1913-14.] Atmospheric Electrical Potential Gradient. 207 
increased potential gradient must produce many more positive than 
negative ions, due to some characteristic of the mechanism of the com- 
bustions investigated. Such is the case, for example, with ionisation by 
certain incandescent particles at moderately high temperatures.* In the 
cases cited the effluent gases would have been subjected to a temperature 
of perhaps from 600 to over 1000° C.+ 
This work was done during the summers of 1912 and 1913 in connection 
with other smoke experiments being conducted at Leeds University. In 
studying the effects of smoke on plant growth it is very desirable to have 
some means of measuring the concentration of noxious smoke gases in the 
atmosphere, and we hoped that this object might be attained by measure- 
ments of the air potential gradient. It does not seem, however, as if these 
would give much guidance. 
Summary. 
The general effect of products of combustion would be to cause a 
transformation of the small ions of the air into large ions, which, acting 
alone, would tend to decrease the air conductivity. Ionisation by flames, 
however, adds to the number of ions in the air, so that the size of the ions 
might be increased without the conductivity of the air diminishing. In 
the case of the fresh smoke direct from the forge or colliery chimney- 
stalks or railway engines of our experiments, it is suggested that com- 
bustion in the furnaces would result in an ionisation producing more 
positive than negative ions. It is only where similar conditions obtain 
that we should expect such large increases in the positive potential 
gradient, due to smoke, as we have recorded. 
* H. A. Wilson, loc. cit. 
t Rusby, Journal of Franklin Inst., July 1913. 
(. Issued separately July 15, 1914.) 
