328 The Mobility of Negative Ions at Low Pressures. 



t = Q to t = T— v reach the plate, so that the total number 



reaching the plate is 



T-d/k.X 



< T -i)+j ^- W -<»* 



If we regard T as constant, then Q the quantity reaching 

 the plate will he represented by (a) from X = ^//: 1 T to 

 X==d/& 2 T and for greater values of X by (/3). The graph 

 representing the relation between Q and X will differ from 

 that represented in fig. 2, in that it will start from zero 

 when X=d/kiT and will not reach a constant value when 

 X = ^//t 2 T, but will asymptotically approach the constant 

 value qT. 



In comparing the preceding theory with experiment it is 

 necessary to remember that we have supposed all the negative 

 particles to leave one plate as electrons ; we could not 

 directly apply it to such a case as that tried by Mr. Wellisch, 

 when the gas was ionized behind the plate by polonium and 

 the negative ions driven through holes in this plate by a 

 small electric field. With this arrangement some of the 

 electrons may unite with the molecules before they get through 

 the plate, and so start on their journey as ions and not as 

 electrons ; it is possible too that some methods of ionization 

 may produce negative ions initially as well as electrons. 

 It would be necessary to test the theory to produce the 

 electrons at the surface of the plate, either by incandescent 

 metal or by allowing ultra-violet light to fall on the plate. 



We have at present, except in the case of flames, no direct 

 determinations of h\ the mobility of an electron : these, how- 

 ever, would be of very great value and would enable us to 

 settle many points in connexion with the theory of the 

 mobility of ions which are at present uncertain. The 

 advantage possessed by the electron is that there is no 

 uncertainty as to its nature : it is the same whatever may be 

 the nature, temperature, or pressure of the gas. The ion, on 

 the other hand, whether positive or negative may vary with 

 each or all of these conditions. This uncertainty as to the 

 size of the ion makes the evidence as to the nature of the 

 action betwoen ions and molecules, afforded by such pheno- 

 mena as the variation of their mobility with temperature, 

 pressure, and the nature of the gas, ambiguous. 



