Meadows & Passmore's French Clock Pendulum Length Calculator ©
How to calcuate the length of a French clock pendulum from the numbers on the backplate
Many French clocks have the pendulum length stamped on the backplate, in the French pre-metric 'Pouce & Lignes' system. You may find a number such as 4.2
If you multiply the first part by 27.07 (the millimetres in a Pouce) and the second part by 2.256 (the millimetres in a Lignes) you will find the original pendulum length.
To take our example of 4.2:
4 * 27.07 = 108.28
2 * 2.256 = 4.512
108.28 + 4.512 = 112.792mm
Of course this is far too accurate a conversion, but we know the pendulum in our example should be between 110 - 115mm long. When cutting a new pendulum rod, it is always wise to cut it on the long side, with the Brocot adjuster up. Drill out the hook to be a tight friction fit and test the formula before finally cutting to length and riveting the hook in place.
The length calculated is from the top of the flexible part of the suspension spring to the centre of mass of the bob. In practical terms this is the centre of the bob, but in theory it will be slighlty above the centre due to the rod.