On the Ionisation of Atmospheric Air. 



153 



Several investigators from the time of Coulomb onwards have 

 believed that there is a loss of electricity from a charged body- 

 suspended in air in a closed vessel in addition to what can be 

 accounted for by leakage through the supports.* In recent years, how- 

 ever, the generally accepted view seems to have been that such leakage 

 through the air is to be attributed to the convection of the charge by 

 dust particles. 



The experiments were begun in July, 1900, and immediately led to 

 positive results. A summary of the principal conclusions then, arrived 

 at was given in a preliminary note " On the Leakage of Electricity 

 through Dust-free Air," read before the Cambridge Philosophical 

 Society on November 26. Almost simultaneously a paper by Geitel 

 appeared in the ' Physikalische Zeitschrift 'f on the same subject, in 

 which identical conclusions were arrived at in spite of great differences 

 in the methods employed. 



The following are the results included in the preliminary note, which 

 I read : — 



(1.) If a charged conductor be suspended in a vessel containing 

 dust-free air, there is a continual leakage of electricity from 

 the conductor through the air. 



(2.) The leakage takes place in the dark at the same rate as in 

 diffuse daylight. 



(3.) The rate of leak is the same for positive as for negative 

 charges. 



(4.) The quantity lost per second is the same when the initial 

 potential is 120 volts as when it is 210 volts. 



(5.) The rate of leak is approximately proportional to the pressure. 



(6.) The loss of charge per second is such as would result from the 

 production of about 20 ions of either sign in each c.c. per 

 second, in air at atmospheric pressure. 



Of these conclusions, the first four were also arrived at by Geitel. 



As Geitel has pointed out, Matteucci,! as early as 1850, had arrived 

 at the conclusion that the rate of loss of electricity is independent of 

 the potential. He had also noticed the decrease in the leakage as the 

 pressure is lowered. § 



The volume of air used in my experiments was small, less than 

 500 c.c. in every case, many of the measurements being made with a 



* Perhaps the most convincing evidence of this is furnished by the experiments 

 of Professor Boys, described in a paper on " Quartz as an Insulator " (' Phil. Mag.,' 

 vol. 28, p. 14, 1889). 



f 'Physikalische Zeitschrift,' 2 Jahrgang, No. 8, pp. 116—119 (published 

 November 24). 



X ' Annales de Chim. et de Phys.,' vol. 28, p. 385, 1850. 



§ This was also observed by "Warburg (' Annalen der Physik u. Chemie,' vol. 

 145, p. 578, 1872). 



