A SOUND PROJECTOR SYSTEM 201 



this resistance a voltage which is proportional to the light falling upon 

 the cell and accordingly bears a direct relation to the varying density 

 of the sound track interposed between the exciting lamp and the 

 cell. 



Fig. 6 — Sound projector equipped with Simplex head. 



The photo cell circuit is inherently one of high impedance. In 

 such a circuit there are two matters which require attention. (1) 

 Local interference— "static," to use the radio expression, is most 

 readily picked up, ^nd at this point where the energy level is low. 



14 



