french clock pendulum calls

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    I have a nice french clock with the pendulum missing. It has 4.8 stamped so using the following link: its length should be 126mm.

    However how do I calc the pendulums weight ??? Suppliers offer length & size of pendulum which alters the weight.
    Any help much appreciated.

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    to calculate the pendulum length you need to know the number of vibrations per minute.
    You can then use the following rule: L = t*t*g/pi*pi (L = t square times g divided by pi square)
    where t is the time of one vibration, g is the acceleration due to gravity (9.81) and pi = 3.14
    This gives you the theoritical length in metres.

    De Carle gives a complete explanation (in inches) as well as a table in his book”Practical clock repairing”.

    To calculate the number of vibrations you will need the teeth count of your wheel train.

    Hope this helps. :)


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    I can’t find anything that shows how to work out the weight needed for your french pendulum. I might have a “4 8” pendulum, I will take a look on Monday and if I have one I will weigh it for you. Worse case scenario you can make something up and just keep adding weight until the clock works. It should be somewhere around 2 1/2 ounces.

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