loNIZATION BY NEGATIVE IONS 5 



,*** 



3. Determination of a from experiments with ultra-violet 



light. 



The principal parts of the apparatus which was 

 used to find the conductivities are shown in figure 2. 

 The currents take place in the gas between the plates 

 A and B. The upper plate from which the ions are set 

 free was of zinc and was fixed to a micrometer screw for 







adjusting the distance between the plates. Insulation 

 was provided by the ebonite pillars E. The lower plate 

 B was of quartz silvered on the upper side so as to have 

 a conducting surface in contact with the gas. A series 

 of fine parallel lines were ruled on the quartz forming a 

 transparent grating over a small area at the centre of 

 the plate. The light from a spark gap S in a leyden 

 jar circuit passed through a quartz lens and the grating, 



