﻿[ 677 ] 



LXIII. The Charges on Ions. 



To the Editors of the Philosophical Magazine. 

 Gentlemen, — 



IN a paper by Mr. J. C. Pomeroy, published in the 

 January number of the Philosophical Magazine, some 

 experiments are described from which the author draws a 

 number of conclusions as to the values of the charges on 

 ions produced by a hot wire in air and in hydrogen at 

 atmospheric pressure. 



The experiments are said to have been made on the 

 same principle as that which I used some time ago for 

 finding the product N . e for ions in different gases, N 

 being the number of molecules per cubic centimetre of a gas 

 at normal pressure and temperature, and e the charge on an 

 ion. There is one fundamental condition on which the 

 success of this method depends, which is that the experiments 

 must be conducted at low pressures, except in the rare cases 

 when such small densities of ionization are used that the 

 electrical measurements can only be made by a sensitive 

 electrometer when the charges on the electrodes have been 

 accumulating for some hours. 



Mr. Pomeroy has made his experiments at high pressures 

 with densities of ionization which obviously are not very 

 small, and the consequence is they furnish no evidence in 

 support of the conclusions he has adopted. 



It may perhaps prevent the recurrence of a similar error 

 if I point out why it is necessary to work at low pressures 

 in order to determine the value of N . e by experiments on 

 the lateral diffusion of a narrow stream of ions moving under 

 an electric force. The principle of the method consists in 

 producing ions on one side of a disk A, the disk being a thin 

 sheet of metal with a circular aperture in the centre through 

 which the ions can pass. On the other side of the disk the 

 ions travel through a field of uniform electric force X, and 

 are received by a circular disk B and a broad ring C sur- 

 rounding the disk, a narrow air-gap being left between them 

 for purposes of insulation. The disk B is directly opposite 

 the aperture in the plate A and is of the same area as the 

 aperture. All the ions are not received on the disk as the 

 stream opens out and part of the charge is received by 

 the ring C. There are two causes which contribute to the 

 opening out of the stream — the diffusion of the ions, and their 

 self-repulsion. 



The effect of diffusion is, in these cases, independent of 



