514 



Mr. H. A. Wilson on the Variation of the 



cit.). But where 



dX 

 dec 



= 0, n x = n 2 ; so that X = 





where X is the electric intensity, and is independent of i. 

 Regarding the ionization as due to the impacts of the negative 

 ions, we should have expected q to vary as n 2 , and hence as 

 c, not as c 2 , in which case X and i would not have been 

 independent. 



2. The Electric Conductivity. 



The discharge-tube already described was also used in the 

 measurements made of the variation of the electric conduc- 

 tivity along the discharge. Small platinum plates each 

 1*5 mm. broad and 5 mms. long were w r elded to the wires W 

 and W, and the tube supporting these was cut off, turned 

 through a right angle, and sealed on again so that the two 

 small plates were fixed in the centre of the discharge-tube 

 facing each other 1*5 mm. apart and with their long edges 

 horizontal. 



Fig-. 9. — Discharge in Oxygen. 

 Pressure T32 mm. Current 1*75 milliampere. 



0-5 



8 0-3 



-, 02 



+ Li 



Faraday Dark Space 



0-5 



0-4 



0* ^ 



8 9 10 II I? 



The current between them produced by one Clark cell 

 whilst the discharge was passing was measured. This current 

 was found approximately proportional to the number of cells 

 used up to ten cells in all parts of an ordinary discharge such 

 as that shown in fig. 5. The current was measured first in 



