101:^1 on Paths of Particles Ejected from Atoms 671 



A> rcyards the uctual mechanism for causing the sudden drop of 

 the Hoor of the cloud chamber, it is sutticient to state that the space 

 below the plunger can be put in communication, through wide tul)es, 

 with an exhausted chamber liy suddenly opening a valve. 



In order that the ionizing particle should leave sharply defined 

 cloud trails, it is necessary that they should traverse the moist gas 

 immediately after this has l)een expanded while the water vapour is 

 still supersaturated to an extent consideral)ly exceeding the minimum 

 which is required to cause condensation on the positive ions (which 

 are more difficult to catch than the negative). Under these 

 conditions, the ions lose their mobility and grow into visible drops 

 before they have had time to diffuse appreciably away from the 

 original track of the ionizing particle. 



If the clouds formed by condensation on the ions are to be 

 photographed, it is necessary to expose them to an instantaneous 

 illumination of great intensity while the camera is in position. The 

 instantaneous illumination is obtained by a Leyden jar discharge, the 

 arrangement being essentially the same as that used by Lord Ray- 

 leigh in photographing jets of water and by Worthington in his study 

 of the splash of a drop. 



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 cur- 

 rents. 



Figs. 2 to 13 are pictures obtained by this method. It is 

 perhaps necessary to point out they are all photographs of clouds 

 consisting of minute water drops condensed upon ions, for many of 

 them have a very uncloudlike appearance. 



P^ig. 2 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 luO,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 super- 

 saturated 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 conden- 

 sation of water upon them. The electric field maintained in the 



