204 



BELL SYSTEM TECHNICAL JOURNAL 



of electrons which appear when sufficiently amplified as noise from a 

 loud speaker. In spite of all precautions there is a certain amount of 

 vibration of the projector when in operation and it has therefore been 

 necessary to design a rather elaborate shock-proof mounting for the 

 photo cell amplifier. 



It is evident from the relative location of apparatus as shown in 

 Fig. 2 that it is not feasible to print the film sound record directly 

 beside the picture to which it applies. As a matter of fact, there is a 

 spacing of 14|" between picture and corresponding sound record and 

 a certain amount of slack is allowed between the sprocket which 

 carries the picture with an intermittent motion before the picture 

 projection lens and the sprocket which must carry the sound record 



30 



20 



-10 



100 



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FREQUENCY 



Fig. 10. 



with a uniform motion in front of the photoelectric cell. In this 

 connection it is noteworthy that special precautions are necessary in 

 order to prevent vibrations and speed fluctuations due to either 

 varying supply voltage or varying load from affecting the uniformity 

 of rotation of this sound sprocket. This is taken care of by the very 

 effective means of automatically controlling the speed of the driving 

 motor and by means of a mechanical device interposed between the 

 sound sprocket and the rest of the moving equipment of the projector 

 which effectively opposes the transmission of any abrupt change of 

 speed to this sprocket. 



The control box which contains the apparatus for governing the 

 speed of the driving motor is arranged to hold the record speed the 



