190 



BELL SYSTEM lECUMC.IL JOi'RXAL 



of the illumination, and the response to variations of illumination 

 is practically instantaneous. A photograph of a photoelectric cell 

 of the type used in the picture transmission apparatus is shown in 

 Fig. 2. This cell is placed inside the cylinder formed by the photo- 

 graphic transparency which is to be transmitted, as shown in Fig. 1. 

 As the film cylinder is rotated and advanced, the illumination of the 

 cell and consequently the current from it registers in succession the 

 brightness of each elementary area of the picture. 



Assuming for the moment that the photoelectric current, which is 

 a direct current of varying intensity, is of adequate strength for suc- 

 cessful transmission, and tliat the transmission line is suitable for 



Fig. 3 — Light valve details: (R) riblxin carrying picture current; (P) pole piece of 

 magnet; (j) jaws of aperture behind ribbon 



carrying direct current, we ma>- imagine the cuneiil Irom tiie [iholo- 

 electric cell to traverse a communication line to some distant point. 

 At the distant point it is necessary to have the third element above 

 mentioned, a device for retranslating the electric current into light 

 and shade. This is accomplished in the present system by a de\ice, 

 due in its general form to Mr. E. C. Wenle, termed a "light valve." 

 This consists essenlialK- of a narrow ribbon-like conductor lying in a 

 magnetic field in >urh a position as to entirely co\er a small aper- 

 ture. The iiuiiniing current passes through this ribbon, whicli is in 

 conse(|uence detlecled to one side l)y the inter-action of the rurreiit 

 with the magnetic fielil, thus exposing the aperture beneath. Light 

 passing through this aiierttire is thtis varied in intensity. If it then 

 falls upon a photographic sensitive film bent into cylindrical form, 

 and rotating in exact synchronism with the film at the sending end, 

 the film will be exposed b\- amounts var\ing in proportion to the 

 lights and shades of the original picture. The ribbon and aperture 

 of the light valve are shown diagraminatically in Fig. 3. F"ig. 4 



