soMF. coxTr..\fi'OR.iKy .iiH'.ixcFs IX ri[ysics-~iii 



285 



I'sually tlu- iIiTtnulc is siret'iKHl hy a family of gau/cs, with tlieir 

 potentials ailjustfd as is iiuliiatcd in Fij;. o (with the paths of intruding 

 ions of lK)th signs, ini-liuling those excited from the outer gauzes 

 themselves, mapjKni out to show how they are rebuffed). Naturally 



Kigs. 5 and 6 — Morton's apparatus for determining excitation-potentials by the 

 "photoelectric method. " (Philosophical Magazine.) 



the arrangement of potentials in front of the electrode must be such 

 that the emitted electrons are all drawn away from it, not driven 

 back onto it. The rate of emission of electrons, the photoelectric 

 current, may lie measured with an electrometer connected either to the 

 sensitive electrode or tfi a gauze so placed as to gather in all the 

 electrons emitted from it. Figs, o and G, the latter of which shows a 



