ELECTRICAL CONDUCTIVITY OF AIR AND SALT VAPOURS. 
431 
attraction between the two charges, then it is easy to calculate tlie distance between 
them in the molecule before it is ionized. We liave, if e is the charge on one ion and 
r the distance required, 
2-6 e 
300 “ r ’ 
and according to J. J. Thomson^'" e = 6 x I0“^° electrostatic unit, hence 
r = 7 X 10~® centim. 
This is of the same order of magnitude as the radius of molecular action iii liquids 
at the ordinary temperature. 
Professor TowNSENof has shown that the potential difference through which the 
ionic charge must fall to have enough energy to ionize a molecule of air at the 
ordinary temperature is less than 5 volts, and probably greater than I volt. My 
result, 2-6 volts for air at temperatures above 1000° C., is therefore nearly the same 
as Townsend’s, in spite of the great difference between the temperatures at which his 
and my experiments were carried out. 
4. Variation of the Current through Salt Vapours ivith the Temperature. 
I 21 the earlier measurements made on the conductivity of salt vapours the con¬ 
ductivity of the air alone without salt at temperatures above 900° was not small 
Diagram No. 11. 
* ‘ Phil. jNIag.,’ December, 1898. 
t “ The Conductivity produced in Gase.s hy the Motion of Negatively Charged Ions,” ‘ Phil. Mag.,’ 
February, 1901. 
