1 1 4 MESSRS. A. SMITIIELLS, H. M. DAWSON, AND II. A. WILSON : ELECTRICAL 



an equation capable of expressing the relationship in question in a remarkahly 

 complete way. In a paper published by Professor THOMSON and Mr. RUTHERFORD in 

 the 'Phil. Mag.' ((V.) vol. 42, p. 392 (1896)), an account is given of the passage of 

 electricity through gases exposed to Rontgen rays. According to the authors, the 

 current through a gas exposed to Rontgen radiation between two parallel plates 

 increases more and more slowly as the potential difference between the plates is 

 increased, until finally a maximum current is obtained which remains constant under 

 all further increase of potential difference. The electromotive force corresponding to 

 the attainment of this state is called a saturating E.M.F. Assuming that the 

 conductivity of Rontgenised gas is due to the presence of free ions, and that the 

 rate of disappearance of these ions by recombination to form neutral molecules is 

 proportional to the square of the concentration of the ions, THOMSON and RUTHERFORD 



iP 



give the following formula, I i = A , where I is the maximum current for a 



saturating E.M.F., i is the current strength for any electromotive force E, and A a 

 constant. 



Rontgenised gases and the gases of a flame exhibit a noteworthy similarity in their 

 behaviour. Thus both rapidly lose their power of conducting electricity when they 

 pass from the source where they have acquired this power. Both also lose their 

 conductivity when passed between a pair of electrodes maintained at different potentials. 

 (GiESE, ' Wied. Ann.,' vol. 17, p. 517, 1882.) That the conductivity in both cases may 

 be due to ions has been suggested by previous investigators. 



A glance at the curves in which THOMSON and RUTHERFORD plot the relation 

 between current strength and electromotive force, will show that there is a general 

 resemblance to the Curves I to V, by which our own results are plotted. The current 

 through the flame, however, continues to increase even when a large E.M.F. is applied, 

 whilst that through a Rontgenised gas reaches an almost constant value. * 



For potentials above one volt our curves are almost rectilinear, so that the relation- 

 ship between current and E.M.F. may, for E.M.F.s above 1 volt, be expressed by the 

 equation 



c = I + *,E . . . . ' (1) 



that is to say, the current strength may be represented as composed of two parts, one 

 being a constant quantity and the other a quantity proportional to the E.M.F. This 

 equation, however, does not give us the value of the current for small E.M.F.s. 



If we represent the relation between the current and E.M.F. at all E.M.F.s by the 

 equation 



c = i + &JE, 





then t is a variable which at high E.M.F. attains a constant value I exactly as the 

 current through a Rontgenised gas attains a constant value. 



From the analogy of the flame conductivity to that of Rontgenised gases, we 



