Silent-Era Film Projector

1. Intermittent film motion. Each frame must remain stationary in the gate during exposure, while the supply and take-up reels operate approximately continuously. The transport therefore converts continuous rotation into indexed frame advances.
2. Continuous side. Toothed sprockets engage the perforations — four per frame on 35 mm film, so a 16-tooth sprocket advances exactly one frame per quarter turn — and pull film off the supply reel at constant speed; the take-up reel winds it on through a slipping belt (its needed speed changes as it fills, ω = v / r).
3. Geneva drive. In this model, a pin on the continuously rotating driver enters one slot of the Maltese cross and indexes it by 90°, corresponding to one frame. A locking surface holds the cross during the remaining dwell interval. The geometry provides zero ideal angular velocity at engagement and disengagement.
4. Slack loops. The feed sprocket supplies film continuously while the intermittent sprocket advances it one frame at a time. Upper and lower loops vary in size to isolate the reels from these acceleration pulses; loss of adequate slack increases perforation tension and the risk of film damage.
5. Shutter timing. A phase-locked shutter blocks projection light while the film advances. The modeled second blade provides two light pulses per frame, so 16 fps produces 32 flashes/s. This reduces perceived flicker, although flicker sensitivity varies and historical projectors used different blade counts and frame rates.
6. Illumination and projection. The exhibit represents an arc lamp, condenser optics, an approximately 18 × 23 mm image area, and a projection lens. The film runs through the gate upside-down; the lens inverts the image again, so the screen shows it upright. The indicated enlargement is illustrative and depends in practice on lens focal length and throw distance. Suspended particles can scatter enough light to make the beam path visible.
7. Apparent motion. The visual system interprets a sufficiently rapid sequence of discrete images as motion. Shutter dark intervals and flash rate affect flicker perception; retinal persistence alone is not a complete explanation. Reduce the model to 1–2 fps to inspect individual frames and pull-down intervals.
8. Source material. The built-in strip is an original frame-by-frame animation. The optional reel streams Charlie Chaplin’s “The Immigrant” (1917) from the Internet Archive item archive.org/details/CC_1917_06_17_TheImmigrant when external media access is available.

Model scope: a composite c. 1915 theatre projector built at real scale. The crank, gear train, Geneva movement, 16-tooth sprockets, rear shutter and 1000 ft reels are driven from their tooth counts (8 frames per crank turn, 4 perforations per frame), and the film is one continuous strip whose loops breathe by exactly the slack. Throw distance and screen size are compressed for display.

Try: run at 2 fps with sound · remove the shutter · advance one frame · inspect the Geneva drive and gear train in X-ray view · vary lamp intensity · load the 1917 reel.