482 SCIENCE PROGRESS 



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-like 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 being connected to the floor. In this 

 way, a nearly uniform vertical electric field is maintained be- 

 tween the roof and floor of the chamber. The floor is virtually 

 a pool of water made solid by the addition of gelatine and 

 blackened 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. 



As regards the actual mechanism for causing the sudden drop 

 of the floor of the cloud chamber, it is sufficient to state that 

 the space below the plunger can be put in communication, 

 through wide tubes, with an exhausted chamber by suddenly 

 opening a valve. 



In order that the ionising particles should leave sharply 

 defined cloud trails, it is necessary that they should traverse 

 the moist gas immediately after this has been expanded while 

 the water vapour is still supersaturated to an extent consider- 

 ably 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 ionising 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 Rayleigh in photographing jets of water and by 

 Worthington in his study of the splash of a drop. 



