Section III., 1910. [asl Trans. R. S. C. 
X.—Electric Potential and Conductivity of the Air at Winter Harbour, 
Melville Island. Lat. N. 74° 47’, Long. W. 110° 48’. 
By W. E. W. Jackson, M.A. 
(Communicated by R. F. Stupart and read September 28, 1910.) 
The instruments used in these experiments were made available 
for the Canadian Government expedition to the Artic in 1908-9 through 
the courtesy of the Director of the Department of Terrestrial Magnetism 
of the Carnegie Institution at Washington, D.C. They included an 
Exner Electroscope No. 981 and Elster and Geitel’s field Collector for 
observations on atmospheric electricity and the Gerdien conductivity 
apparatus for conductivity experiments. 
Observations were carried on simultaneously for the measurement 
of the potential of the Atmosphere and the conductivity of the air 
whenever conditions would permit, and a considerable series of values 
were obtained. For the measurement of potential an Elster and Geitel 
flame collector was used, which was connected by insulated wire to 
an Exner aluminium leaf electroscope. Insulating supports of various 
heights were available, and the height of the collector was adjusted 
to keep the potential within the range of the electroscope. For the 
measurement of specific conductivity of the air, the instrument used 
was that devised by Dr. Gerdien, and consists of a cylindrical tube 
about 20 cms. in diameter, in the centre of the lower side of which 
is a side tube connecting with a aluminium leaf electroscope. The 
inner electrode consists of a thin brass tube with hemispherical ends 
about 1.4 cms. in diameter, placed in the axis of the large cylinder 
and connected with the electroscope. In the rear part of the tube 
is a fan driven by hand by means of a worm gear, which draws a cur- 
rent of air through the tube. A charge of electricity placed on the 
inner electrode is then dispersed by the ions of opposite sign, which 
are drawn through the tube, and the reduction in potential on the 
electrode, as indicated by the electroscope, is a measure of the dis- 
persion. 
Dr. Gerdien! has shown, both theoretically and practically, that 
great variations may be made in the rate of flow of air through the 
tube without affecting the value of the specific conductivity. During 
the experiments here noted, however, uniform rotation of the fans was 

1 Terrestrial Magnetism, Vol. X., No. 2. 
