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Clocks & Pocket Watches

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Before Marv chimes in, I'll do so from my Physics 101 classes many years ago.

The period of a frictionless pendulum is dependent only on the length from the pivot point to the center of gravity.  Since the angle of swing does not affect the period, a clock pendulum doesn't need to swing very far from side to side.

Because temperature causes clock pendulums to expand and contract, you often see them made of several metals in an attempt to counteract each other's expansion.

Since we're getting a bit technical here, did you know a pendulum clock will ont operate correctly in a tall building in the upper floor. The natural sway of a building in the wind will cause a pendulum clock to not hold the right time or will cause it to go out off beat and stop all together.


Dong!  (Sound of Marv chiming in)

--- Quote ---Since the angle of swing does not affect the period, a clock pendulum doesn't need to swing very far from side to side.

--- End quote ---

It's a bit more subtle than that.  If you write the differential equation for a simple pendulum, you can only get a simple solution (the one where the period is given by T=2*pi*sqrt(L/g)) if you make the assumption that the angle through which the pendulum swings is small enough that you can safely make the approximation sin(theta)=theta, where theta is the excursion angle.

Thus, if you keep the pendulum excursion small, it will act like a true sinusoidal oscillator, which is what you want.  Since, in a clock, there's no need for large excursions in order to drive the escapement, designing for stable predictable performance is easier with small excursions.

The Dent that Bernd posted is what is known as a "Fusee" movement. The fusee was to keep the spring power constant as it unwound. Donald de Carle, In his book, Practical Clock Repair, devoted several chapters to the building of a "Fusee". The machining of the fusee is somewhat challenging. Most, if not all, mass produced fusee movements were made in England.


I have both of  ' de Carle's books, clock repair and watch repair. Infact, I have two copies of Practical Clock Repair, I believe my older addition to be worth a bit of money as an older book as well as the content. It has many adverts in the front on glossy paper which is not in the newer version (reprint 8 of Second Edition) . I`ll dig it out as I can`t remember what edition it is, I tend to keep it safe.



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