180 Dr. H. A. Wilson on the Electric Intensity 



added in such small quantities as not to alter the mean free 

 path. This may possibly account for the effect produced on 

 the positive point-discharge in air by a small admixture of 

 CO2 and other gases ; but it does not explain the different 

 effect obtained when the point is negative. 



It may also be noted that under the influence of the not 

 uniform field a gas of higher specific inductive capacity, as 

 for instance CO2, will tend to concentrate round the point 

 and to displace the air, so that the discharge may really pass 

 through a mixture containing a larger proportion of CO2 

 than observed in the experiments. 



I am at present investigating the oscillatory discharge 

 from a point, as observed by Himstedt, in gaseous mixtures, 

 w^hich may throw some more light on the subject. 



Through the kindness of Professor J. J. Thomson I was 

 enabled to carry on these experiments in the Cavendish 

 Laboratory, Cambridge, and I take this opportunity of ex- 

 pressing my sincerest gratitude for the kind interest he has 

 shown in my work. 



XXI. The Electric Intensity in the Uniform Positive Column 

 in Air, By Harold A. Wilson, B.A., B.Sc, Fellow of 

 Trinity College, Cambridge'^. 



IX the following paper some measurements of the electric 

 intensity in uniform positive columns in air at low^ 

 pressures are described, the object of w^hich w-as to determine 

 the variation of the electric intensity with the current density 

 when the latter is very small. 



In a previous paper + by the writer it was shown that the 

 electric intensity in a uniform positive column in air falls off 

 slowly wdth increasing current. When the current was less 

 than 10"* ampere, however, the intensity appeared to be 

 nearly independent of the current. The value of this maxi- 

 mum intensity was found to be given by the equation 

 X = 35\/p where X is the electric intensity in volts per cm. 

 and p the pressure in millimetres of mercury. 



When the current through an ordinary vacuum-tube con- 

 taining air at about one mm. pressure is diminished much 

 below 10"* ampere, the discharge becomes intermittent, and 

 on further diminishing the current goes out altogether. 



In order to make measurements with very small currents 

 it was therefore necessary to use a special device to prevent 

 the discharge going out. 



* Oommimicated by the Author. 



t " On the Hall Eft'ect in Gases at Low Pressures," Proc. Camb. Phil. 

 Soc. vol. xi. pt. iv. 



