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THE ROYAL SOCIETY OF CANADA 



ionisation tracks of alpha, beta and X-ray tracks in air. The apparatus 

 which was made by the Cambridge Scientific Instrument Co. under 

 his direction is shown in Fig. 1., and in diagram in Fig. 2. For 



— • TO PUMP 



Fig. 2. 



complete details of the construction and operation of this apparatus 

 the reader is referred to C. T. R. Wilson's original paper^ or to the 

 descriptive catalogue of the Cambridge Scientific Instrument Co. 

 It will suffice here to say that the cylindrical cloud chamber A. Fig, 

 2 was about 16-5 cms in diameter and 3-4 cms in height. The walls 

 and roof of this cloud chamber A were coated inside with gelatine 

 containing a trace of copper sulphate and the plate glass roof, B, of 

 the piston or plunger, C, was coated with gelatine blackened with 

 ink and containing 2% of copper sulphate. The expansion was effected 

 by opening the valve E and so connecting up the air space below 

 the plunger with the evacuated chamber F. In all the experiments 

 the apparatus was arranged so that the ratio of the final to the initial 

 volume V2/V1 of the gas in the expansion chamber above the plunger 

 was about 1 -36 : 1. 



As the plunger was always at the bottom of the chamber when the 

 expansion was completed, the value of the ratio V2 /Vl depended upon 

 the height to which the plunger was raised initially. It was raised 

 therefore to any height desired by simply removing the stopper K from 

 the expansion chamber and blowing into the tube H. The reading 

 taken on the gauge depicted in Fig. 1 after expansion showed when the 

 proper ratio for V2/V1 was secured. 



^ loc. cit. 



