared with other places in South Africa, and the cause is 
clearly due to the steam and dust. 
On returning from Lourengo Marques on September 30th, the observa- 
tions were repeated, but with very different results. There was no tipping 
of the sand going on, and very little wind, with the consequence that 
the electrification was everywhere positive, and also of high value. At 
the School of Mines the value was +200 at 10 a.m., and +250 at the 
Observatory at 4 p.m. On returning from Durban on October 5th 
the conditions were the same as on September 24th, viz., positive to the 
north, negative to the south of the town, with very high value near to 
the mine heap, from which clouds of dust were rising. The general 
results of the Johannesburg observations show to what a great extent the 
potential gradient in the atmosphere is influenced by steam and dust, and 
also that the influence is a local one. The various factors which influence 
the electrification of the atmosphere have been considered in a communica- 
tion made to the South African Journal of Science.-'' An extended series 
of observations on the electrification caused by raising clouds of dust is 
now in progress, and will be published shortly. 
* March, 1912. 
