[time-nuts] Hello

Tom Van Baak tvb at LeapSecond.com
Wed Mar 28 06:14:28 UTC 2007


Hi Neville,

Welcome to the world of extreme accuracy... Not to
discourage you; but to challenge you -- see answers
below.

> My work with a pendulum seems to point to the need to know the exact  
> phase and amplitude of the pendulum as being a sticking point.

Right, but calculate this to the microsecond in time or to
the micron in displacement. It's a really hard problem.
There is white noise. There is drift. It will affect the stability
of the pendulum. There is no "exact" measurement; even
if it's time or if it's distance.

> The pendulum can be mounted firmly, with no yielding or flexing of  
> brackets on a masonry mount bedded well into the ground.

Sorry, there is no such thing as "firm" and at some level
everything flexes or bends. Do the math. A highway I-beam
looks super super strong. Until you see one transported on
a truck and you notice it bends maybe an inch over a span
of 80 feet. That means it flexes by 0.1%! That's a micron
per millimeter. Can't have that in a pendulum. Or consider...

A pendulum with a 10 pound bob on a ten pound frame will
cause the frame to move as much as the bob. Clearly, this
is bad. So, hey, use a 100 pound frame. Now the support
moves only 1/10th as much. Still bad. Use 1000 pounds;
No, use a house foundation that's 10,000 pounds, that's
5 tons. You've still only attenuated the movement of the
frame to 1/1000th the lateral movement of the bob. Do you
see the problem? The bob moves 1 cm but your frame
moves 0.01 mm. Yikes. Do you need 50 tons? Or 500?

When dealing with precision pendulums you cannot say
something has "no effect". You have to calculate what the
effect actually is. It might be small but it is never zero. It
will affect your accuracy.

> The pendulum can be made mechanically stable with accurately fitted  
> joints between shaft and bob, and shaft and suspension.

There's several hundreds years of history of attempts to
achieve this. Very hard problem.

> The pendulum can be excited with a minute magnet and an air cored  
> coil to drive it with a few nanowatts.

Careful with the magnet. The earth is a magnet also and
the local field changes over time; both over the long-term,
and over the short-term (e.g., depending on solar activity).
At the level of a precision pendulum, electromagnets are
also highly voltage, current, and temperature sensitive. Do
the math to see what your lower bound is. There is also
drift and perhaps hysteresis in magnetic systems.

Nanowatts of magnitude are fine. But can you hold it constant
to picowatts over the duration of the experiment?

> The pendulum can be shielded from air currents and vibrations.

Shielding is relative. Most places on planet earth experience
vibrations. Air currents are usually solved only by putting the
pendulum in a vacuum. However, that might introduce other
challenges. Note that Shortt did not operate in a high vacuum.

> However accurate signals for phase and amplitude measurement with  
> accuracy of microseconds for phase and microns for amplitude seem to  
> be the challenges that must be met, this information is necessary to  
> generate the pendulum drive so that the pendulum is the only  
> frequency determining element.

Correct. However, the danger here is that many modern
methods to determine precise and accurate pendulum phase
and amplitude depend, directly, or indirectly on clever
digital electronics, precise timing, or microprocessors;
electronics whose very accuracy is driven by, you guessed
it, quartz oscillators.

When you say "information" you imply a measurement
system that "knows" the behavior of the pendulum. So what
is the "real" clock then: the mechanical pendulum or the
electronic measurement system that provides feedback
to the pendulum? That's a hard question.

You have to be very careful that your pendulum is still free
and not benefiting, in any way, from the precise measurement
and feedback afforded by modern quartz timekeeping. Otherwise
what you a building an "almost-free" pendulum clock that is simply
governed, or stabilized, by a quartz oscillator.

> Compensation for temperature and barometric pressure are do-able,  
> although the legendary clocks were in a vacuum (more or less) and in  
> underground clock vaults kept at constant temperature.

Nothing is constant. Being underground just means that your
temperature doesn't vary by degrees, it varies by tenths of
degrees. Again, do the math; determine the actual tempco
of your system.

> I still have hopes of getting great performance from a room  
> temperature clock at ambient pressure.

I applaud your efforts. The proof will be in the data you get
from your clock. It might be possible. It certainly would be
very interesting. Expect to collect a year of data to be sure.
Keep us, or the members of NAWCC HSN 161 updated.
Several HSN members, I think, are trying to match the
performance of the 1920's Shortt pendulum clock. If you
want to write more, perhaps time-nuts isn't the best forum;
I think we're mostly modern electronics timekeepers here.

/tvb

> See
> http://ph.groups.yahoo.com/group/synchronome/photos/view/c1ba?b=1
> 
> cheers, Neville Michie
>

Can't see that URL from here. Can you email it to me, offline?





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