60 McCiettanp—Jonization in Atmospheric Arr. 
3.—Nature of the Conductivity in Atmospheric Arr. 
It is scarcely necessary to repeat observations showing that the conductivity 
in atmospheric air is of the nature resulting from ionization of a gas. The 
relation between current and E.M.F. is such as is produced by ionization. One 
curve is given (fig. 2), showing the variation of the current with the potential 
difference of the plates between which the current passes through the air. 
In this connexion we may describe another experiment. The glass case PQ 
(fig. 1) is removed, and replaced by a case of perforated zine (fig. 3); a second 
case LS, similar, but slightly larger, also of perforated zine, is placed enclosing the 
first, and resting on paraffin supports. The whole apparatus is placed where a gentle 
draught of air is blowing, so that a stream of air passes through the perforated 
covers, and reaches the plates Z and M. The plate WM is charged positively, 
and both covers connected to earth; the current observed is 5 (in arbitrary 
units). ‘The inner cover PQ is kept to earth, and RS joined to the storage cells 
so as to be 160 volts positive; this produces an electric field, assisting the positive 
carriers to reach the inside of the cover PQ, and the current is found to be 
increased to 7. The inner cover is kept to earth, and the outer kept at 160 volts 
negative. ‘This prevents the positive carriers from traversing the space between 
the covers with the stream of air entering the inner cover, and the current between 
M and L is now found to be only 0.5 on the same scale as before. Similar 
numbers are obtained when the plate J is charged negatively, only now the 
small current occurs when the outside cover is positive, and the large current 
when the outside cover is negative. Such an experiment is easily understood 
when we consider that the observed conductivity is due to positively and 
negatively charged particles, but would otherwise be difficult to explain. 
