1909-10.] Observations of the Earth-air Electric Current, etc. 463 
guard-ring separated from it by a gap O' 7 5 cm. wide. The surface of this 
test-plate was within 5 cms. of the ground ; it was supported by insulating 
rods of sealing-wax, and could be connected to the test-plate of the electro- 
meter by a shielded wire. Alternate observations were made of the charge 
on the small test-plate and on the large, care being taken that the opera- 
tions were carried through quickly enough to prevent any gain of charge, 
due to atmospheric leak, from affecting the results. Observations for this 
purpose were taken only on days on which the earth’s electric field was 
found to be comparatively steady : the average value of the ratio of the 
charge on the large test-plate to that on the small was found to be 6’50, 
this being the mean of twenty-nine comparisons. As the effective area of 
the small test-plate is 404 sq. cms., and that of the large one is 966 sq. cms., 
the surface density of the electrification on the small test-plate to that on 
906 1 
the large was — — x -r = 3'7. Wilson, who devised and used this method, 
& 40-4 6-50 
got the value 4*2, the dimensions and height of his instrument not being 
the same as ours. 
Constancy of the Dissipation Factor for Different Surfaces 
and Different Heights. 
Currents as measured directly by the large test-plate did not give so 
consistent results as with the small test-plate ; but if the dissipation 
factor is constant throughout a height of 65 cms. (the height of the instru- 
ment above the ground), the currents can be at once deduced from those 
through the small test-plate. To test this, observations were taken alter- 
nately on the roof of the Physical Laboratory and on the grass in front of 
the Laboratory with the small test-plate, the difference of height being 
about 50 feet. The average dissipation factors for nine measurements of 
each did not vary by more than 10 per cent, of their mean ; and as the 
difference of height is about twenty times the height of test-plate from the 
ground, it seems legitimate to conclude that the variation of dissipation in 
a height of 65 cms. would, if it exists at all, not affect the results. 
Wilson has shown that the dissipation factor under given conditions is 
the same whether the surface be the metal of the test-plate or turf, or a 
cylindrical conductor of the same shape as the turf. 
We have made further observations on this point in town atmosphere, 
in which, as will be shown later, the mobility of the ions is much less than 
in country air. We obtained our results by means of an artificial field. 
The instrument was placed on a table under a sheet of tinned iron, which 
was insulated and kept charged to a definite constant potential of about 
