280 Prof. J. 8S. Townsend on Charges on Ions. 









Value of ¢ in | Method of generating Observe | 
electrostatic units. the ions. seer 
TUE Ue Ue eee | Ions in oxygen pre- | J.S. Townsend, Proce. 
| pared by electro- Camb. Phil. Soe. | 
17 Aysis: vol. ix. pt. v., 1897. 
OO nce hee ad | Rontgen rays. J. J. Thomson, Phil. 
Mag. Dec. 1898. 
| SMO Wie 4, a Negative ions pro- | J. J. Thomson, Phil. 
| duced by ultra- Mag. Dec. 1899. 
violet light. 
oA NOS10i), Seen Radium. J.J. Thomson, Phil. 
Mag. March 1903. 
OL 100 ee eee | Rontgen rays. H. A. Wilson, Phil. | 

| | | "Mag. April 1903. 



With regard to the differences between his determinations, 
Prof. Thomson states in his recent paper: ‘‘The mean of 
these values gives 3°4X107!° as the charge in electrostatic 
units of the gaseous ion. This is about half the value 
6°35 x 10—!° I found in the earlier experiments. The difference 
is, as I have already explained, due to the expansions in the 
earlier experiments practically catching only the negative 
ions; this made the calculated value ot » little more than 
half the true value, while it made the value of e twice as great 
as it ought to have been.” The number n denotes the number 
of drops in the cloud formed by expanding the moist gas 
containing the ions. It appears from Mr. C. T. R. Wilson’s 
experiments, that condensation of moisture takes place round 
negative ions for a slightly smaller expansion than is required 
to produce condensation round positive ions*. This may be 
the cause of the discrepancy between the experiments with 
radium radiation which give the value 3:4 10-'° and those 
which were first made by Prof. Thomson with Réntgen rays. 
The explanation does not, however, explain the difference 
between the value 3:4 10-1 and the value 6°8 10—!° which 
was obtained for the charge on the ions produced by the 
action of ultra-violet light. In the latter case no positive 
ions were present in the gas, and the number 6°8 10-!° was 
obtained from considerations of the presence of negative 
ions alone. 
The mean of the values, omitting the value 6°5 x 10—"° for 
Réntgen rays, comes to 4:1 x 10~™. 
Of the numbers in the table, that found by Dr. H. A. 
Wilson is probably the most reliable, and more weight ought 
to be given to his determination. By the method which he 
used he avvids the necessity of finding the numker of drops 
in the cloud formed by the expansion of the conducting gas, 
* ©. T. R. Wilson, Phil. Trans. vol. exciii. p. 289. 
