36 THE THEORY OF IONIZATION OF GASES 



It thus appears that with a given potential v the best 

 conductivity is obtained with those gases for which V is 

 small, as with helium, hydrochloric acid, and argon. 

 The second of these gases would give results in accord- 

 ance with the formulae over large ranges of potentials, 

 since positive ions do not produce appreciable effects in 

 this gas until fairly large potentials are attained. 



16. Comparison of the velocities of ions and molecules. 



It is interesting to see how the velocity acquired by 

 an ion moving freely under an 'electric force compares 

 with the velocity of agitation when its motion is in 

 equilibrium with the surrounding molecules of a gas 

 at normal temperature, 15 C. This point has been 

 examined by Professor Schuster in one of his Bakerian J 

 lectures, and the following is essentially the method 

 adopted. 



If v is the velocity acquired by an ion of mass M in 

 travelling freely along a path over which the potential 



0V 



difference is V, then %M.v 2 =:, the charge e being 



expressed in electrostatic units, and V in volts. The 

 velocity of agitation u is given by the equation 

 JMm^ 2 =10 6 where ra is the number of molecules in a 

 gas at normal pressure and temperature. Hence 



me being 1'23X10 10 . 



Hence in order to generate ions by collision it is 



necessary that the velocity of the negative ion should be 



1 Proceedings of the Royal Society, Vol. xlvii., 1890. 



