230 Dr. L. B. Loeb on the Relative Affinity 



It has been shown * that the current i between two con- 

 denser plates as a function of the alternating potential 

 difference V between them when one of them is illuminated 

 by ultra-violet light is represented by the equation 



1 ==e nK'K 1 (. V : ~ ' N ) /n 



with a fair degree of approximation. In this equation p is 

 the pressure of the gas in mm., V is the alternating potential 

 difference of square wave-form in volts, d is the plate dis- 

 tance, N is the frequency of alternation, K is the mobility of 

 the negative ions at 760 mm., K' is the electronic mobility, 

 V the mean free path of the electron at 760 mm., W the 

 energy of thermal agitation of the electron, i/i the fraction 

 of the maximum current possible existing under the given 

 conditions, and n the constant of attachment. It was also 

 shown that if the mobility of the electron K' be assumed 

 equal to 200 t cm. /sec. volt/cm., and if it be assumed that 

 the mean free path of the electron is 4.^/2 of that of the gas 

 molecules, then by inserting the value of W, the velocity of 

 thermal agitation of the electron, into the equation, it became 

 capable of experimental verification in the form 



l -osxio o \ d "(nu) _ K \f6o) I 

 % -r=e^~~ l v " * 3 ; . . . (2) 



? 



n may be evaluated by choosing a point neat the feet of one 

 of the current-voltage curves and placing the values of the 

 various quantities into the equation above. The n so obtained 

 was found to be constant in order of magnitude for a con- 

 siderable range of pressures and frequencies for any given 

 gas ; in fact, n for oxygen was found to lie between 2 x 10 4 

 and 3 x 10 5 in gases which varied in oxygen content from 

 that of pure oxygen to a mixture of 745 parts of nitrogen to 

 one part of oxygen. The range of pressures covered in this 

 set of determinations varied from 15 mm. to 750 mm., while 

 N varied from 70 alternations per second to 750 alternations. 

 This variation in n between 2 X 10 4 and 3 X 10 5 seems to 

 be a definite variation in n with some of the experimental 



t This was the most reliable value of electronic mobility in nitrogen 

 available at the time of publication of the original paper. 



