122 BBLL SYSTEM TECHNICAL JOURNAL 



nearly-simultaneous collisions hapf)en often, ionization will begin at 

 the resonance-potential. In a profuse electron-stream, the threshold 

 potential for ionization is the lowest resonance-potential. Another 

 feature of the profuse discbarge is, that when ionization does com- 

 mence the current leaps up much more suddenly and violently than 

 it does in the scanty discharge. This is because the electron-current 

 is depressed at first by the space-charge effect, the repellence which 

 the electrons crossing the gap exert against the electrons which are 

 on the verge of starting; when positi\e ions first appear in the gap, 

 they cancel the action of a great number of the traversing electrons, 

 and the flow of electrons from the cathode to anode is immensely 

 increased. I shall speak of this more extensively further on. 



We return to the case of the feeble electron-stream. We have con- 

 sidered various things which an electron may do to a helium atom 

 which it strikes — bouncing off harmlessly, or putting the atom into 

 an excited state, or ionizing it; we have mentioned that each of the 

 two latter actions commences at a critical value of energy, at the so- 

 called resonance or ionizing potential, respectively; we have con- 

 sidered the effect of each of these actions upon the discharge. Have 

 we listed all the possible interactions between atoms of matter and 

 atoms of electricity, when electrons flow across helium? and if we 

 knew all the resonance potentials and all the ionizing potentials' 

 of helium, could we predict all the features of all electrical discharges 

 in pure helium, whether in rarefied gas or in dense, whether the elec- 

 tron-stream be scanty or profuse? This is the general belief; whether 

 justified, it is impossible to say. We e\idently need another Maxwell 

 or anotJier Boltzmann, somebody e.xceedingly skilful in statistical 

 reasoning, al)le to take the information we can provide about the 

 possibility or the probability of various kinds of impacts, and deduce 

 the slate of affairs in the mixture of atoms, ions and electrons without 

 getting hopelessly entangled in the frightful maze of equations into 

 which his very first steps would certainly lead him. While awaiting 

 him we have to content oursehes with our successes in interpreting 

 the (low of electrons through very rarefied helium and the other noble 

 ga.ses and the metal vapors; and as for the discharges in denser gases 



' I have simpli(ii.-(J this passage somewhat so as not to retard the exposition. We 

 know that an electron may "excite" a helium atom if its energy exceeds 19.75 volts, 

 but this docs not prove that it must do so; it is more reasonable to suppose that it has 

 a certain chance of exciting the atom, zero when its energy is less than 19.75 volts, 

 but greater than zero, and a certain function of its energy, when the latter exceeds 

 19.7S volls. We should know these functions for all the resonance-potentials and for 

 the ionizing-|K>tential; inde|>endent experiments to determine them have been per- 

 formed, and no doubt will be multiplied. 



