5 ms·
> The clock is altered by adding and removing old coins from its pendulum to change the rate at which it swings. British engineering at its finest. I understan
by m0nty 11y ago
> The clock is altered by adding and removing old coins from its pendulum to change the rate at which it swings.
British engineering at its finest. I understand they have a stock of discontinued six-pences and ha'pennies which weigh only a few grammes each and are used for these fine adjustments.
- Milner08 11y agoApparently so! I love the idea of them taking off a coin to speed it up and adding it back again once its in time. For such a famous clock you would think they'd create some kind of perfectly accurate weight, but I guess this gives them more control (Easier to add and remove weight, and why make custom weights when you have a few in your pocket?!)
- ealexhudson 11y agoIt's basically the fixed-chord problem, plus changes in friction / temperature / humidity etc. over time probably means the thing runs at varying rates.
- rurounijones 11y agoDo they literally have to put these and take them of as the thing is swinging back and forth? I am also thinking of unfortunate scenario where someone adds /removes momentum accidentally while trying to place pennies.
- CiaranMcNulty 11y agoI don't believe a pendulum's period is affected by its momentum at any point in its arc, just the length and its mass.
- joe5150 11y agoJust the length, rather. The mass has no effect on the period.
- mintplant 11y agoSo how does adding a coin change the clock's speed? The extra mass can't be changing the period of its pendulum, right? This is the bit of the article I didn't understand.
- IanCal 11y agoThe "length" that's important will be the distance to the centre of mass of the pendulum. By adding weights part-way along the pendulum, you move the centre of mass just like if you'd found a good way of adjusting the length.
- richmarr 11y agoFor a frictionless pendulum in a vacuum this is certainly true. The article repeatedly says they add weights to the pendulum though, which makes me wonder whether either (a) it's some connected mechanism they add weights to, or (b) this is somewhere reality diverges from the simple model.
- deleted 11y ago[deleted]
- rtkwe 11y agohttps://youtu.be/5qEPDXDVxvw?t=169 https://youtu.be/5qEPDXDVxvw?t=169 They're added directly to the 'pendulum shelf' which looks like it's right at the top of the pendulum. That would adjust the length up (shorter) by a tiny amount by shifting the center of mass of the pendulum up. This squares with the simple model of a pendulum.
- joe5150 11y agoIt's true the weight and shape of the bob will have an effect on the air friction applied to the system, but for a pendulum of this scale, the effect of adding a coin or two will be a negligible change in drag compared to the relatively significant change in the pendulum's center of mass.
- mkj 11y ago
- mapt 11y agoThe pendulum's period isn't affected by its bob mass at all, in the simple models of a pendulum, only length from axle to center of mass of the bob. I invite an armchair physics teacher to explain why this is different. Probably something to do with shifting the center of mass closer to the axle.
- makomk 11y agoYeah. They're actually putting them on a shelf a fair way up the pendulum itself, so I imagine that adding them shifts the center of mass noticeably. (Though we're talking single-digit parts per million corrections here, so they don't need to shift it by a huge amount.)
- tfm 11y agoThe "length of the pendulum" (for the purposes of calculating its period) is specifically the distance between the the pivot and the pendulum's centre of mass. Adding coins to the pendulum moves the centre of mass ever so slightly!
- ldite 11y agoYup - see https://youtu.be/5qEPDXDVxvw?t=169 https://youtu.be/5qEPDXDVxvw?t=169
- rurounijones 11y agoAh, that is much less dramatic than I thought. Thanks for the video.
- pavel_lishin 11y agoI'm thinking of the scenario where that guy loses his balance, falls onto the pendulum, and scatters all of the coins into the mechanism below.
- Sarkie 11y agoIt might be to do with the crack in it?
- lighthawk 11y agoIt seems like adding an electromagnet, whose power is adjusted by something tracking how well the clock is keeping to another more accurate source of time, could be employed to help fix this. It might require changes to the pendulum, but they might be able to be made without much interruption.
- TylerE 11y agoOne of the key points to a reliable clock is not requiring electricity. Big Big is all mechanical. Would be a pity to start adding active components.
- VLM 11y ago> It might require changes to the pendulum Not necessarily, tweak it to run slightly fast and rely on the natural dampening ability of a magnetic field to generate a slowing force on moving conductors, then apply a (an extremely) modest (AC to demagnetize) and controlled magnetic field to slow it down to dead on accurate (probably as determined by a $10 GPS module, or I guess a 7 or so pound gps module). Also you can sense pendulum position with the usual optical technologies, so no contact with historical relics needed there either. Given the "fun" of controlling precision currents vs the ease of controlling precision times/durations its probably easier to run a standardized constant current thru the correctional electromagnet for a varying X nanoseconds just before the top of the hour rather than trying for a continuous 8 bit precision field that'll never really settle down. A better design than running the correction routine hourly would be continuous position/time monitoring and run the correction field every time the phase variance between the clock and a GPS receiver exceeds a tenth of a second absolute difference or whatever, then monitor how often that correction happens (and alert when broken, etc). The best part of doing it non-contact is other than dust not settling where the apparatus is sitting, hundreds of years from now researcher types will have no evidence the apparatus was there. Which I guess would be good if they're researching 1800s clocks and bad if they're researching 2000s microcontrollers... hmm.