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CVI. The Ionizing Potential of Positive Ions. 

 To the Editors of the Philosophical Magazine. 



Gentlemen, — 



N a paper on impact ionization by low-speed positive 

 H ions in hydrogen, published in the November number 

 of the Philosophical Magazine, p. 806, Mr. A. J. Saxton 

 gives a general account of the results that have been 

 obtained on the subject of ionization by positive ions. 



As an example of some of the experiments which I made, 

 Mr. Saxton refers to one where the ionization is produced 

 by electrons and positive ions moving in hydrogen at 

 8 millimetres pressure under a force of 700 volts per centi- 

 metre (' Electricity in Gases,' p. 317), and states that " the 

 M.F.P. of a positive ion would be very small at tins high 

 pressure, so that it could not obtain an unimpeded fall 

 through more than a fraction of a volt," and makes some 

 suooestions as to how the ions were generated in the^e 

 experiments. Mr. Saxton's conclusions appear to be based 

 on the supposition that all free paths are nearly equal to the 

 mean free patli and that the number of molecules ionized 

 by single collisions with positive ions is too small to be 

 taken into consideration. 



The experiment in question was published in the Philo- 

 sophical Magazine, vi. p. 607, Nov. 1903, wbere I gave a 

 method of estimating approximately the ionizing potential 

 of positive ions from the determinations of the number of 

 molecules ionized by a positive ion in moving through a 

 centimetre of the gas in the direction of the electric force. 

 The ionizing potential of positive ions in hydrogen was found 

 to be 20 or 30 volts. The voltage depends on the length of 

 the mean free path, and it was also stated in the original- 

 paper that a somewhat lower voltage would be obtained if 

 the mean free path of a positive ion was the same as that 

 given by Meyer for a molecule of the gas. For hydrogen 

 at normal pressure and temperature the mean free path 

 given by Meyer is 17*8 xl() -6 cm. ('Kinetic Theory of 

 Gases/ p. 192). 



The free paths were not known very accurately at that 

 time, but as a result of recent investigations a more exact 

 formula connecting the viscosity of a gas and the mean free 

 path of a molecule has been obtained. It is therefore of 

 interest to calculate the ionizing potential of positive ions in 



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