BELL SYSTEM TECHNICAL JOURNAL 



ing voltage, the electrometer reports no influx of electrons. Does this 

 really mean that there are no electrons with more than just the speed 

 necessary to o\erpass a retarding voltage of just that critical value? 

 Or does it merely mean that the electrons flying with more than that 

 critical speed are plentiful, but not quite plentiful enough to make 

 an impression on the electrometer? Is there any topmost speed at 



/^DVLE^E: VOLT/qGL 



Fig. 1 — Curves showing thfrniioEiic flfitri)n-ciiircnt \er.sus opposing voltage, ileinon- 

 st rating a distribution-in-specd extending over an unlimited range of speeds. Multiply 

 the ordinates of the middle cur\e by 100, those of the right-hand curve by 10,000, to 

 bring them to the same scale and make them merge into a single curve. (L. H. Gernier) 



all, or should we find, if we could replace the current-measuring 

 de\'ice with other and progressively better ones ad infinitum, that the 

 apparent maximum speed soared indefinitely upwards? 



Absolute decisions cannot be rendered in a question of this kind; 

 but it is possible, under the best of circumstances, to pile up indica- 

 tory evidence to such an extent that only an imusually strong will- 

 to-disbelieve would refuse to be swayed by it. Tiie judgment depends 

 on the shape of the curve which is obtained by plotting the electro- 

 meter-reading vs. the retarding potential — in other words, the fraction 

 y of the electrons of which the encrgj' of motion surpasses the amount 

 X, determined from the relarding-voltage by the relation x = eV. 

 Look for example at the curves of Fig. 1, which fefer to the electron- 



