THE SELENIUM CELL 5 



to a distant receiving station. t The portrait is pro- 

 jected on to a screen, where light and shade and 

 varying tones are produced. Now suppose this 

 screen divided up into a thousand square sections, 

 each one the size of a selenium " cell," the cell 

 being an arrangement made with selenium, which 

 varies in its electrical resistance according to the 

 strength of the light illuminating it. This cell is 

 held in one section of the bright image on the 

 screen, then in the next, then in the next, and so 

 on, until finally it has been held in the whole thou- 

 sand sections. But each section is of a different 

 brightness, according to what portion of the image 

 is projected upon it. In each section, therefore, 

 that the cell was held, its electrical resistance 

 varied. 



Now imagine you could record these variations 

 in resistance on a similar screen at the receiving 

 station. When the selenium cell was held over 

 section i of the image of the screen, its resistance 

 was, let us say, r\ ; using a battery of 100 volts, 

 and neglecting the resistance of any connecting 

 lines, the current at the receiving station would be 



. Let this regulate, in any imaginable way, 

 r\ 



the strength of an electric light, which is shining 

 on a similar screen at the receiving station, on the 

 screen being placed a sheet of sensitive photo- 

 graphic paper. Next let the selenium cell be held 



