Theory and Low Voltage Arcs in Ccesium Vapour. 93 



can be shown that most of the electrons possess an initial 

 velocity of about 1 volt. Thus Foote, Rognley, and Mohler*, 

 using a 3-electrode tube, found the initial potential to be 

 1*3 volts in rubidium, and I/O volts in caesium, and Tate and 

 Footef observed similarly high initial potentials in sodium 

 and potassium. These high-velocity electrons are not pro- 

 duced photo-electrically, because they appear at accelerating- 

 voltages below which any radiation is excited. Thus in 

 ceesium the first resonance collision occurs at about 0*5 volt 

 applied potential and the second at about 2*0 volts, instead of 

 1*5 volts and 3*0 volts. Neither can they be accounted for 

 by velocity distribution due to temperature of the cathode. 

 Thus the fractional number F of emitted electrons having 

 a velocity greater by V volts than the applied poten- 

 tial V, on the basis of the Maxwell distribution of velocities, 



is Y = erf X +- 2 / - xe-* 2 , where % 2 = 11600V /T and T = abso- 



V IT 



lute temperature of the cathode. In the present case but 

 3 electrons per thousand could have a velocity 0*5 volt greater 

 than the applied voltage and but 5 per 10,000,000 a velocity 

 1*0 volt greater. The intensities of the lines are thousands 

 of times too great at the applied potential 2*9 volts to be 

 explained by the presence of 5 electrons per 10,000,000 with 

 velocities sufficient to ionize. The high-speed electrons are, 

 however, present. Possibly their initial velocities are due 

 to contact potentials occasioned by the presence of an alkali 

 metal. Accordingly the curves of fig. 4 should be cor- 

 rected by adding to each abscissa the initial potential 1*0 volt 

 determined from the total current curve shown in fig. 5. 



It is noted that the intensity of A,8521 and \8943 begins 

 to decrease at 3*3 volts true potential, reaching a minimum 

 at 3*6 volts. This would follow directly from Bohr's theory. 

 Thus, when the true accelerating voltage exceeds 1*45 volts 

 the lines Vhs — 2pi are excited by electronic impact, and 

 these lines continue to be produced until the ionization 

 potential is reached. Above this latter voltage, however, 

 electrons which at a slightly lower velocity would have given 

 rise to 1*55— 2/n now produce ionization and the complete 

 series spectrum. Any line of the principal series, for which 

 m>2, is excited at a sacrifice in intensity of X8521 and 

 X8943, since each line of this series requires that electrons 

 fall from the nip ring into the 1*55 ring. The subordinate 



* Loc. cit. 



t Phil. Mag. xxxvi. p. 64 (1918). 



