Dust and Smoke on the Ionization of Air. 6^1 



positive ions, makes the measurement of their relative mag- 

 nitudes a difficult and intricate matter. 



In order to investigate this point further, apparatus of the 

 following type was used (fig. 2). 



Fig. 2. 



Fourteen tng. of pure radium bro-mide in two test-tubes 

 and a cylinder of lead about, 2 mm. thick, were placed about 

 10 cm. beneath the tinned iron cylinder V (17 cm. long, 

 17 cm. diameter), from which the ionized air passed along 

 the brass tubes (3 cm. diameter) and round an insulated 

 brass central cylinder T" (0*7 cm. diameter, 12*3 cm long), 

 which could be charged to any desired potential so as to 

 remove some or all of the small ions in the stream of air. 

 Three testing vessels, A, B, C (0, which was just below B, is 

 not shown in the figure), with their corresponding electro- 

 scopes, could be used either separately or together. 



A had an axial rod, 8 cm. long, 0*2 cm. in diameter. 

 B had a comparatively large cylinder, 8 cm. long, 2*2 cm. in 

 diameter, whilst the outer tube was 31 cm. The third 

 testing vessel, C, and electroscope were those of the El-ert 

 apparatus. The hand-wound spring tnrbine of the instrument 

 was used to cause the current of air, about a litre per second, 

 moving with a velocity of about 120 cm. per second along 

 the tubes, so that the air passed from the ionizing chamber V 

 to the testing vessels in about one second. 



It was found that the ionised air on arrival at A caused 

 the electroscope leaf to move at the rate of 130 divisions a 

 minute when T was earthed, but a moderate voltage on T, 

 about 70 volts, reduced this to about 1 div./min. A larger 

 voltage, say 400 volts on T, produced little further decrease, 

 so that the ions which move to the axial rod of A were mainly 

 small ions (see curve 2, tig. 3, p. 602). 



In the same way, if the central rod of A was kept at 

 230 volts, B caught but one per cent, of the ions detained 

 when A was earthed. 



