﻿Ionization by Collision in Helium. 839 



The Electrical Measurements. 



The experimental arrangements are shown in the figure. 

 The parallel plate electrodes were contained in a glass bell- 

 jar, and negative ions were set free initially by the incidence 

 of ultra-violet light on the top plate. The metal base had a 

 hole bored in it covered by a quartz plate, through which 

 the light entered. The upper plate, of zinc, was fixed to a 

 micrometer-screw which could be turned from the outside, 

 and by means o£ which the plates could be put accurately at 

 any desired distance apart, from 1 to 11 mm. The lower 

 plate, also of zinc, had holes cut in it through which the 

 light might paes, and was attached to the top part of the 

 apparatus by four ebonite pillars. To improve the insulation 

 these pillars were encircled about hallway up by tinfoil 

 connected to earth. The upper plate was insulated from the 

 turning-handle by ebonite and connected to the electrometer ; 

 the lower plate was connected to the base, which in turn 

 could be put to any desired potential by connecting to a 

 battery of small storage-cells. 



The wires leading to the upper plate and to the tinfoil 

 over the ebonite pillars were taken out of the jar through 

 two ebonite plugs fixed to the glass. 



The joints were made air-tight with a plastic cement, 

 while the turning-handle passed out through a tightly fitting 

 brass collar made to fit the top of the bell-jar and rendered 

 air-tight by being covered on top with vaseline. 



The ultra-violet light was produced by a spark-gap in a 

 leyden-jar circuit, and the method of experiment was to 

 turn on the coil for 20 seconds and measure the resulting 

 current through the gas by means of the electrometer and an 

 electrostatic induction-balance *. 



The Purification of the Gas, 



As the effects of purifying the gas are considerable, it will 

 be well to explain the methods adopted for getting the gas 

 as pure as possible, and to discuss the impurities necessarily 

 present owing to the form of the apparatus. 



The helium, which was very kindly given to us by 

 Sir William Eamsay, was kept in the left of the two 

 chambers shown in the diagram [B], being always pumped 

 there at the conclusion of an experiment. The right-hand 

 chamber was reserved for the purification and contained 

 coconut charcoal from which moisture and other occluded 



J. S. Townseud, Phil. Mag-. [6] vol. vi. 1903, p. -V, 



>8. 



