RADIOACTIVITY VISUALISED 483 



I have, however, allowed the spark to traverse mercury vapour 

 at atmospheric pressure instead of air, the brightness being 

 thereby greatly increased. 



The spark, of course, has to be suitably timed, so that the 

 cloud trails may be illuminated after the drops composing 

 them have grown sufficiently to scatter plenty of light but 

 before there has been any appreciable disturbance of the air by 

 convection currents. 



Figs. 1 — 12 are pictures obtained by this method. It is 

 perhaps necessary to point out that they are all photographs 

 of clouds consisting of minute water drops condensed upon 

 ions, as many of the clouds have a very uncloudlike 

 appearance. 



Fig. 1 is a photograph of the tracks of some alpha-particles 

 shot out from a minute quantity of radium placed within the 

 cloud chamber, the camera looking down through the plate- 

 glass roof. From the atoms of radium, alpha-particles are 

 continually being projected with velocities of many thousands 

 of miles per second, each producing more than 100,000 ions in 

 the course of its flight. Under ordinary conditions the trail of 

 ions left behind by each particle is invisible ; those formed 

 by particles which have traversed the supersaturated air of the 

 cloud chamber immediately after its expansion, however, are at 

 once converted into visible cloud trails. These form the sharply 

 defined spokes or rays of the picture. The more diffuse cloud 

 rays are the tracks of particles which have traversed the air 

 before its expansion, the ions having thus had time to wander 

 out of the original track before losing their mobility through 

 the condensation of water upon them. The electric field 

 maintained in the cloud chamber fixes a limit to the age and 

 hence to the diffuseness of the trails which are rendered visible ; 

 under the actual conditions any free ions would be driven by 

 the electric force to the roof or floor within less than a fifth of a 

 second after being set free. None of the ions made visible has 

 had a free existence exceeding this limit. 



It is clear that an ionising particle, while traversing or even 

 passing near to an older trail of ions on which a cloud has 

 already formed, will not find the vapour supersaturated to the 

 extent necessary to cause condensation on the ions ; it will 

 therefore fail to leave a visible trail in this region. This is 

 doubtless the reason why the sharply defined trails only appear 



