with Electron Currents in Different Gases. 439 



The smallest potential difference which we have recorded 

 between the stage when the glow starts on the anode and that 

 at which it crosses to the cathode is 0*5 volt, the largest about 

 6 volts. These observations were made with a high pressure 

 of mercury vapour comparable with 1 cm. The smallest 

 potential differences were required with the smallest filament 

 temperatures and correspondingly small discharge currents, 

 and vice versa. In this respect the behaviour of the glow is 

 similar to that of the positive column in the ordinary low- 

 pressure discharge-tube, which is gradually forced back on 

 to the anode as the strength of the discharge current is 

 increased. 



We have examined the glow carefully with the spectro- 

 scope in the initial stages to see if there is any difference in 

 the potentials necessary to excite the different lines of the 

 mercury spectrum. We have not been able to detect any 

 such effect, either when the glow just starts or when it is 

 made to die out. The strong lines of wave-lengths 4358 

 and 5460 have been examined with especial care in this 

 respect. So far as we are able to observe, the potentials 

 required to excite these two lines are identical, certainly to 

 within one-tenth of a volt. The equivalent voltage difference 

 of the two lines by the quantum relation is about eight-tenths 

 of a volt. This indicates that the lines of the mercury arc 

 spectrum are not excited separately by single electron 

 impacts, one for each line, but suggests that they are the 

 result of a complex change set up in the mercury atom, and 

 affecting a large number of electrons simultaneously. 



The value 11*5 volts of the smallest potential difference 

 which will start the glow is less than the value 12*5 volts 

 given by McLennan and Henderson* for this quantity. The 

 precise interpretation of this voltage is a difficult matter 

 without a knowledge of the distribution of potential along 

 the discharge. If it is interpreted as the potential difference 

 equivalent to the energy necessary for an electron to disrupt 

 the mercury atom or the mercury ion in a definite way, we 

 have to face the difficulty that, according to the experiments 

 of Franck and Hertz, it is impossible for an electron travelling- 

 through mercury vapour to acquire kinetic energy in excess 

 of that equivalent to 4'9 volts. This might be overcome if 

 there is a sharp drop of potential sufficiently close to the 

 cathode to carry a considerable number of electrons through 

 the critical region without atomic impact, on the further 

 assumption that the collisions become elastic again after 

 passing the critical velocity. 



* Roy, Soc. Froc. A. vol. xci. p. 485 (1915). 



