[time-nuts] Non electrical time-nuttery

Tue Jan 12 10:19:26 UTC 2010

These are both good books, in fact the best out there, but both have  
significant flaws.
For instance they cannot unequivocally accept that high Q is  
fundamental to good time keeping
in a clock.
They look at an un-sustained pendulum and say it only oscillates for  
10 minutes, so
obviously it cannot measure beyond that time. They fail to realise  
that in a clock,
losses are made good so the oscillator has infinite Q. And an  
oscillator with infinite Q
can define any length of time.

Any form of friction or loss is bad as it increases the amount of  
make-up energy
that must be supplied. That can only increase the possibility of  
frequency or phase drift.

There are two main barometric pressure effects. First is buoyancy,  
the pendulum losing
apparent weight by floatation in the air. The inertia stays the same,  
but the attraction to
the centre of the earth is decreased. That attraction is the  
restoring force.
The second effect is that the pendulum bob displaces a mass of air  
that sloshes around
the clock case. This air has momentum and adds slightly to the  
momentum of the pendulum.

The air will also cause the increase in damping that effects  
amplitude, and via circular error,
change the frequency, but this is a lesser effect or non-existent  
when electronic feedback
is used control amplitude. Only clock people think they can run an  
oscillator without
amplitude control.
In my experimental clock I control amplitude and use circular error  
to adjust timing.

You may have gathered that my time nut activities are directed to  
pendulum clocks,
in particular an improved pendulum.

Buoyancy can be neutralised in a suitably shaped compound pendulum.   
Consider a light
cylinder pivoted in the middle.
No matter how dense the medium it is in it will experience no torque.  
Put a weight in one end
and you have a buoyancy compensated pendulum.

Circular error is more difficult, but if you have a knife-edge  
suspension with a finite radius,
the circular curve crosses the cycloid (the curve of an isochronic  
pendulum) at a low amplitude,
  and like a quartz crystal at its thermal turnover, you have an  
amplitude where small changes
in amplitude have no effect on time.

Torsion pendulums are also sensitive to gravity. When you twist the  
suspension it gets slightly shorter.

The best mechanical timekeeper is probably the quartz crystal, it has  
stable mass, stable elasticity,
stable dimensions and very low damping losses. Put it in a vacuum,  
keep its temperature constant,
make good its losses  and you have a good clock.

cheers, Neville Michie

On 12/01/2010, at 7:41 PM, Don Latham wrote:

> I'm not absolutely certain, Hal, but I think high Q has to do with  
> the spectral purity of the source, but not necessarily with its  
> long term stability, which depends on other design factors. I can  
> only say about the damping that I dimly remember it in connection  
> with the Shortt clocks but cannot remember where.
> This track caused me to go into my stacks (I really thought I was  
> free of the pendulum compulsion, but apparently 'taint so). There  
> are two books i'd recommend:
> Mathys, Robert J, "Accurate Clock Pendulums",Oxford University  
> Press, NY, 2004, ISBN0 19 852971 6 (Hbk) ; this is a real bible  
> about vertical pendula. Amazon lists this one, both used and new
> And for a personal, very entertaining, refreshing, and thorough  
> examination of pendulum clock building,
> Woodward, Philip, "My own Right Time : An Exploration of Clockwork  
> Design", Oxford University Press, NY, 1995, ISBN 0 19 856522 4 I  
> really enjoyed this book! Amazon has this one both used and new.
> If you get these used, you'll spend about $170, but you will avoid  
> endless hours of fooling around. OTH if the fooling around amuses,  
> then. . .
> BTW, Invar has a really low tempcoefficient, but tends to random  
> shifts in its structure, yielding to abrupt length changes. Can't  
> have everything, I suppose...
>
> Best to all,
> Don Latham
>
>
>
>
> ----- Original Message ----- From: "Hal Murray"  
> <hmurray at megapathdsl.net>
> To: "Discussion of precise time and frequency measurement" <time- 
> nuts at febo.com>
> Sent: Monday, January 11, 2010 8:04 PM
> Subject: Re: [time-nuts] Non electrical time-nuttery
>
>
>>
>> djl at montana.com said:
>>> Also, according to some info I read on pendulum clocks, vacuum does
>>> not work because some viscous drag is needed to damp the free
>>> pendulum;
>>
>> Why do I need damping?
>>
>> I thought the general idea on oscillators was to get the highest Q  
>> (lowest
>> damping) that was reasonable.  But now that I think about it, I'm  
>> not sure
>> why.
>>
>>
>> -- 
>> These are my opinions, not necessarily my employer's.  I hate spam.
>>
>>
>>
>>
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