[MCLENNAN IONISATION TRACKS OF ALPHA RAYS IN HYDROGEN 213 

 mercer] 



In order to obtain distinct ionisation 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 be- 

 tween 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 con- 

 nected by means of a ring of tinfoil cemented between the glass 

 cylindrical wall afnd 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 

 vapourized 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 discharge 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 cham- 

 ber 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 connections are shown 



Fig. 3. 



for producing the expansion and exciting the spark. The Holtz 

 machine was joined to the insides of two sets of Leyden jars at P, 



