G12 Prof. J. S. Townsond mi tlip Genesis of Tons 



8. A comparison between the curves shows that it requires 

 much hirger forces to dcA^elop new ions by the motion of 

 positive ions in air than in hydrogen. I£ the positive ion be 

 regarded as being approximately the same size as the molecule 

 of the gas in which it is generated, the free paths of a posi- 

 tive ion in hydrogen would be 1*85 times as long as the free 

 paths of a positive ion in air at the same pressure, this number 

 being the ratio of the mean free paths of molecules in 

 hydrogen and air (Meyer, ' Kinetic Theory of Gases '). It 

 is interesting to compare the number of ions that would be 

 produced per centimetre by a positive ion in hydrogen at 

 1-85 millimetre pressure and the number that would be pro- 

 duced per centimetre in air at 1 millimetre pressure w^hen the 

 same electric force is acting. The lengths of the free paths 

 and the number of collisions per centimetre are the same 

 in the two gases ; also the falls of potential along the free 

 paths would be the same if the same electric force is 

 applied. As an example the force of 370 volts per 

 centimetre may be taken. For air at a millimetre pressure 



13 =-01S when X = 370 and 2^ = 1. For hydrogen ^ = -09 



when— =200; so that when X = 370 and ^ = 1*85, yS is '166. 



The ratio of the values of jS gives the ratio of the numbers of 

 ions produced per centimetre in the two gases ; so that for 

 this particular force there are about nine times the number 

 of ions produced in hydrogen as in air for the same number 

 of collisions and the same falls of potential between the 

 collisions. This result justifies the assumption that the positive 

 ion in hydrogen is of smaller mass than the positive ion in air, 

 so that the former acquires a greater velocity in travelling a 

 given distance under a given potential- gradient. The result 

 could also be explained on the hypothesis that the positive 

 ions are of the same mass in air and in hydrogen, and that 

 the velocity required to generate ions from molecules of air 

 is greater than that required to produce a similar effect on 

 molecules of hydrogen. This explanation is not in accordance 

 with what we should expect from considerations of the action 

 of the negative ions in the two gases. 



It can be seen from the previous researches * that the 

 molecules of air are ionized by negative ions with somewhat 

 smaller velocities of impact than the velocities required to 

 ionize molecules of hydrogen to the same extent. 



9. The fall of potential necessary to give sufficient velocity 

 to a positive ion in order to produce new ions by collision is 

 * J. S. Townsend, Phil, Ma^. April 1903, p. 397, 



