Resonance Potentials for Electrons in Metallic Vapours. 37 



these points of resonance gives the resonance potential 

 directly. Since we are concerned only with differences of 

 points similarly selected, the arbitrary method of selection 

 can introduce no error. The total current curves, for 

 example, fig. 1, curves 1 and 4, show a rapid increase in 

 current at an applied potential of 7*3 volts, due to ionization. 

 These points are selected in the same manner as the points 

 of resonance, both curves being plotted on the same scale of 

 coordinates. In general the total current is from 10 to 

 50 times greater than the partial current. The initial 

 velocity of the electrons is determined from the partial 

 current curves. Thus, referring to Table I., the first reso- 

 nance occurs at 2'6 volts, for curve 2, whereas if the initial 

 velocity were zero it would occur at 2*69 volts. The initial 

 velocity of the electrons is accordingly 0*09 volt. The mean 

 value of the initial potential for any one group of curves is 

 added to the mean value of the applied potential at ionization 

 to obtain the true ionization potential. The data represented 

 by Tables I. and II. were obtained with entirely different 

 construction of apparatus, and the agreement in the mean 

 values of the resonance and ionization potentials illustrate the 

 reproducibility of this work. The final mean values for 

 magnesium are 7*75 volts for ionization and 2*65 volts for 

 resonance. 



Table I. 



Ionization and Resonance Potentials for Magnesium 

 Referring to fig. 1. 



Applied Resonance Potential. 



_ , Potential „ * — ^ Initial 



Curve, a. b. c. for Potential. 



Ionization. 



1 73 



2 2-6 5-5 ... ... 2-9 ... 0-09 



3 24 52 28 ... 03 



4 7-3 



5 2-4 48 ... ... 2-4 ... 03 



6 20 46 ... ... 2.6 ... 0-7 



7 20 46 ... ... 2*6 ... 0-7 



8 20 46 68 ... 26 22 0*7 



... 7*3 269 volts 05 



Mean Initial Potential ... 0*5 



Ionization Potential 7*8 volts. 



