Electrical Conductivity of Atmospheric Air. 



701 



Fig. 1. 



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ebonite ping about 5 cms. in diameter, a brass tube B was 

 passed through this, and into it a second ebonite plug was 



tightly fitted. This second plug- 

 carried a brass rod C, which 

 extended almost to the bottom 

 of the cylinder. The brass 

 tube B, which was earthed 

 throughout the measurements, 

 served as a guard-ring, and 

 prevented any leak from the 

 vessel to the rod C across the 

 ebonite plugs. 



The conductivity was mea- 

 z^Msztt*^, Sllrec l by placing the cylinder 

 upon an insulated platform, 

 charging it by means of a set of 

 small storage -cells to a poten- 

 tial of 165 volts, which sufficed 

 for the saturation current, and 

 observing the rise in potential 

 of the electrode D, which was 

 joined to a quadrant electro- 

 meter in the usual manner. The sensibility of the electrometer 

 was such as to produce a deflexion of 1000 mms. on a scale 

 at one metre distance for a potential-difference of one volt 

 between the quadrants. 



3. Conductivity Measurements. — Time Effect. 



Before inclosing air for examination the cylinder was 

 placed in an open window in the laboratory, with the ends 

 removed and the air allowed to blow through it for some 

 time. The top and bottom were then replaced, cemented in 

 position, and the cylinder connected with the electrometer as 

 quickly as possible. 



Measurements on the conductivity were made at intervals 

 of a few minutes at first, and it was invariably found that a 

 rapid decrease in the ionization took place until a minimum 

 value was reached. The conductivity then slowly increased 

 and approached a limiting value in the course of two or 

 three days. 



In repeated tests carried out in this manner with the zinc 

 cylinder it was found that, while the initial conductivity 

 varied from day to day, there was always observed a rapid 

 decrease to a constant minimum, followed by a gradual rise 

 to a constant limiting value. 



A typical set of values for the conductivity of air confined 



