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XXXVII. Velocity of Electrons in Gases. 

 To the Editors of the Philosophical Magazine, 

 Gentlemen, — 



IN a paper in the Jalirbuch der lladioactivitCd und Electronik 

 (vol. xviii. p. 201, April 1922) H. F. Mayer gives an 

 account of some of the formula? obtained by different 

 physicists for the velocity of ions or electrons in gases due to 

 an electric force, and concludes that a formula recently 

 given by Lenard is more correct than the others. 



Among the other formulae which are discussed, the author 

 gives what purports to be an account of a formula for the 

 velocity of an ion which I published in the ' Proceedings of 

 the Royal Society' (A. vol. lxxxvi. p. 197, 1912), and states 

 that this formula is so incorrect that it does not even give 

 the right order of the velocity. I should like to draw 

 attention to the way in which Mayer has misinterpreted the 

 matter, and to quote the formulae as I gave them for the 

 different cases in which the mass of the ion is small or large 

 compared with the mass of a molecule of the gas through 

 which it moves. 



On pages 199, 204, and 206 of my paper, three formula? 

 are given for the velocity U of an ion in the direction of the 

 electric force X in terms of the mean free path I of the ion, 

 its mass m, charge <?, and velocity of agitation u which is 

 supposed to be uniform and large compared with U. 



The first of these is 



JJ = Xel/mu, (1) 



and applies only to cases in which the mass of the ion is 

 small compared with that of a molecule of the gas (an electron 

 for example), since it is here assumed that after a collision 

 with a molecule all directions of motion of the ion are equally 

 probable. 



I pointed out that when the mass of the ion is larger than 

 that of a molecule of the gas, all directions of motion of the ion 

 after a collision are not equally probable, and that in this 

 case an ion travels a considerable distance (having an average 

 value X) after a collision in the direction in which it was 

 moving before a collision. A more general formula for the 

 velocity was given, which is 



XJ = Xe(l+X)/mu. . . ! . . (2) 

 If the mass m of the ion is so large compared with the 



