CH. XI] 



MOVING PICTURE FILM AND MECHANISM 



417 



The essential part of the mechanism consists in three sprocket 

 wheels, a, b, and c, (fig. 231), the two wheels a and b move con- 

 tinuously at the average rate at which the film is passing (30 cm., 

 i foot, per second), and serve to unwind the film from the upper 

 reel F and feed the film to the take-up reel G at a uniform rate. 

 The sprocket wheel c, located between the other two, is inter- 

 mittent in its movements, being stationary for about % of the time 

 and being in rapid motion for about J"6 of the time. The effect 

 is, that after the film has been in position for exposure on the screen 

 this sprocket wheel jerks the small section of film between L and M 

 forward to the next picture. In fig. 232 is shown one form of 

 mechanism for causing the intermittent movement of the sprocket 

 wheel. 



When the film is stationary it is projected on the screen by the 

 objective, but during the short time when the film is in motion a 

 shutter either before or behind the objective cuts off the light and 

 prevents any blurring due to the movement of the picture. 



The films are made in such a way that if the pictures are right 

 side up, the later picture will be below the earlier ones, but as in 

 passing through the machine the pictures are upside down, the 

 later pictures are above and it is necessary to move the film down- 

 ward to bring the pictures on 

 .fiMMtfAtefc*, the screen in due order. 



584. Threading the film in 

 the machine. The film as 

 wound on the reel usually is 

 wound in the correct direction, 

 so that the first pictures are on 

 the outside. If this is not the 

 case, the film must be rewound 

 on another reel to reverse its 

 direction. If the direction is 

 correct the pictures will be up- 

 side down when the film is in 



FIG. 232. INTERMITTENT MOVEMENT the machine, that is, when the 



OF POWER'S No. 6 CAMERAGRAPH. ,-, -, -, c 



,,, , , , , ., ... , , film is passing downward from 

 (L ut loaned bv the Nicholas Power 



Company). F (fig. 231). 



