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BELL SYSTEM TECHNICAL JOURNAL 



field-free region R, to be swung around in senii-circular arcs by a mag- 

 netic field i7 applied normally to the plane of the paper over the region 

 Q; thus they arrive at the ion-collector behind the slit Si. The major 

 experimental difficulty consists in maintaining simultaneously a gas- 

 density between F and Eo high enough to afford plenty of ions, and a 

 gas-density in R and Q low enough so that the ion-stream is not dis- 

 persed. This is effected by feeding in the gas through A and applying 

 powerful pumps to draw it out through B, C and D. 



Varying H and plotting against it the current into the ion-collector, 

 one obtains a curve with peaks, such as the one in Fig. 4. This is, how- 



<J 2 



5.9 



6.1 fc.9 



ATOMIC WEIGHT 



7.0 



Fig. 4 



ever, a curve obtained by Dempster with ions issuing from a hot fila- 

 ment. The charge-to-mass ratio for the kind of ion producing each 

 peak is calculated from the accelerating-voltages, the deflecting field, 

 and the diameter of the circular arc through which they swing. 



The use of this method in determining ionizing-potentials may be 

 illustrated from the work of H. A. Barton on argon. ^^ Observing at 

 values of Vi superior to some 50 volts a two-peaked curve with the 

 M/E values of the corresponding ions standing in the ratio 2:1; and 

 observing at values of Vi inferior to some 40 volts only one of these 

 peaks, the one with the greater value of M/E; he inferred that this 

 peak was due to A+ ions and the other to A"^"*" ions. Plotting the heights 

 of these peaks or the areas under them as functions of Vi he obtained 

 curves such as those shown in Fig. 5. From many such curves as these 

 he deduced that the energy of electrons just able to produce doubly- 

 ionized argon atoms exceeds that of electrons just able to produce 



" Phys. Rev. (2) 25, pp. 469-483 (1925). 



