670 



Mr. C. T. R. Wilson 



[March 7, 



It is hardly necessary to say that the cloud chamber must be freed 

 from dust particles and all nuclei on which water readily condenses. 

 This is easily done l:)y repeated expansions, each too small to cause 

 condensation on the fons, any cloud formed Ijeing always allowed to 

 settle before making another expansion. 



The cloud chamber must be free from ions other than those 

 produced by the ionizing agent under investigation. Since ions are 

 always being produced even under normal conditions within a closed 

 vessel, it is necessary to maintain an electric field between the top 

 and bottom of the cloud chamlier, so that they may be removed as 

 fast as they are produced. 



Fig. 1. 



One very practical point in connexion with the cloud chamber 

 remains to be mentioned. It is necessary that the interior should be 

 maintained in a nearly saturated condition and yet that the roof and 

 walls should be transparent and admit of a clear and undistorted 

 view of the contents. A glass vessel containing moist air soon 

 becomes coated internally with a dew-hke deposit of minute drops. 

 This difficulty is completely avoided by covering the inner surface of 

 the glass with a film of gelatine. 



The moist gelatine under the plate-glass roof of the cloud 

 chamber forms a conducting film which is connected through a 

 marginal ring of tinfoil with one terminal of a battery of cells, the 

 other terminal l)eing connected to the floor. In tliis way, a nearly 

 uniform vertical electric field is maintained between the roof and 

 floor of the chamber. The floor is virtually a pool of water made 

 solid by the addition of gelatine and l^lackened by means of ink so 

 that it forms a dark background for the clouds. It is supported by 

 a glass plate which forms the top of a hollow cylindrical plunger 

 working; in water. 



