Theory and Low Voltage Arcs in Ccesium Vapour. 85 



inelastic, resulting in a displacement of a bound electron to 

 the 2/>t and ocy> rings respectively: at other voltages, such 

 as 3*18, the velocity condition is not fulfilled. This assump- 

 tion, as far as displacement from the l'5s ring is concerned, 

 is, however, scarcely justifiable, since the range between 1"45 

 and 3'88 volts is small ; and since it appears that when the 

 electron possesses 3'18 volts velocity, 1*45 volts velocity may 

 be absorbed at the impact, this going into increased total 

 energy of the atom and the remainder into kinetic energy of 

 the colliding election. Also ionization is believed to occur 

 from the 2/> ring, since arcs may be operated below the 

 ionization potential. At some stage, however, the mass of 

 the colliding particle is effective, since the quantum relations 

 do not appear to hold for low-speed positive or negative 

 ions. Indeed, it is questionable whether a definite ionization 

 potential for ions exists. 



An electron of 1*45 volts velocity colliding with a caesium 

 atom ejects an electron from the l'5s ring to the 2p 1 ring. 

 In returning to equilibrium the atom emits the frequency 

 1*5*— 2/>ii This radiation, however, is capable of being 

 absorbed by a neighbouring atom (resonance radiation effect) 

 resulting in a displacement of an electron to the 2p l ring. 

 Hence, in an electron-tube there may be a building-up of 

 the number of electrons in the 2p 1 ring, and if all radiation 

 emitted were absorbed, every atom might have electrons in 

 this ring. Thus further collision with T45-volt electrons 

 would result in an ejection of an electron to an extreme 

 outer ring. A collision with a 2*4-volt electron would ionize 

 the atom. Van der Bijl* has suggested as an explanation 

 of low-voltage arcs operated below the ionization potential, 

 a theory of successive impact. The atom collides with an 

 electron, causing a displacement of a bound electron to the 

 2y> l ring. Before it is able to return to the 1*55 ring 

 it collides again with a second electron, and the atom is 

 ionized. The probability of a second collision under such 

 conditions is, however, very small. The above suggestion of 

 the absorption of radiation which was first proposed by 

 Compfonf is a much more plausible explanation of arcs below 

 the ionization potential. In fact, one must consider how 

 ionization could take place otherwise than from the 2^ ring. 

 It must be accordingly necessary that only a small proportion 

 of the radiation emitted when electrons fall from the 2/> x to 

 1"5* ring is absorbed by the vapour. This is a reasonable 



* Physical Review, x. p. 546 (1917). 



t K. Compton, Chicago Meeting Am. Phys. Soc. 1919, Phya. Rev. 



