Stages of Ionization by Collision. 975 



In the case of - =120, and to a smaller degree in-- =100 

 P G p 



for p = 3 mm., the positive ions are probably producing ions 

 by collisions to an appreciable extent. This would account 

 for the value of p, obtained from (1), being smaller than the 

 value found from (3), for expression (4) may be written 



p=l 



.2a 



so that 



, 2tt -l 1+p' 



and §p is positive if the positive ions generate ions by 

 collisions. 



When the apparatus was tested for small potential dif- 

 ferences between the electrodes it was found that in some 

 cases the electrodes 1 and 3 received negative ions. These 

 ions must have diffused to 1 and 3 before they had time to 

 acquire sufficient velocity in the direction of the electric 

 force. _ The following table shows some of the results obtained 

 tor various pressures. V represents the potential difference 

 between the electrodes in volts, 8 the deflexion of the electro- 

 meter when connected to the electrodes 1 and 3, and I the 

 length m cm. of the mean free path of an electron in the 

 gas. The ultra-violet light remained on for one minute in 

 each case, and the deflexion was about 300 when the elec- 

 trodes 1, 2, and 3 were all connected to the electrometer. 











Table V. 













P—2 mui. 1=-01Q cm. 



^ = •34 mm. l=-l 



1 

 lp='044mn 



./="8 



p= 017mm 



. 1=2 



V 



Z 



V 



10 



3 

 --4 



V 



13 



z 



p 



20 



8 



__ 

 -7 



V 



1 z 

 P 



8 



V 



10-5 



z 



V 



309 



8 

 -1 



40 



105 



120 



-s 



80 



20 



1 



21 



32 



-3 



17 



193 



-1-5 



17 



500 







120 



30 



2-5 



26-5 



40 



-1-5 



21 



238 



1-5 



19-5 



573 



•75 



160 



40 



9-5 



30-5 



45-5 







25 



283 



8 



21 



617 



2 









34-5 



52 



2 









25 



735 



7 









40 



60 



6 I 















It appears that as the pressure decreases a larger value of 

 - is necessary to prevent the negative ions from diffusing 

 to the electrodes 1 and 3. This result is in accordance with 



