TATE AND FOOTE I RESONANCE POTENTIALS 



521 



The correct value for the critical potential is obtained by 

 taking the difference in potential between successive points, 

 thus eliminating any effect of initial velocity. For reasons to be 

 discussed later only the points a and b were used. The average 

 value obtained for the difference in potential between a and b 

 on curves 3 to 7 was 2.12 volts. Curves 1 and 2 were prelimi- 

 nary curves and not enough points were observed to permit of 

 an accurate determination of the critical potentials. 



TABLE 1 

 Summary of Data on Sodium 



The initial velocity of the electrons may be obtained from 

 these curves directly. For example, in curve 6, the first reso- 

 nance occurs at 1.3 volts and the second at 3.4 volts. The cor- 

 rect resonance potential is the difference of these two values or 

 2.1 volts. Hence the first resonance must have occurred at a 

 true potential of 2.1 volts. The initial potential is the correc- 

 tion necessary to add to the applied potential of 1.3 volts to give 



