Dust and Smoke on the Ionization of Air. 663 



This shows that diffusion is an important factor in the 

 value of the ratio. This has long been recognized. 



With the longer tube (2 metres) a loosely wound ball of 

 fine wire was inserted midway along the tube. 



Ratio. 



Humidity -}- to centre — to centre 

 (relative). D/ra. D/m. 



No wire ball... 50 90*5 66 1\37 



Wire ball 50 56 36 1*56 



Wire ball 90 57 45 1*27 



The wire ball increased the loss due to diffusion, especially 

 that of the negative ions. An increase of the humidity of 

 the room, however, while affecting the positive ions little, 

 reduced the velocities of the negative ions, and therelore 

 their loss by diffusion was lessened, and the ratio n + jn — 

 was decreased. Tubes of zinc, aluminium, and brass were 

 tried, but the material made no difference in the ionization 

 current. 



Variations due to Atmospheric Conditions. 



Observations were next made on different days to find the 

 influence of various atmospheric conditions on the ionization 

 current at B. The room was well ventilated with open 

 windows, the velocity of the air current was maintained con- 

 stant at each reading, and the radium was left in situ. There 

 was no variant except the type of air examined. The voltage 

 on B's central system was about six hundred volts, and the 

 results obtained for the ionization current expressed in 

 divisions a minute were : — 



-f ions to centre, —ions to centre. Ratio «4 /n— Weather. 



17-3 17-3 1-00 Very clear. 



16-0 15-0 1-07 Clear. 



13*5 12*8 1*05 Smoke and mist. 



12-3 11-3 1*09 „ 



With high voltages the ratio u + jn — is generally nearer 

 to unity than with low voltage, due to increased approxi- 

 mation towards saturation. The above results are remarkable, 

 because with an electric force of the order 1000 volts/cm. in 

 the testing vessel B, not only was the ratio w+/n— not 

 unity, but the number of ions caught when the air was clear 

 was about 50 per cent, greater than when the air was smoky 

 or misty. 



