200 



BELL SYSTEM TECHNICAL JOURNAL 



the bright filament of the exciting lamp is focused as a very narrow 

 line upon the film by passing through a system of lenses and an 

 aperture plate. The lamp filament is focused upon a slit of dimensions 

 .0015" X 3/16". The image of this slit is then brought to focus upon 

 the film as a .001" line whose length has been reduced in passing 

 through the aperture plate to .080". This reduction in length allows 

 .010" on either side for variations in position of the .100" sound track. 

 The position and focus of the lens tube are fixed, but the carriage of 



Fig. 5 — Amplifier used at projector. 



the exciting lamp is movable so that when replacing lamps the filament 

 may be properly brought on focus. 



A photoelectric cell of the type used is shown in Fig. 3. The 

 characteristic of this device is that when it is polarized by a proper 

 voltage and is used within proper limits the current through it is pro- 

 portional to the incident light. The circuit is shown in Fig. 4, It is 

 to be noted that the polarizing voltage is supplied to the photo cell 

 through a very high resistance and there is, therefore, obtained across 



1 



