678 Prof. McLennan and Mr. Mercer on Ionization 



the evacuated chamber F. In all the experiments the appa- 

 ratus was arranged so that the ratio of: the final to the initial 

 volume Vg/V] 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 

 V 2 / V 1 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 V 2 /Vi was secured. 



In order to obtain distinct ionization tracks it was necessary 

 to have all ions in the cloud-chamber removed before the 

 expansion took place, and this was effected by establishing 

 an electric field between the top plate of the plunger and the 

 roof -plate of the cloud-chamber: to do this the gelatine layer 

 under the roof -plate was connected by means of a ring of 

 tinfoil cemented between the glass cylindrical wall and the 

 roof-plate to one terminal of a battery of storage-cells whose 

 other terminal was connected to the metallic base of the 

 expansion-chamber, and through the brass cylindrical portion 

 of the plunger to the roof-plate of the latter. 



In taking the photographs the method followed was also 

 that adopted by C. T. R. Wilson, and consisted in passing 

 the discharge from a set of leyden-jars charged with a 

 Holtz machine through some vaporized mercury, in a set of 

 four quartz tubes joined in series and placed close to the 

 walls of the expansion-chamber. The light from these dis- 

 charge-tubes was projected into the expansion-chamber in 

 beams of parallel rays by a set of four plano-convex strips of 

 glass. The camera was directed vertically down upon the 

 roof-plate of the cloud chamber. In setting up the apparatus 

 it was found convenient to separate the vacuum-chamber 

 somewhat from the expansion-chamber, and the manner in 

 which this was done is shown in the upper part of fig. 3. 

 In the lower part of the figure the connexions are shown for 

 producing the expansion and exciting the spark. The Holtz 

 machine was joined to the insides of two sets of leyden-jars 

 at P, NN' was a wet string short-circuiting the outsides of 

 the jars, and S represents the discharge-tubes in which the 

 secondary spark occurred. In taking the photographs the 

 string joined to T (fig. 3) was cut, and the valve B was 

 opened by the falling weight W. When this weight reached 

 the limit of its fall a second weight, in the form of a brass 

 ball and supported by a slender cord, broke away and fell 



