GIG 



Prof. J. S. Town send on tlie Genesis of Ions 



the distance />2 a spark a|)j)eared, and at the shorter distance 

 Z>i no spark appeared when the ultra-violet light was acting. 

 a' is the sparking dii^tance for the force X and pressure />, 

 as calculated from the values of a and yS. The quantities a' , 

 Z^i, and />2 ^1'^ ill centimetres. 



The sparking potentials X x 



2 



determined experi- 



mentally are plotted on the curve, fig. 4, and marked by 

 small circles to distinguish them from the points determined 

 by calculation. 



The experiments with hydrogen were made a little dif- 

 ferently. The values of a' were first calculated for the 

 different forces and pressures from the numbers given for 

 oL and /3. The plates were then set at this distance apart and 

 the potential at which a spark appeared was found experi- 

 mentally. A comparison between the potentials thus found 

 and the potentials v' — ^a' gives a method of testing the theory. 

 The potentials V found experimentally are given in Table IX., 

 and are also marked on the curve, fig. 5. 



Table IX.— Hvdrooren. 



p. 



X. 



a'. 



v'. 

 472 



Y. 



8 



525 



•90 



465 



8 



700 



•57 



398 



385 



4 



350 



1-14 



393 



385 



8 



1050 



•306 



322 



322 



4 



525 



•613 



322 



323 



4 



700 



'405 



283 



282 



2 



350 



•810 



283 



287 



2 



525 



•501 



269 



273 



1 



350 



•806 



282 



289 



It will be seen from the diagrams that the sparking 

 potentials as given by the theory are in accurate aoreement 

 with the potentials determined experimentally. The latter 

 determinations were made w hile the light from the external 

 source was acting on the negative electrode, so that the 

 potentials are a little lower than those which would have 

 been obtained if no radiation had been falling on the electrode. 



This effect of ultra-violet light on the sparking potential 

 was first observed by Hertz in air at atmospheric pressure. 

 At pressures of a few millimetres the eflf^ect of ultra-violet 

 light on the sparking is not so great as the effect at 

 pressures. 



liuh 



