420 BELL SYSTEM TECHNICAL JOURNAL 



E. The Stationary States 

 E 1. The Direct Evidence for the Stationary Slates 



Imagine a tube filled with gaseous helium, and containing a hot 

 filament from which electrons emerge. By means of an accelerating 

 potential applied between the filament and a fine-meshed gauze close 

 in front of it, the electrons are speeded up, and pass through the gas 

 with an energy which is accurately controlled by the accelerating- 

 potential. A third electrode is maintained at a potential only slightly 

 higher than that of the filament. To reach this electrode, the elec- 

 trons must sacrifice nearly all of the energy which they acquired in 

 coming up to the gauze. If they lose little or no energy in their 

 progress through the gas, they can win their way to the third elec- 

 trode, like water rising again to the level of its source. If, however, 

 they lose a notable amount of energy to the atoms with which they 

 collide, they cannot reach the third electrode, as water which has 

 turned a mill-wheel cannot climb again to the level whence it fell. 



By measuring the current into the third electrode in the helium- 

 filled tube, it is found that if the electrons ha\e an amoimt of energy 

 lower than 19.75 equivalent \olts, they lose scarcely any of it in 

 their progress through the gas; but if the energy of an electron is just 

 equal to 19.75 e(|uivalent volts, it may and frequently does lose its 

 energy altogether; and if the energy of an electron surpasses 19.75, 

 it may and frequently does surrender just 19.75 equivalent volts to 

 the gas, retaining the residuum itself. Imagining that the electron 

 collides with atoms of helium on its way across the gas, we conclude 

 that the helium atom can receive exactly 19.75 of these units of 

 energy, no lesser quantity and (within certain limits) no greater. 

 From similar e.xperiments it appears that the mercury atom can 

 receive 4.GG equivalent volts of energy, no smaller amount and (within 

 certain limits) no larger. It appears that the sodium atom can 

 receive 2.1 equivalent Nolts, no less and (within certain limits) no 

 more — and the list can be extended to some thirty elements. 



Another way of saying the same thing is this: the helium atom may 

 exist (at least transiently) in its normal state, or also in a second 

 state in which its energy is greater by 19.75 equivalent volts than in 

 its normal state, — but not, so far as we can find evidence, in any stale 

 with any intermediate value of energy. Let us call this second 

 state an "excited state." The mercury atom then has, in adtiilion 

 to its normal state of undefined energy, an excited state of energy- 

 greater by 4.()() equivalent volts. The sodium atom has, in addition 

 to its normal stale, an excited state of energy greater by 2.1 equivalent 



