John Spradley wrote:

> I have often wondered why wagon wheels on the silver screen
> often went backwards!  Can't always believe my eyes.  Can you?

It is caused by the stroboscope effect, since the movie is projected
at 24 frames per second.  For each frame, the wheel rotates a bit, but
at certain speeds it will look like they turn backwards when the wheel
rotates nearly the distance between two spokes for each frame.

Let's say the wheel has 12 spokes, that would give us the angle
(360/12)=30! between two of them.

Now, if the wheel rotates two rounds per second, e.g. 720 degrees, we
should divide that with 24, since there's 24 film frames per second.
That gives us 30!, which is the number of degrees that the wheel
rotates for each frame.

This will make us believe that the wheel isn't rotating at all since
next spoke is at the same place as the previous one, for each new frame
of film projected.  Now, if the wheel rotates a tad slower, it will
look like the wheel rotates backward slowly since "next" spoke seem to
be a little behind the previous one, and vice versa when the wheel
rotates a bit faster than two rounds pr second.

The example I made after visiting http://www.emainc.com/radnor/wagon.htm
It seems that 12 spokes in a wagon's wheel is a "standard"..? :-)

The same strobe effect you can use to measure exactly if your
gramophone turntable revolves at the right speed. I have once seen a
small round paper disc the same size as a LP label, with radial
patterns for 16, 33-1/3, 45 and 78 rpm.  It simply uses the fact that
the light from a bulb varies with the AC-periods.

Stroboscope discs from USA cannot be used in Europe and vice versa
since the mains frequencies aren't the same (60 Hz vs. 50 Hz).  When the
turntable rotates at the right speed the corresponding pattern to the
speed set on the turntable should appear to stand still.  Some modern
turntables also have built in "strobe lines" and a fine adjustment knob.

This measurement method has one major disadvantage:  If the gramophone
went at the wrong speed because of wrong AC mains frequency (probably
won't happen nowadays, but could happen before) you couldn't detect it
with the stroboscope effect.  The AC frequency would be too fast/slow,
so also the gramophone motor speed, _and_ also the blinking of your
bulbs.

Thomas Henden

 [ Musicians who enjoy really old gramophone recordings complain that
 [ the disks often weren't recorded at a standard speed.  They note that
 [ the important thing is to reproduce the sounds at the same musical
 [ pitch which the artists played.  Some modern CD anthologies make the
 [ necessary speed/pitch corrections.  -- Robbie