120 HILL sySTf.M TECIIMC.IL JULKS'AL 



is raised just past 19.75 \olts there is a sudcien falling-off in llie num- 

 ber of electrons arriving at the third electrode. The curve in Fig. 3, 

 obtained by ICinsporn, shows the current into such an electrode in 

 mercury-vapor rising and falling again and again as the voltage 

 passes through the values which arc integer multiples of 4.9 volts, 

 the least resonance-potential of mercur\-. Heliiun has a second 

 resonance-potential, at 20.45 volts; neon has two, at 1G.65 and 18.45 

 volts rcspectisely; argon three, at 11.55, 13.0 and 14.0 \olts;' mercury 

 two, at 4.9 and 0.7 volts. It is almost certain that in each case these 

 are only the most conspicuous among many, but the lowest men- 

 tioned is the lowest of all. 



Up to this point we find the gas acting as a mere inert obstruction 

 to the discharge; e\ery collision of an electron with an atom inter- 

 rupts the progress of the electron toward the anode and to that extent 

 impedes the discharge. Past the resonance-potential the same action 

 continues, although the interruption is doubtless less severe when 

 the electron is depri\ed of part of its energy of forward motion than 

 when it is flung backward with its motion reversed in direction and 

 its encrg\' intact. .At the resonance-potential, the gas does begin 

 to assist the discharge in an indirect way. Atoms which are put 

 into an "excited state" b\- a blow from an electron revert of them- 

 selves to the normal stale, some time later; in so doing they emit 

 radiation, some of which falls upon the cathode; some of this is ab- 

 sorbed in the cathode metal, and exjjels electrons which go along 

 with the maintained electron-stream as extra members of it. Thus 

 the gas hel[)s in increasing and maintaining the discharge; this effect 

 is of great theoretical importance, and I will return to it later; but 

 in these actual circumstances it is not \ery prominent. 



The really powerful cooperation of the gas in the discharge com- 

 mences when the electrons are given so great an energy that they dis- 

 rupt the atoms which they strike, tearing off an electron from each and 

 leaving a posiliveb-charged residue, an ion which wanders back 

 towards the cathode while the newly-freed electron and its liberator 

 go on ahead towards the anode. The onset of this ionization may 

 be detected by inserting a third electrode into the gas, it being charged 

 negatively to such a degree that no electrons can reach it, but only 

 p(»siiive ions; or by the increase in the current between cathode and 

 ano<le, for the current increases very suddenly and very rapidly 

 when the energy of the primary electrons is raised past the threshold- 

 value, the ionhiiiR-poleiiliul of. the gas; 24.5 volis for helium, 21.5 for 

 neon, 15.3 for argon, 10.4 for mercury. Consider for example the 



* I take the wiluex for neon .tnd ,irK<iii (roin Hertz' I.Uesl puliliration. 



