PICTURE TRANSMISSION Oll-.R TELEPIIONF. LINES 189 



inti'rior of the cylinder then varies in intensity with the transmission 

 or tone value of the picture. The optical arrangement by which a 

 small spot of linht is projectetl upon the pholn^r.ipiiic transparency 

 is shown in section in Fig. 1. 



The task of transforming this light of varying intensity into a 

 variable electric current is (H?rfornK'<l by means of an alkali metal 



Kig. 2 — Photograph of photoelectric cell of type used In picture transmission 



photoelectric cell. This device, which is based on the fundamental 

 discovery of the photoelectric effect by Hertz, was developed to a 

 high degree of [>erfection by Elster and Geitel. It consists of a 

 vacuum tube in which the cathode is an alkali metal, such as potassium. 

 Under illumination, the alkali metal gi\es off electrons, so that when 

 the two electrodes are connected through an external circuit, a cur- 

 rent flows. This current is directly proportional to the intensity 



