1898.] Electrification by Ultra-violet Light. 409 



through a plate fixed to the rod. The plate AB could be raised 

 or lowered from the outside of the vessel by a screw. The bell- 

 jar was fitted down to the base plate with sealing-wax, and the 

 whole was placed on insulating blocks over a source of ultra-violet 

 light S, which was either an arc light or a spark gap. The ultra- 

 violet light first passed through a quartz plate Q lf fixed over an 

 opening in a large metal screen, then through the quartz plate 

 Q., covering the opening EF, and then fell on the metal plate AB. 

 The wire -gauze performed the double function of allowing part 

 of the light to pass through and yet acting electrically as a plane 

 metal surface. The plate AB was generally of polished zinc and 

 was accurately levelled to the base plate. A delivery tube T was 

 let into the base plate and the whole vessel was made air-tight 

 to allow of exhaustion. 



In order to determine the velocity of the carrier, the rod L 

 which was insulated by means of the ebonite stopper was con- 

 nected to one pair of quadrants of the electrometer by the wire 

 M, the other pair being to earth. The base plate CD was then 

 connected to one terminal of a 100 volt transformer, worked from 

 the town mains, the other terminal of which was connected to 

 earth. When the base plate is charged positively, the plate AB 

 is charged negatively by induction, and the negative carriers, set 

 free under the action of the ultra-violet light, start travelling 

 towards the base plate under the influence of the electromotive 

 force acting between the two plates. If the plate AB is close to 

 CD, a large number of the carriers are able to reach CD and give 

 up their charge before the electromotive force is reversed. All 

 the carriers distributed between the plates at the instant of 

 reversal travel back to the top plate, and since the top plate is 

 charged positively by induction no more carriers are produced 

 during that half alternation. Experimentally it was found that 

 there was no leak to the top plate when the base plate was 

 negatively charged and the ultra-violet light was in action. We 

 see therefore that when the plates are close together the plate 

 AB loses a negative charge. This rate of leak will evidently 

 decrease as the distance between the plates is increased, until a 

 certain distance is reached, when the plate AB shows no loss of 

 charge, although the ultra-violet light and alternating e.m.F. are 

 both acting. When the plate is at this distance the first carriers 

 liberated when AB becomes negative by induction are able to 

 travel nearly to the base plate, but before any can give up their 

 charge, the E.M.F. is reversed and they travel back to the plate 

 from which they came. All distances greater than this give us 

 no rate of leak, but it is the object of the experiment to determine 

 the shortest distance between the plates for which AB shows no 

 loss of charge. This point is in general fairly sharply defined, as 



