ELECTRON THEORY KLOEFFLER 255 



PHOTOELECTRIC CELLS 



It was suggested earlier that electrons may be ejected from some 

 solids by the impingement of light rays. It appears that the high- 

 frequency electromagnetic wave (visible or invisible) has the power 

 of imparting energy to the electrons in the outer orbits of atoms of 

 certain photosensitive materials so that these electrons are released 

 or torn from the atom and fly out into space. This phenomenon 

 forms the basis of action of the photoelectric cell or the "electric eye" 

 which is so widely used in many control applications. The more 

 common light-sensitive materials are sodium, potassium, barium, 

 strontium, a*nd some of their compounds. The photoelectric cell 

 usually consists of an evacuated tube containing a large area of light- 

 sensitive material and a small anode or wire near the center. The 

 cell is connected into a circuit so that a positive potential is applied to 

 the anode. Whenever light falls on the light-sensitive cathode, 

 electrons are emitted and attracted to the anode. The actual current 

 flowing in the cathode-anode circuit is very small and amounts to only 

 a few microamperes at the best. This small current produces a small 

 voltage drop across a resistance. This potential is amplified by means 

 of other electron tubes to produce a signal for performing the purpose 

 desired. The number of electrons emitted from a light-sensitive 

 surface is directly proportional to the intensity of light falling upon 

 that surface. Thus a variation of light intensity may be transformed 

 into an electric signal to perform a desired purpose. This principle 

 is used in the sound production for the talking picture where a sound 

 track on the film varies the light intensity falling on a photoelectric 

 cell. The photoelectric cell may also be used for picture transmission, 

 for television, and for all kinds of signal applications. 



Photoelectric cells may have a high vacuum or may contain gas 

 under low pressure. The emission of electrons by the impingement 

 of light rays is the same in both types. The resulting current to the 

 anode in the vacuum type is equal to the electron emission, but in the 

 gaseous type the current is amplified several times (in practice not 

 exceeding 10 times) by the addition of current from the gas ions which 

 are formed during the transit of electrons from the cathode to the 

 anode. 



