RADIOACTIVITY VISUALISED 



481 



left behind along the course of any ionising particle is con- 

 verted into a visible line of cloud of which a photograph is 

 secured. In this way a record is obtained of the path of each 

 projectile by making visible the atomic wreckage it has caused 

 in its passage through the air or other gas. In many cases 

 the individual ions produced along the tracks are visible in the 

 photographs. 



In order that undistorted pictures showing the result of the 

 passage of the various rays may be obtained, it is essential 

 that the expansion should be effected without stirring up the 

 gas. This condition is secured by using a wide shallow cloud 

 chamber of which the floor can be made to drop suddenly and so 

 produce the desired increase of volume (fig. x). 1 





Fig. x. 



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 by repeated expansions, each 

 too small to cause condensation on the ions, any cloud formed 

 being always allowed to settle before making another ex- 

 pansion. 



The cloud chamber must be free from ions other than those 

 produced by the ionising 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 chamber, so 

 that they may be removed as fast as they are produced. 



1 The apparatus is described in the Proceedings of the Royal Society ', A., vol. 87 

 (1912), p. 277. 



