EXPERIMENTAL STUDY OF LARGE IONS IN THE AIR. 55 



A CONTRIBUTION to the EXPERIMENTAL STUDY 

 of the LARGE IONS in the AIR. 



By S. G. Lusby, m.a. 



(Communicated by Professor J. A. Pollock, d. so.) 



[Read before the Royal Society of N. S. Wales, June 2, 1909.'] 



1. Introduction.— In addition to the well known small 

 ions naturally present in the air, with a mobility of about 

 1*5 in practical units, M. Langevin 1 has discovered others 

 whose mobility is only of the order of 1/3000. M. Langevin 

 calls these the 'large ions' of the air, and finds that their 

 number under natural conditions considerably exceeds that 

 of the small ones. Few particulars of these slowly moving 

 ions in connection with tlie ordinary ionisation of the air 

 have as yet been published ; I give here some observations 

 of their mobility, of their absorption by hygroscopic sub- 

 stances, and of their rate of reproduction, the results being 

 the outcome of experiments carried out a little time ago 

 at the Physical Laboratory of the University of Sydney. 



2. The mobility of the ions.— The apparatus used in the 

 experiments, which will be referred to as the testing pipe, 

 is similar to that employed by Professor Zeleny 2 in his 

 determination of the mobility of the small ions; it consists 

 of a long brass pipe through which a steady stream of air 

 is passed, the pipe being provided with an axial electrode. 

 The electrode is divided into two sections insulated from 

 each other ; the portion at the mouth of the pipe, 3*6 cm. 

 long, is earthed ; the other part 156 cm. long is connected 

 to a pair of quadrants of a Dolezalek electrometer. The 



1 Langevin, C. B., t. 140, p. 232, 1905. Le Kadium, 4, p. 218, 1907. 



2 Zeleny, Phil. Trans., A. 195, p. 193, 1900. 



