Re:'Temperamental' Big Ben was ringing up to six seconds late
by tobylane on Wednesday August 26, 2015 @05:17AM
(#10121907)
It's had all sorts of challenges over time. The Circle and District are just under the surface nearby, the Jubilee line is deep but still affected the foundations. The building across the road was demolished down to about six levels below ground level. The buildings it is attached (including Parliament and a school) to are up to 1000 years old and need to be vacated for major works from every type of damage, including WWII bombings. Also it's on the edge of what was once an island, and there's an underwater river nearby.
Re:'Temperamental' Big Ben was ringing up to six seconds late
by m0nty on Wednesday August 26, 2015 @03:48AM
(#10121666)
> 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.
Re:'Temperamental' Big Ben was ringing up to six seconds late
by lighthawk on Wednesday August 26, 2015 @08:10AM
(#10122351)
It 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.
Re:'Temperamental' Big Ben was ringing up to six seconds late
by VLM on Wednesday August 26, 2015 @09:06AM
(#10122593)
> 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.
Re:'Temperamental' Big Ben was ringing up to six seconds late
by tfm on Wednesday August 26, 2015 @06:07AM
(#10122026)
The "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!
Re:'Temperamental' Big Ben was ringing up to six seconds late
by makomk on Wednesday August 26, 2015 @05:17AM
(#10121905)
Yeah. 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.)
Re:'Temperamental' Big Ben was ringing up to six seconds late
by richmarr on Wednesday August 26, 2015 @04:47AM
(#10121813)
For 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.
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.
Re:'Temperamental' Big Ben was ringing up to six seconds late
by joe5150 on Wednesday August 26, 2015 @01:10PM
(#10124445)
It'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.
Re:'Temperamental' Big Ben was ringing up to six seconds late
by mintplant on Wednesday August 26, 2015 @04:44AM
(#10121804)
So 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.
Re:'Temperamental' Big Ben was ringing up to six seconds late
by IanCal on Wednesday August 26, 2015 @06:39AM
(#10122097)
The "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.
Re:'Temperamental' Big Ben was ringing up to six seconds late
by Milner08 on Wednesday August 26, 2015 @04:01AM
(#10121697)
Apparently 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?!)
Re:'Temperamental' Big Ben was ringing up to six seconds late
by ealexhudson on Wednesday August 26, 2015 @04:13AM
(#10121730)
It's basically the fixed-chord problem, plus changes in friction / temperature / humidity etc. over time probably means the thing runs at varying rates.
Re:'Temperamental' Big Ben was ringing up to six seconds late
by sdoering on Wednesday August 26, 2015 @05:18AM
(#10121908)
Damn it. Knowing how hard it can be to tune a mechanical wrist watch to be within some seconds of atom time in 24 hours, this kind of precision on a clockwork that big is really amazing.
Would never have thought that. I am totally impressed.
Re:'Temperamental' Big Ben was ringing up to six seconds late
by BobLaw on Wednesday August 26, 2015 @06:43AM
(#10122104)
Definitely a neat fix for something that worked for 150+ years and only needed to be checked a couple times a week.
Now that it has to be manually checked at least once a day though ... I don't know. Seems like a lot of trouble for something that could be solved with an Arduino and a water pump.
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.
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.
I am also thinking of unfortunate scenario where someone adds /removes momentum accidentally while trying to place pennies.
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.
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.
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.
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?!)
Would never have thought that. I am totally impressed.
Now that it has to be manually checked at least once a day though ... I don't know. Seems like a lot of trouble for something that could be solved with an Arduino and a water pump.
I would love to automate this with a nagios-like alert system and perhaps a robot arm to add/remove coins.