Question about clock pendulums

I'm just completing my first clock, and puzzling over pendulums.
Does anyone in the group know the significance of the weight of a
clock pendulum bob? I know that the ideal pendulum has a weightless string and a point mass, so the heavier and denser the bob on a real pendulum, the closer it gets to perfection, but what else determines the weight and form of the bob?
Is the shape of the bob important (for aerodynamics perhaps)?
TIA
Mike
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Well if wasn't asleep in school physics lessons 40 years ago, istr period = 2 pi root (l/g) so is independant of the pendulum mass. If the pendulum and its support offer more air resistance presumably you have to put more energy 'in' at each tick to keep it swinging, hence the disc shapes often seen that present a thin edge to the air. My instinct (not very scientific!) tells me that the more energy you have to add to keep the pendulum swinging, the less dominent will be the natural resonance of the pendulum, so the timing will be affected by whatever is adding the energy (escapement wheel, electric impulse etc). Certainly this effect is noticable on balance wheel French clocks (I suppose that the balance wheel is analogous to a pendulum) where they go faster when fully wound.
AWEM
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On Sat, 1 Sep 2007 13:10:31 +0100, "Andrew Mawson"

The motion of a pendulum isn't as nicely linear as school physics would have us believe - see:
http://physics.usask.ca/~hirose/ep225/animation/pendulum/anim-pendulum.htm
So as the amplitude goes up, the frequency goes down (i.e., the clock runs slower). The better spring-driven clocks counteract this by use of a "fusee" that attempts to give a constant torque output from the drive to the clock train.
So, curiously, a cheap clock should actually run faster as its spring winds down.
Using a larger bob supposedly improves the "Q" of the pendulum (i.e., it will take longer for its amplitude to reduce to zero from a given starting amplitude) and hence the timekeeping properties also improve; however, you also need more drivibg force to achieve the same amplitude with a heavier bob.
Regards, Tony
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In crude terms, the mass of the pendulum is irrelevant providing the suspension is flexible enough to have no significant influence on the rate, i.e. the restoring force on the pendulum is predominantly gravity. The rate of the clock is then governed by the effective length of the pendulum. Aerodynamics, buoyancy, temperature, and air pressure really only become important in very accurate clocks; they can be ignored for most domestic clocks
Cliff Coggin (at last making a contribution to the list in my role as a horologist.)
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On 1 Sep, 12:47, snipped-for-privacy@virgin.net wrote:

The ideal pendulum has the minimum interference from outside influence. Even draughts will affect a pendulum. A heavier bob will be less affected by any influences that you can't avoid.
Reseach has shown that a rugby-ball shape is the best, mounted sideways, so the "points" are to the left and right. Also good is a disc with flat front and back and shamfered edges, rahter like a throwing discus. But both these types have to be mounted very accurately, so they are not swinging at a slight angle. A sphere avoids this problem and is only a few percent worse than these two. Easist to make is a cylinder, but not quite as good as the others. But the pendulum rod has an aerodynamic effect, and thin rod makes very significant improvement.
Dont neglect the pendulum suspension. This can have more influence on the timekeeping of the clock than any other aspect of pendulum design.
If you can borrow a copy of "Accurate Clock Pendulums" by Robert J Matthys, all your questions will be answered. Buy a copy if you are really keen, and have 50.
Wilfrid Underwood
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Andrew, Tony, Cliff and Wilfrid
Many thanks for the concise and helpful replies. Between them I have all the information I could wish for!
Regards
Mike
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