A Mini Tower Clock

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Because the weight of a bob does not affect the period of the pendulum.  Only changing the length does.

Theoretically that is correct, as every physics student learns.

However, in the real world the pendulum experiences frictional forces and air resistance forces. A change in weight (alone) can make a small difference. Of course the adjustments are for very small corrections.

Gene

[crueby]

Kim, they did not add weight to the bob, they were adding it somewhere up near the top, not sure exactly where - might have been off center near the top? Would need to do some internet searches on that.

EDIT: found the explanation - they add/subtract pennies up at the top, which lowers/raises the center of mass of the pendulum, which changes the effective length, which is what determines the period. When you lower the bob, you lower the center of mass. Raise the bob, raise the center of mass. Same effect, two different methods. Their pendulum is 13 feet long and weighs 660 pounds, so adding/removing pennies is a really tiny change, but its a lot easier than trying to adjust the bob position when its moving, and stopping starting that pendulum would be a major undertaking.  One penny changes the period by 2/5 of a second per day, quite fine adjustment.

[Sanjay F]

Here you go chaps - I remember seeing the pennies mentioned on a science programme when I was a young'un

https://www.nbcnews.com/video/big-ben-clock-corrected-using-old-pennies-512956995562

[crueby]

Thats the one! I think I saw that same video years ago - thanks for finding it! 

I looked once to see if I could find plans for that clock, never found any - would make a really cool model. That clock is bigger than my shop!

[Sanjay F]

I was tryng to work out how long the pendulum would need to be on your potential model as The Big Ben pendulum is 4.4 meters (approximately 14 feet) long and weighs 310 kg! 

[crueby]

I was tryng to work out how long the pendulum would need to be on your potential model as The Big Ben pendulum is 4.4 meters (approximately 14 feet) long and weighs 310 kg! 

I'd have to scale things down quite a bit just to fit the gears on my mill! Some of the ratios would have to change to handle a shorter pendulum, had to do the same for the current model build since Wildings clock had a 3 foot pendulum. Hmm, a 14 foot pendulum would require cutting a slot in the floor, and some new beams to handle the extra weight!  That much moving mass in a normal house would probably shake it off the foundation... 

[cnr6400]

  If you ever built the full size clock with the 14 foot 660 lb pendulum, a scrap 1970 Cadillac 500 cu in V8 would be almost the exact weight needed...you could take off / put on one exhaust manifold nut to adjust the time - no pesky finding of a penny coin needed! 

How tall's the hay stack   pile of brass swarf in the back yard now that the gears and pinions are completed? 

[crueby]

If you ever built the full size clock with the 14 foot 660 lb pendulum, a scrap 1970 Cadillac 500 cu in V8 would be almost the exact weight needed...you could take off / put on one exhaust manifold nut to adjust the time - no pesky finding of a penny coin needed! 

How tall's the hay stack   pile of brass swarf in the back yard now that the gears and pinions are completed? 

I'm  gonna need to beef up my Ohio crane model...




I wonder how many spoonfuls  of brass swarf equal the weight of one British pre-decimal penny? "I'll  take Useless Conversion Factors for 100, Alex"

[uuu]

Accurate clock Pendulums by Robert Matthys is a terrific book.  It explores pendulum design in great depth.  But it's out of print, I think, and second-hand copies are rare and expensive.

For example, if you support the bob at the bottom, then a change of temperature will have the bob expand upwards.  But the pendulum expands to get longer, and if you arrange it right, the two effects cancel out.

But - the pendulum is thin and absorbs heat and expands quickly - the bob is fat and expands slowly.  So how to arrange things to sort that out?  It's all in the book.

Wilf

[John MacArthur]

In almost all high grade regulator clocks the pendulum rate is changed by adding or subtracting minute weights either on top of the bob or on a small tray part way up the rod. As mentioned this raises the center of mass.  Additionally, stopping and starting a high grade pendulum almost always changes the timekeeping, so one adds or subtracts weights with tweezers while not stopping it.  Finicky stuff.  I have one of my clocks dialed in so that it lost 8 seconds from the first of Jan '25 until summer, then it gained it back and is now 5 seconds fast.  People have gone crazy over the last 300 years trying to improve accuracy of mechanical clocks, and improbably, are still at it.
Johnny

[crueby]

Great information  guys, thanks!

[mklotz]

I've seen numerous clocks with vials of mercury as the pendulum weight.  I used to think that mercury was used because it is very dense but in light of what I'm reading here it might be for reactive adjustment of the length with temperature changes.  As the rod expands and lengthens the mercury expands and grows slightly higher in its glass container thereby raising the center of mass ever so slightly.

[crueby]

Sounds like there have been numerous methods to maintain pendulum period over the centuries, more than I ever imagined. For those who have not read it, I  heartily recommend  reading Dava Sobel's book Longitude, about Harrison's quest to come up with a clock that could be used in navigation at sea. Brilliant work early on in clockmaking. His four timepieces for that are on display over at the Greenwich  Maritime Museum by London. A big reason I took a trip over there in 1996. Plus the gazillion  other things to see there!

[Sanjay F]

That is one of my favourite books of all time and the Greenwich museum is somewhere I visit over and over. Also nice to have your picture taken with one leg each side of the Greenwich meridian.

Mr Harrison didn't get his £20,000 prize straightaway as the Royal Navy held the money back; I can't imagine how much that would be worth now ........ maybe enough to buy one of his original timepieces.........hmmmmmm

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Sounds like there have been numerous methods to maintain pendulum period over the centuries, more than I ever imagined. For those who have not read it, I  heartily recommend  reading Dava Sobel's book Longitude, about Harrison's quest to come up with a clock that could be used in navigation at sea

Of course the entire reason for Harrison's quest, the prizes, etc, is that pendulum clocks don't work on a moving ship.

Land-based pendulum clocks could be made quite accurate long before Harrison.

Gene