SOME CONTEMPOR.IRY ADVANCES IN PHYSICS— XI 465 



electron having a kinetic energy of one e(iiiivalent volt as a "one-volt 

 electron." 



The ionizing-energy of an atom is usually given in equixalent volts, 

 whence the name ionizing-potential. 



Occasionally one meets with a value stated for an ionizing-potential 

 in terms of a unit known as the wave-number ("equivalent wave- 

 number" would he better) which amounts to 1.968.10"^" erg. 



lONIZATION-POTENTIALS 



The ionizing-potential of a monatomic gas is usually measured 

 by projecting electrons with controllable kinetic energy K into the 

 gas, and determining the value of K at which current begins to flow 

 into an electrode inserted into the gas and maintained at such a 

 potential that positive ions, but no electrons, can reach it. 



This method requires more elaborate apparatus than the outline 

 suggests. The experimenter must guard against an effect w^hich 

 was not suspected by those who first worked with the method. Elec- 

 trons having kinetic energy less the ionizing-energy of the gas may 

 cause the atoms which they strike to emit radiation. Some of this 

 radiation falls upon the electrode arranged to collect positive ions, 

 and expels electrons from it. The field around the collecting-electrode, 

 being such as to draw positive ions toward it, drives these electrons 

 away; and so there is a continuous current of negative charge out of 

 the electrode into the gas, which is quite indistinguishable from a 

 current of positive charge out of the gas into the electrode. Thus 

 the value of K at which positive charge first seems to flow into the 

 electrode from the gas is the "critical" electron-energy (as the phrase 

 is) not for producing ionization but for producing radiation. The 

 earliest determinations of what were thought to be ionizing-potentials 

 were vitiated by this effect. 



To avoid or recognize the influence of radiation several schemes have 

 been devised.^ For example, if two collecting-electrodes are used in 

 alternation, one having a large area and the other being small, much 

 more radiation will fall upon the larger one, and there will be a cor- 

 respondingly great difference between the currents of negative charge 

 out of the two; but if ions are being formed in the gas, the difference 

 between the numbers of these which find their way to the large and to 

 the small electrode will be much less pronounced. A slender collecting- 

 electrode may record only a very small current due to radiation, but a 



^ For a detailed account of the methods developed up to 1924, consult K. T. Comp- 

 ton and F. L. Mohler: "Critical potentials" {Bull. Nat. Res. Council, No. 48). 



