Threading the 16mm Projector ~ 1950s Iowa State University

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'How-to film produced to ensure quality shows and prevent destruction of films.'

Originally a public domain film from the Library of Congress Prelinger Archives, slightly cropped to remove uneven edges, with the aspect ratio corrected, and one-pass brightness-contrast-color correction & mild video noise reduction applied.
The soundtrack was also processed with volume normalization, noise reduction, clipping reduction, and/or equalization (the resulting sound, though not perfect, is far less noisy than the original).

en.wikipedia.org/wiki/16_mm_film
Wikipedia license: creativecommons.org/licenses/by-sa/3.0/

16 mm film is a historically popular and economical gauge of film. 16 mm refers to the width of the film; other common film gauges include 8 and 35 mm. It is generally used for non-theatrical (e.g., industrial, educational) film-making, or for low-budget motion pictures. It also existed as a popular amateur or home movie-making format for several decades, alongside 8 mm film and later Super 8 film. Eastman Kodak released the first 16 mm "outfit" in 1923, consisting of a camera, projector, tripod, screen and splicer, for $335. RCA-Victor introduced a 16 mm sound movie projector in 1932, and developed an optical sound-on-film 16 mm camera, released in 1935...

en.wikipedia.org/wiki/Movie_projector

A movie projector is an opto-mechanical device for displaying motion picture film by projecting it onto a screen. Most of the optical and mechanical elements, except for the illumination and sound devices, are present in movie cameras...

Projection elements
As in a slide projector there are essential optical elements:

Light source...

Reflector and condenser lens...

A positive curvature lens concentrates the reflected and direct light toward the film gate.

Douser
(Also spelled dowser.)

A metal or asbestos blade which cuts off light before it can get to the film... Dousers protect the film when the lamp is on but the film is not moving, preventing the film from melting...

Film gate and frame advance

If a roll of film is continuously passed between the light source and the lens of the projector, only a continuous blurred series of images sliding from one edge to the other would be visible on the screen. In order to see an apparently moving clear picture, the moving film must be stopped and held still briefly while the shutter opens and closes. The gate is where the film is held still prior to the opening of the shutter. This is the case for both filming and projecting movies. A single image of the series of images comprising the movie is positioned and held flat within the gate. The gate also provides a slight amount of friction so that the film does not advance or retreat except when driven to advance the film to the next image. The intermittent mechanism advances the film within the gate to the next frame while the shutter is closed. Registration pins prevent the film from advancing while the shutter is open. In most cases the registration of the frame can be manually adjusted by the projectionist, and more sophisticated projectors can maintain registration automatically.

Shutter

It is the gate and shutter that gives the illusion of one full frame being replaced exactly on top of another full frame. The gate holds the film still while the shutter is open. A rotating petal or gated cylindrical shutter interrupts the emitted light during the time the film is advanced to the next frame. The viewer does not see the transition, thus tricking the brain into believing a moving image is on screen. Modern shutters are designed with a flicker-rate of two times (48 Hz) or even sometimes three times (72 Hz) the frame rate of the film, so as to reduce the perception of screen flickering. (See Frame rate and Flicker fusion threshold.) Higher rate shutters are less light efficient, requiring more powerful light sources for the same light on screen.

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