I I t MR. J. S. TOWNSEND ON THE DIFFUSION OF IONS INTO GASES. 



The potential of A is then changed to 80 volts negative, and the same experiment 

 is repeated and a negative deflection, n\, is obtained. 



The tubes, T,, are then removed, and the short tubes, T 2 , are put in their place as 

 already described. Two similar experiments are then made, and larger deflections, 

 nj and n' 2 , are obtained when the rays are turned on the same stream of air for the 



same time. 



These four experiments are then repeated several times, and the mean value of the 

 observations is taken in order to eliminate errors arising from variations in the 

 strength of the rays. It was found that the constancy of the bulb was improved by 

 allowing a fixed time (3 minutes) to elapse between each experiment. When this 

 precaution was taken, it was possible to get rays which remained constant within 

 5 per cent, for the space of an hour. 



When working with the second apparatus the numbers n l and n\ are obtained by 

 sending the gas along the tube A], and finding the deflections when the rays fall on 

 the window, W,, for 20 seconds. In order to obtain n* and n' 2 it is only necessary to 

 move the apparatus along the top of the box till the window in A 2 comes over the 

 bulb, and to make similar observations with the electrode E 2 joined to the quadrants 

 of the electrometer and the stream of gas passing along A 2 . 



CORRECTION FOR' KECOMBINATION. 



Before the coefficients of diffusion can be calculated from the above observations it is 

 necessary to make a correction for the loss of conductivity due to recombination. 

 Let us denote by en the number of ions which, when collected on the electrode, 

 give a deflection of n divisions on the electrometer scale. The above experiments 

 show that there are c?i 2 positive ions which pass the section of the tubes, Tj, at a 

 distance of 1 centim. from the end near the window, W,. Of these en* ions c (n 3 n,) 

 are lost in the remaining 9 centims. of the tubes. The loss is principally due to the 

 ions coming into contact with the sides of the tubes ; but the loss is also to a small 

 extent due to collisions between positive and negative ions ; it is necessary to find 

 how much the observed value of HI must be increased in order to compensate for the 

 loss of ions arising from recombination. If cM is the number of positive ions which 

 encounter negative ions and do not come into contact with the sides, then the ratio 



- = y is the number which is required in order to calculate the coefficient of 

 "i 



diffusion from the curve given in Section I. Recombination also takes place in the 

 short tubes T 2 , but this effect is too small to take into account. 



In order to find M it is necessary to find the rate at which the gas loses conduc- 

 tivity due to recombination, and this can be easily done by making a change in the 

 arrangement of the apparatus. The tubes T, and T 2 were removed from A, and A 2 , 

 and the electrodes were placed in the positions shown in fig. 2, the wire grating in front 

 of the electrode E, being 12 centims. from the window in A,, and the grating in 



