198 



BELL SYSTEM TECHNICAL JOURNAL 



all the variations in pitch and loudness are accurately represented, to 

 suitably amplify this current, to effect its conversion into sounds 

 approximating those from which the records were made, and to so 

 direct those sounds as to reasonably create the illusion that sound 

 and picture are cognate. 



The disc records do not differ essentially from those used in the 

 ordinary phonograph except that they are considerably larger and 

 run at a much slower speed so that a single record will play throughout 

 an entire reel. The reproducer used is in some ways similar to that 



PROJECTOR HEAD 

 FILM 



TENSION PAD 



GUIDE ROLLER.., 

 APERTURE PLATE 



LENS TUBE 

 EXCITING LAMP- 

 STRIPPER 



LIGHT GATE 



FILM PICK-UP 

 AMPLIFIER 



' \PHOTO-ELECTRIC 

 ■'"' CELL 



SPROCKET 

 GUIDE ROLLER 



LAMP SOCKET 



Fig. 2 — Diagram of motion picture projector equipped for reproducing sound from 



film. 



used on the acoustic phonograph, the needle holder being connected 

 to a clamped diaphragm. This diaphragm is of highly tempered 

 spring steel and to it there is fastened an armature made of a special 

 high permeability alloy and so arranged that as the diaphragm vibrates 

 the flux in the air-gap of a permanent magnet varies correspondingly, 

 thereby inducing in appropriately placed coils currents which are the 

 electric representation of the wavy groove which the needle travels. 

 This reproducer is shown in Fig. 1. Although the energy delivered 

 by this instrument is comparatively low it has a very uniform response 

 over a wide frequency range. This result is largely brought about 



