602 



Prof. J. H. TowiiJjcnd on, the Genesis uf Ions 



upper plate A was ot" quartz silvered over so as to obtain a 

 flat conducting surface. A series of line parallel lines, half 

 a millimetre apart, was ruled on the silver near the centre of 

 the plate with the point of a needle so as to allow the light 

 to ]3ass through and fall on the movable plate B which was 

 of zinc. The distance between the plates was adjustable by 

 means of the micrometer-screw to which the lower plate was 

 attached. The plates were four centimetres in diameter and 

 the light fell on a circular area of one centimetre diameter 



Fig. 1. 



i_s^_L. 



at the centre of the zinc plate. The conductivity thus takes 

 place in a field of imiform electric force, since the largest 

 distances between the plates in the experiments was not much 

 more than a centimetre. The brass plate C, in which the 

 micrometer-screw worked, was fixed firmly by four ebonite 

 pillars to the disk D which held the quartz plate. The disk 

 was fixed by four brass pillars to the large brass plate E, the 

 hole at the centre of E being covered with a quartz lens and 

 the joint made airtight with a plastic cement. 



The apparatus was constructed very carefully by Mr. Bush, 

 instrument-maker in the laboratory, so that the plates were 

 accurately parallel to each other at all distances apart. 



The glass cover G fitted into a groove in the plate E and 

 the joint was made airtight. A wide barometer-tube was 

 connected to the lower end of the glass cover, and the micro- 

 meter-screw was turned by a long rod coming up through 

 the mercury column. 



Connexion was made with the plate B by a rod passing 

 through an ebonite plug in the side -tube T, and the tube U 



