I don't know this for an absolute fact but most of the
machines I worked on had various forms of the mechanism for
showing individual frames as stills. One would move a lever and
the shutter and claw mechanism stopped. That's when the filter
would drop down automatically. The machine I mentioned where the
still filter was wandering down in to the light path was a
classroom projector, possibly a Singer or maybe a Bell and
Howell. The mechanism was pretty clever. The drive shaft that
spun the shutter wheel and claw mechanism also had a slip clutch
which had a driven part inside of a sealed cup of grease. As the
driven part turned, the grease made the outer part turn with it
but not with much force. The filter was built to be heavy so if
the clutch wasn't being spun, the filter arm would fall straight
down, putting the filter in the path of the light.

	When one started rolling the film either forward or
backwards, the clutch began to spin again and gently raise the
filter out of the way. It would hit a stop and just rest there as
long as the film was moving.

	In the caseof the broken machine, the grease had
gradually leaked out or evaporated over many years and it no
longer pulled the outer part along.

	When I was working as a technician, we got machines that
were as old as I was, in my thirties at the time.

	We frequently got Bell and Howell projectors that were
new in the fifties. They had great sound but could shred film
like nobody's business.

	There was another rather expensive machine called a
Kelhart-Victor which was also a very good machine but as gentle
to the film as the Bell and Howells were rough. They had a trip
lever that stuck out in to the bottom loop below the film gate.
If the film so much as brushed this lever, a mouse-trap like
mechanism tripped and the machine stopped cold.

	The Victors also confused some folks because their supply
and takeup reels were reversed so they appeared to work backwards.
I hate to say it, but it wasn't too hard to confuse some folks.

Martin

Tom Fowle <[log in to unmask]> writes:
> Knowing how things work, or used to work is never useless.
> At the least it's interesting.
> 
> Used to run 16 MM projectors in highschool in the 60s, but don't remember
> the filter. Maybe those school units didn't have the freeze frame.
> thanks for the fun
> tom Fowle WA6IVG