Dust and Smoke on the Ionization of Air. 669 



Effect of Smoke. 



(Divisions per min.). 

 Elect. A (low pot.). Elect. B (high pot.). 



Air without smoke . . . 140 4 



Air with smoke 10 to 30 30 to 60 



Thus the introduction of tobacco-smoke decreased the ioniza- 

 tion current in A and increased it in B. The radium was 

 then removed to a distance and placed behind a thick lead 

 screen. 



Smoke, without Radium. 



Elect. A (low pot.). Elect. B (high pot.). 

 Air without smoke ... 5*2 3'0 



Air with smoke 1*5 10*0 



The magnitude of the effect is, of course, dependent on the 

 amount or! smoke introduced, and for the present it is 

 sufficient to note that in the absence of radium it was found 

 that the smoke was electrified and contained large slightly 

 mobile ions of each kind. 



This result is in accordance with the observations of 

 Broglie, who has made with the ultra-microscope an inter- 

 esting study of the charges on smoke particles {he Radium, 

 vi. p. 203, July 1909). He first calculates the rate of fall 

 of small particles by Stokes' law, and also deduces the mobi- 

 lities in a field of a volt/cm. The figures he gives are : — 



Kadi us of particle. Time of falling 1 cm. Mobility cm. /sec. 

 10- 3 cm. 0-9 sec. 3-5xl0" 7 



10-4 cm. 90 sec. 3*5 xlO" 6 



10- 5 cm. 2-5 hours. 3'5 x 10~ 5 



10- 6 cm. 10 days. 3*5 x 10~ 4 



He points out that recombination must be extremely slow 

 among the large charged ions, and by studying the Brownian 

 movements he has formed an estimate of the ionic charge. 

 and of the radii of the smoke panicles. 



For example, smoke from a cigarette consists of particles 

 whose radii are 30 to 300 fifi, that is, 3 X 10 -6 to 3 x 10~ 5 cm. ; 

 the corresponding mobilities are 10~ 3 to 10~ 4 cm./sec. volt/cm., 

 and a very strong electric field is necessary to obtain a satu- 

 ration current. 



In order to examine the effects due to smoke with mv 

 apparatus, it was essential to obtain a steady source, and it 

 was found that a smouldering pipe or cigarette was too 

 variable. After some trials it was found that one or two 



