On the question of using the frequency of the mains (at least in the
U.S.) for timing purposes, I operated an earth satellite tracking
camera during the IGY (International Geophysical Year) that had attached
to it a synchronous motor that rotated a chopper paddle across the
aperture of the camera once per second based on the 60 Hz frequency
of the mains.  [The chopper introduced breaks in the streaks on the
negatives left by satellites, which could later be used to time the apparent
position of the satellite as it moved across the sky].

The chopper also generated a short pulse that I could align on an
oscilloscope with the 1 second time tick from the WWV time signal.
To my dismay, the chopper time ticks often drifted an unacceptable
amount from the WWV ticks in just a few minutes of operating the
camera.  The problem was _not_ that the chopper motor was slipping,
but that the frequency of the mains was not that stable.

When I contacted the power company about it, I received the (to me)
now famous reply, "We sell power, not frequency!".

Thus, I learned that, at least within the U.S., the frequency of the
mains is generally not at all stable and cannot, on a short term
basis, be counted on to be an accurate 60 Hz standard.  What the
power comanies do attempt to do over a period of hours, days and
longer is guarantee that the _average_ frequency is very close to 60
Hz.  This is why ordinary wall clocks driven by synchronous motors
appear to be so accurate (in the U.S.) over periods of years
(provided, of course, there have been no losses of power).

Cheers,
--- Warren Davis
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