1909-10.] Observations of the Earth-air Electric Current, etc. 465 
Table I. contains the observations made on the roof of the Physical 
Laboratory, reduced to corresponding values at ground-level. Table II., the 
results of similar observations made on the grass in front of the Laboratory. 
The results in Tables III. and IV. were got from observations made respec- 
tively at Waverley Park, Newington, and the Blackford Hills, in the 
vicinity of the Royal Observatory. The values of the charges, etc. are the 
means for the group of observations denoted by the first three columns, 
while the numbers at the end of each table are the averages of the column. 
The tables show a gradual increase in the dissipation factor as the 
distance from the central part of the town increases. The much smaller 
values of the dissipation factor in town air as compared with country air 
are probably due to the loading up of the ions by their becoming attached 
to dust particles, etc. 
After thunderstorms we have observed the reversal of charge and of the 
direction of the field (this, of course, being well known in connection with 
the ordinary measurements with the water-dropper). Wilson has also 
noticed this on several occasions, as well as sudden jumps in the field each 
time flashes take place. 
If the conductivity of the air be estimated from the measurements of 
current and potential gradient a value of about 2 x 10~ 5 E.S.U. is got, which 
is in agreement with the results of Pollock,* who, working in Sydney, has 
estimated the conductivity of the air there as of the order of 10~ 5 E.S.U. , 
while Wilson’s and Gerdren’s values are about 10 -4 E.S.U. for country air. 
It is possible from the measurements of the charge to obtain values 
which give the order of the potential and charge of the surface of the 
earth. 
Taking the average potential gradient as 190 volts per metre (the 
weighted mean of Wilson’s observations and those in this paper), we have 
charge of the earth =47rrV(r = 6360 x 10 5 cms.)= — 2 56 x 10 15 E.S.U., and 
potential = — 4U3 x 10 6 E.S.U., or —1-21 x 10 9 volts. 
For the electrometer used in this investigation we are indebted to a 
grant from the Carnegie Trust, and for subsidiary pieces of apparatus to 
the Tait Memorial Fund; and we have to thank Mr C. T. R. Wilson, F.R.S., 
and Professor MacGregor, F.R.S., for kindly criticism. 
Physical Laboratory, 
University of Edinburgh. 
* Australian Assoc, for the Adv. of Science Address, sect. A, p. 40, 1909. 
( Issued separately July 28, 1910.) 
