56 S. G. LUSBY, 



length of the brass tube and of the inner rod is 160 cm., 

 the inside diameter of the pipe 3*44 cm., and the diameter 

 of the rod 0*69 cm. When the positive terminal of a battery- 

 is attached to the tube, the other end being connected to 

 the earthed quadrants of the electrometer, the positive 

 ions in the pipe travel towards the rod, the negatives to 

 the tube, and vice versa if the tube is negatively electrified. 

 With an instrument of the dimensions given, all the large 

 ions of one sign entering the apparatus can be forced to 

 the inner electrode with convenient values of the voltage 

 and of the air stream. 



An experiment to determine the mobility consists in 

 taking a series of observations, with an appropriate air 

 stream, of the ionisation current between the tube and the 

 inner rod, for increasing values of the potential of the tube, 

 until the current becomes constant. Prom such a series 

 of measures the minimum potential for which the current 

 has its constant value may be found, and the mobility 

 calculated by the method developed by Professor Zeleny 

 (loc. cit.). Owing to the variability of the number of the 

 ions in the air under natural conditions, only a few such 

 series of observations are accordant enough for the purpose 

 of determining the mobility, and the investigation is con- 

 sequently tedious. 



The air used was from the compressed supply of the 

 laboratory which is fed by a Sturtevant blower ; before 

 reaching the testing pipe it passed through several metres 

 of 1 inch iron piping, the source of the supply being the 

 air of the laboratory workshop. The humidities were 

 determined from the readings of the wet and dry bulb 

 thermometers. 



The results which have been obtained are given in Table 

 I., the mobilities (k) being velocities in centimetres per 

 second due to an electric force represented by a potential 



