JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. VIII SEPTEMBER 19, 1918 No. 15 



PHYl^ICS. — Low voltage discharge in sodium vapor. Paul D. 

 FooTE and F. L. Mohler, Bureau of Standards. 



The question of separate excitation by electronic impact of 

 the doublet lines of the principal series of thallium was raised by 

 the writers at the New York meeting of the American Physical 

 Society, April 1918. The resonance potential for electrons in 

 thallium vapor was observed to be 1.07 volts. This corresponds 

 on the basis of the quantum relation, hv = eV, to a frequency 

 V = 8683 or to a wave length X = 11513 A, the shorter wave- 

 length member of the first term of the principal doublet series. 

 No indication of resonance at 0.95 volts could be detected cor- 



o 



responding to X = 13014 A, the other member of the doublet. 

 The question therefore suggests itself as to whether the line X 

 = 13014 is excited at the resonance potential of 1.07 volts. The 

 behavior of thallium in this regard may be very likely similar 

 to that of sodium. Accordingly we might expect the shorter 

 wave length D2 of the sodium spectrum to appear at the reso- 

 nance potential 2.10 volts and the wave length Di not to appear 

 until the ionization potential of 5.13 volts was attained. 



Figure 1 represents the apparatus employed. A is a lime- 

 coated Wehnelt cathode, C the anode, and B a glass tube sealed 

 at both ends with glass plates through which the arc discharge 

 was viewed. The object in using such a tube was to permit 

 observation directly upon the arc \Wthout the presence of the 

 absorbing unexcited sodium vapor. The tube was electrically 



513 



