Daily Range of Atmospheric Potential Gradient. 
79 
1 cm., a difference of potential of about 30,000 volts must exist between 
the balls, and if the height of the aerial collector above the ground was 
2 metres, this would give a potential gradient of 15,000 volts per metre 
approximately. This is by no means the maximum value, for on occasions 
I have seen sparks over 2 cm. passing between the dischargers. 
Thp] Curves. 
The instrument was arranged to make a record of the potential at 
minute or half-minute intervals, and, in addition, to make marks at the 
end of each hour at the edge of the band of paper. The curves given 
have been made by connecting all the points by a thick ink line, and are 
therefore slightly smoothed, but not to such an extent as to change their 
character. The selected curves are typical of the rest. 
(1), (2), and (3) are fine-weather curves, i.e., days on which no dust was 
blowing. Wind, unaccompanied by dust, has practically no influence. 
(4), (5), and (6) are curves taken on days when some dust was blowing, 
but not in sufficient quantity to destroy completely the positive charge 
present in the air. (7), (8), and (9) are for very dusty days. It will be 
seen here that the dust usually occurs between the hours of 8 a.m. and 
