Electricity carried by a Gaseous Ion. 347 



gas, e the charge carried by an ion, u the mean velocity ot 

 the positive and negative ion in a given electric field, the 

 current through unit area o£ the ionized gas in this field will 

 be new, hence if we measure this current we can determine 

 the value of ne, as u the velocity of the ions in a field of 

 known strength has been determined by Zeleny and Ruther- 

 ford. The number of ions n was measured by depositing by 

 C. T. R. Wilson's method a cloud on the ions and deter- 

 mining the number of particles of water vapour in unit 

 volume of the cloud. 



Since these experiments were made, the progress of our 

 knowledge of the electrical properties of gases has made 

 several improvements in the method possible. The discovery 

 of radium has furnished us with a constant source of radiation 

 very much easier to work with than the variable Rontgen rays, 

 which were the source of ionization in the earlier experiments. 

 The introduction of the Dolezalek electrometer, which can 

 be made to give a deflexion of 20,000 or more scale-divisions 

 for the potential-difference of one volt, makes the determi- 

 nation of the exceedingly small currents passing through the 

 ionized gas much more accurate than was possible with an 

 electrometer of the type used in the earlier experiments, 

 which only gave a deflexion of 50 divisions for a volt ; and 

 although the Dolezalek electrometer has a much greater 

 capacity than the older form, the difference is not sufficient 

 to neutralize the advantage gained by the increased sensi- 

 tiveness. 



The consideration which influenced me most in repeating 

 the experiments was the increase in our knowledge of the 

 laws governing the deposition of the cloud round the charged 

 ions. The cloud is made by cooling the air by a sudden 

 expansion. I had noticed in the earlier experiments that 

 when this expansion was increased so that the ratio of the 

 volume of the air after expansion to that before exceeded the 

 value t"3, there was a noticeable increase in the number of 

 particles in the cloud. Mr. C. T. R. Wilson, who has made 

 (Phil. Trans, cxciii. p. 289) a systematic study of the relative 

 efficiency of negative and positive ions as nuclei for conden- 

 sation, found that while a cloud begins to be deposited round 

 negative ions when the expansion is 1*25, it is not until the 

 expansion is equal to 1*31 that the positive ions are caught 

 by the cloud ; thus if all the particles are caught by the 

 cloud, the number of water particles in the cloud formed with 

 the larger expansion when both positive and negative ions 

 are caught ought to be twice that with the smaller when only 

 the negative ions are caught. In my earlier experiments the 

 increase in the number of particles with the larger expansion-. 



