THE AIEASUREMENT OF THE NATURAL lONISATION 



OF THE AIR. 



By Edouard Jacot^ B.A. 



Various types of apparatus have been designed for observa- 

 tions on the natural ionisation of the air. Elster and Geitd, 

 who were the first to attempt such observations, used a very 

 simple form of Apparatus. A charged conductor, carefully 

 insulated, was exposed in the open air and connected to an 

 electroscope. The rate at which the conductor lost its charge 

 was taken as a measure of the state of ionisation of the air near 

 the conductor. The apparatus is still used, but the results have 

 little useful meaning. Meteorological conditions have an im- 

 portant influence on the measurements. The presence of dust, 

 or moisture, or wind of any kind, seriously affects the observa- 

 tions. Thus, the ionisation measured in fog or a dusty atmos- 

 phere is unnaturally small, as there is necessarily an unnaturally 

 rapid rate of recombination between ions when particles suitable 

 for nucleation are distributed in the atmosphere. 



Other methods of measuring the ionisation have since been 

 devised. The Gerdien and the Ebert forms of apparatus are 

 the best known. The principle of both is essentially the same. 



The Gerdien consists of a central horizontal metal cylinder, 

 some 25 cms. long and i . 5 cms. in diameter, mounted within a 

 large cylinder 56 cms. long and 16 cms. in diameter. The inner 

 cylinder is connected to an electroscope and insulated from the 

 outer, which is earthed. A current of air is drawn through 

 the apparatus by means of a fan ; and, the inner cylinder having 

 been charged, the rate at which the electroscope leaves collapse 

 is noted. The conductivity of the air can then be calculated. 



Gerdien's expression for the conductivity is as follows : — 





12 /o-e ''ol 



To and /'i are the radii of the outer and inner cylinders respect- 

 ively ; / the length of the inner; C is the capacity of the inner 

 cylinder and the electroscope ; n is the number of ions per c.c. 

 of air, positive or negative, according to the charge of the inner 

 cylinder ; and v is the mobility of these ions ; c is the electronic 

 charge. If the potential of the inner electrode falls from l\ to 

 V. in a time r, the expression gives the (juantity ncv. the con- 

 ductivity of the air. If the inner electrode is initially charged 

 positively, the quantities n, e, v would refer to negative ions ; 

 and vice versa. 



