530 Prof. J. J. Thomson on the Charge of Electricity 



usual formula connecting the size of the rings with the size 

 of the drops. 



The method finally employed to measure the size of the 

 drops was to observe the rate at which the cloud sank and 

 then to determine the radius of the drops from the formula 



2ga 2 



where v is the velocity with which the drops fall, a the radius 

 of the drop, /x the coefficient of viscosity of the gas through 

 which the drops fall, and g the value of gravity. 



The velocity was determined by observing the time the top 

 layer of the cloud, which was illuminated by an arc light, took 

 to fall a given distance; observations made on the times taken 

 to fall different distances showed that the rate of fall was 

 uniform, so that the drops had reached their limiting velocity. 



I began by making experiments to test whether the drops 

 in the cloud formed by expansion were deposited round the 

 ions which gave to the gas its electrical conductivity ; this 

 point is fundamental, as the method used in this paper to 

 determine the charge carried by an ion depends on the 

 assumption that it is the ionization of the gas which causes 

 the fog produced by expansion, and that each ion can act as 

 the nucleus for a water drop. 



In the first place we have direct evidence of the power of 

 an electrified particle to act as a nucleus for a drop of water, 

 inasmuch as condensation takes place in a steam-jet when 

 placed near an electrode from which electricity is escaping, 

 and, further, Mr. Wilson has shown that a cloud is produced 

 by expansion in dust-free air when an electrode discharging 

 electricity is placed in the air. A more direct proof of the 

 point under consideration is afforded by the following experi- 

 ment:— If the ions produced by the Eontgen rays act as 

 nuclei for the drops, then, since these ions can be withdrawn 

 from the gas by applying to it a strong electric field, it follows 

 that a cloud ought not to be formed when the air which is 

 expanded is exposed to a strong electric field while the ravs 

 are passing through it. This was found to be the case, and 

 the experiment is a striking one. Tw r o parallel plates were 

 placed in the vessel containing the dust-free air; these plates 

 were about 5 centim. apart, and were large enough to include 

 the greater part of the air between them. The plates could 

 be connected with the terminals of a battery of small storage- 

 cells giving a potential-difference of about 400 volts. Eontgen 

 rays passed through the gas between the plates : this gas had 

 previously been freed from dust. When the plates were dis- 



