Electric Wind in Hydrogen containing traces of Oxygen. 459 



There are two other ways in which these large values may 

 possibly arise : — 



(i.) The ions from the point may travel an appreciable 

 distance before growing large enough to produce much 

 wind. 



(ii.) The gas may be ionized for an appreciable distance 

 from the point — this being equivalent to a lengthening of 

 the point so far as wind production is concerned. 



It is possible that both these processes may be involved in 

 the explanation of z Q . There is at any rate definite evidence 

 available for (i.), since Franck has shown * that when 

 discharge occurs in air from the surface of a fine wire in a 

 strong field, it is extremely probable that the ions, whether 

 positive or negative, do not reach their full size while 

 travelling a distance comparable with 7 mm. when driven by 

 the strong fields he used. 



Now if z is to be due to (i.), we should expect its value in 

 air to be less than 7 mm. for the weaker fields which 

 accompany discharge at a sharp point ; and in fact we find it 

 to be about 3 mm. for both positive and negative "|\ This is 

 consistent with Franck' s result, and makes it likely that for 

 air z is what it is owing to the growth of the ions after 

 leaving the point. 



For negative discharge in hydrogen containing more than 

 1 per cent, of oxygen, and for positive discharge at all 

 percentages, the mean value of z is 3 to 4 mm. It is possible 

 that these numbers may be a little too low as the point (in 

 the case of the hydrogen experiments only) unfortunately 

 projected from its glass sheath rather further than it ought 

 (about 3 mm.), and there may have been a little discharge 

 from its sides. The error cannot, however, amount to much 

 more than a millimetre. Taking z as 4 or 5 mm., it is com- 

 parable with the value for air, and thus presumably explainable 

 in the same way. 



But for lower percentages of oxygen and negative dis- 

 charge the case is different. z is so many times larger that 

 it appears necessary to invoke some special process to account 

 for it. 



The emission of corpuscles from the point seems to us to 

 furnish such a process. It is known that oxygen tends to 

 prevent this emission, and we now find that z is only large 

 when the amount of oxygen is small. If the removal of the 



* J. Franck, Ann. der Phys. Vierte Folge, Bd. xxi. p. 984. 



t This determination was made with the present apparatus, in which 

 the substitution of a plate for the ring renders dependable measurements 

 of z possible for the first time. 



