1908-9.] A Simple Radioscope and a Radiometer. 
471 
XXX. — On a Simple Radioscope and a Radiometer for showing 
and measuring Radioactivity. By Dr John Aitken, F.R.S. 
(MS. received May 1, 1909. Read May 17, 1909.) 
While working at another subject I evolved a very simple piece of 
apparatus which on trial was found to be very sensitive to the penetrating 
rays of radioactive substances, so I have recently diverted my work from 
its original direction to study and improve this piece of apparatus. The 
principle of the action of the instrument is not new, as it depends on the 
fact discovered by Professor C. T. R. Wilson,* that ions in air super- 
saturated with water vapour become centres of condensation when the 
supersaturation is sufficiently great. 
Before going further it may be as well that a few remarks be made 
on the condensation of water in supersaturated air. If ordinary air 
saturated with water vapour be cooled by expansion, the vapour, it is 
well known, condenses on the dust particles in the air and a fog is formed. 
This fog is the denser the more numerous the dust particles, and if the 
particles be few only a rainlike condensation results on expansion ; and if 
no dust particles be present, then no condensation takes place, unless the 
expansion be great. In my early experiments on this subject it was shown j* 
that only a very slight expansion was necessary to make all the dust particles 
active as centres of condensation, an expansion of being sufficient to 
cause condensation to take place on even the smallest of them. It was 
further shown that higher expansions than might be made without 
any condensation taking place, but if the expansion was great, and caused 
to take place very rapidly, then condensation took place in dustless air ; 
but the subject was not further investigated. Here the matter rested till 
1897, when C. T. R. Wilson took up the investigation and by means of most 
ingenious apparatus, in which the air was very rapidly expanded, he 
showed that there were always nuclei in moist air that became active 
centres of condensation when the supersaturation was great enough. If 
v 1 be the initial volume of the air and V 2 the final volume after expansion, 
he showed that condensation began in dustless air when vjv 1 = 1*250. 
When the expansion is that amount the condensation is slight ; that is, only 
a few cloud particles are seen falling as a fine rain, and that as the 
* Trans. Roy. Soc ., Series A, 1897. 
t Trans. Roy. Soc. Edin., vol. xxxv. part i., 1888. 
