/■/( /7A-/. iK.ixsMissiox ori.R iri.i.fiioxi- i.ixr.s 



191 



>h<>\vs a st'ction of tin- riTt'i\iii)L; riul of a systi'in of (lii- sort |)oslulal(.-(|, 

 with its liuhl xiiinv, tfu- lij;hl \al\i', ami the ri'ffi\iiii; rylindtT. 



.\i>\i-i.\ HON i>i S< III MK ID TiiiiiriioM. I.IM-: Tkansmissiux 



The simple stiieine of picture Ir.iiisiiiissiou jiisl outlined must be 

 miHlitieii in order to adapt it for use on commercial electrical coni- 

 iiumication systems, which have heen developed |)rimarily for other 

 puriMtses than picture tr.msmissiou. Of existing electrical means of 

 communication, which include land wire systems (telegraph and 

 telephone), submarine c.iMe, and radio, ihe wire s\stem, as developed 



III rLcei\ ing ciul optical system: (l.j ll^ht source; (I)) condensing 

 lens; (V) light valve; (S) projection lens; (Cj sensitive film 



for the telephone, offers great advantage when all factors are con- 

 sidered, including constancy-, freedom from interference and speed. 

 The picture transmission system has accordingly been adapted to it. 

 In the simple scheme of picture transmission outlined in the pre- 

 ceding section, the photoelectric cell gives rise to a direct current 

 of varying amplitude. The range of frequency components in this 

 current runs from zero up to a few hundred cycles. Commercial 

 long distance telephone circuits are not ordinarily arranged to transmit 

 direct or very low frequency currents, so the photoelectric currents 

 are not directly transmitted. Moreover, these currents are very 

 weak in comparison with ordinary telephone currents. On account 

 of these facts, the current from the photoelectric cell is first amplified 

 by means of vacuum tube amplifiers- and then is impressed upon a 

 vacuum tube modulator jointly with a carrier current whose fre- 

 quency is about 1,300 cycles per second. What is transmitted over 



' For a very full description of the standard telephone repeater the reader is 

 referred to "Telephone Repeaters," (iherardi and Jewett, Trans. A. I. E. E., .Nov., 

 1919. Vol. 38, part 2, pp. I287-1J4S. 



