130 PELL SYSTEM TECHNICAL JOURNAL 



in so returning it emits a quantum of radiant energy which may 

 strike the cathode, and be abs<)rl)ed b\- it, and cause another electron 

 to leap out of the cathofie and follow the first one. There are two 

 other concei\able processes, which have the merit that they can not 

 only be concei\ed but also witnessed in operation iiy themselves when 

 the right conditions are provided. Positive ions flung violently 

 against a metal plate drive electrons out of it, as can be shown by 

 putting a positively-charged collector near the bombarded plate and 

 noticing the current of negative charge which flows into it; and posi- 

 ti\-e ions flowing rapidly across a gas .ionize some of the atoms in 

 it, as may be shown by sending a beam of such ions across the inter- 

 space between two metal plates, with a gentle crosswise field between 

 them which sucks the freed electrons into the positive plate. The 

 mechanism of the first process is not understood, except when the 

 positive ions are so many and so swift that they make the metal hot 

 enough to emit thermionic electrons, which does not happen in the 

 cases we are now considering. The mechanism of the second process 

 is only dimly understood, but it is clear enough that a positive ion 

 driven against an atom is much less likeh' to ionize it, than an electron 

 of equal energA- would be.'- Either of these two processes is very 

 inefficient, at least at the comparatively low speeds with which posi- 

 ti\e ions move under the circumstances of these experiments; but they 

 are probably efficient enough to do what is required of them. No 

 doubt all three of them contribute to the discharge; but the relative 

 proportions in which they act certainly differ ver>- nuicli from one sort 

 of discharge to another, and will furnish research problems for years 

 to come. 



Returning to I-'ig. 5, we note once more that as the electrodes are 

 moved farther and farther apart while the density of the gas and the 

 fielfl strength arc held constant, the current at first rises exponentially 

 (linearly in the logarithmic plot) as it should if the free electrons and 

 onl>' the free electrons ionize; but eventually it rises more rapidly 

 and seems to be headed for an uncontrollable upward sweep. Town- 

 send attributed this uprush to the tardy but potent participation of 

 the i>osilive ions, either ionizing the molecules of the gas by impact 

 after the fashion iA the negati\e ions, or driving electrons out of the 

 cathode when they strike it, or both. F^ilher assumption leads to 



"If iiKiiiicntuin is rinisorvrd in the iiii|>art liotwoc-n ion and atom, the ion must 

 retain ,1 larKe part o( its kinetic energy after the cullision, or else the struck atom 

 must lake a kir^e part iif it as kinetic energy of its own niotiun; it is not possible 

 for tlie striking partiile to spend nearly its entire energy merely in liberating an 

 elerlron from the struck one. Conservation of momentum |x'rliaps does not pre- 

 vail on the atomic scale; but of all the principles of classical ilvnaniics, it is the one 

 which the reformers of physics most hesitate to lay violent hands upon. 



