THE ALKALI METAL I'lIOTOELECTRlC CELL 



323 



exciting light. This is shown in the figure by the several curves for 

 different wave lengths, varying from Xi, representing short wave 

 energy, such as blue light, to X3, long wave energy such as yellow. 

 The shorter the wa\e length (the higher the frequency), of the exciting 

 light, the higher must be the positive potential necessary to prevent 

 or stop the emission of electrodes under illumination. As the positive 

 potential is reduced, the photoelectric current increases, until the 

 applied field (or the eft'ective field if contact potential differences are 

 present) becomes zero. At this point, the current becomes saturated, 

 that is increase of voltage in the negative direction fails to increase 

 the current. This means that the applied field does not penetrate to 

 any appreciable depth into the photoelectric material. 



Characteristic curves of the type shown in Figure 3, have played a 



X, 



54321 



+V0LT5 ON CATHODE 



12 3 4 5 

 -V0LT5 ON CATHODE 



Pig, 3 — Voltage-current curves for typical central cathode vacuum photoelectric cell. 



very important part in the development of photoelectric theory, and 

 particularly of the quantum theory. If V is the voltage applied to the 

 cell, e the charge on an electron, h the quantum- constant, v the fre- 

 quency of the exciting light, Einstein predicted and Millikan has 

 shown experimentally that the following relationship holds: eV=hiv- 

 vo), where vq is the limiting frequency corresponding to the long wave 

 length limit, beyond which the photoelectric emission does not occur. 

 If m is the mass of the electron, and v its velocity, the above relation 

 can be written 1/2 m v^ (Velocity^) =]i{y-v^. From this expression 

 it is evident that the greater the interval between the frequency of the 

 light used, and the limiting frequency, the higher is the velocity of 

 emission of the photoelectrons. 



When instead of being highly exhausted, the cell has an atmosphere 

 of gas at a low pressure (a few tenths of a millimeter of mercury) the 



