Pendula

[John HaineParticipant @johnhaine32865]

Posted by Mick B1 on 18/11/2022 13:14:32:

Without reading the book – why were they messing about with coppers to set the pendulum period?

Even quite modest pendulum clocks I've seen have a screw-ajustable bob, which would presumably be positionable within fine limits. You'd expect Big Ben's to have an ultra-fine thread adjustment with vernier graduation to set it within tenths…

Well, two things. First, adjusting the bob with a screw is surprisingly coarse when you are chasing ~0.5 s/day. Second, you have to stop the pendulum and fiddle with it! It can take days to settle down after that. The Big Ben pendulum weight 203kg, so there's a LOT of weight on any screw thread that has to be slightly lifted and held to allow the rating nut to turn.

You can place a penny on top of the bob, or on a weight tray further up, while the pendulum is swinging, and remove it with tweezers or a soft brush. If you've got an electronic period monitoring system you can see the effect almost immediately.

[duncan webster 1Participant @duncanwebster1]

It would have to be very fine indeed to have the same effect as adding one old penny weighing 9.4 gm to a pendulum weighing 203kg. One penny speeds the clock by 0.4 seconds per day. Second advantage is you can add pennies without stopping the clock

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Mick B1 on 18/11/2022 13:14:32:

Without reading the book – […]

.

I would recommend reading the book

Oh yes … silly me, I did that already

MichaelG.

[Martin ConnellyParticipant @martinconnelly55370]

Chris, the affect of gravity is to cause an acceleration, the speed referred to is the speed of propagation of gravity waves across a distance which was not specifically stated in the post above. So no issue with the use of speed in this case and an understandable misunderstanding in your case.

Martin C

[Mick B1Participant @mickb1]

OK, fair answers.

There was me wondering if maybe it'd never occurred to 'em…

[SillyOldDufferModerator @sillyoldduffer]

Posted by ChrisLH on 18/11/2022 12:48:15:

SOD,

Did you mean "speed" of gravity. ? My contact with same was always in terms of "acceleration due to ….". I've never even wondered about its speed ! Perhaps it comes into astronomy and is not important for my main interest, mechanical engineering.

Just pick a nit to death, are not all pendulums "compound" since we can not pack mass into a point suspended on a weightless rod as demanded by the definition of a simple pendulum ? Just asking.

I did mean speed, but you're right, for all practical purposes gravity is a force understood in terms of 'acceleration due to'. I don't think gravity having a speed effects mechanical engineering at all, but it's about the nature of gravity, which is unclear, and adds 'what, where, when' knowledge that gets science closer to the 'how and why'. Understanding might have a practical application in future; it often does, as with electrons.

Speaking of electrons, they too are mostly understood in terms of their effects, but it seems they have mass and no dimensions, that is they're a point mass ideal for making a perfect pendulum. Job done, except we also need a weightless rod…

Even though gravity is a weak force, we're all familiar with it because it stops us and stuff from floating away. Everyone is being pulled towards the earth's centre. This begs the question, 'what stops us from being pulled through the floor and into the core?', Most worrying because matter is mostly empty space, there's literally almost nothing in a steel block.

We're saved from being horribly sucked under by Coulomb's Force. The electrons spinning around the atoms making up my feet interact with electrons spinning in the floor, and stop me sinking in. Not many have heard of Coulomb Force but it's considerably more powerful than gravity. Falling 20 metres onto concrete stops the victim dead in his tracks. Mechanical engineers allow for the effect of the force (hardness and softness), and don't normally worry about the details. Matters very much to anyone doing high-technology though, but high-tech is laboratory and R&D territory, rarely tackled in a home workshop.

Dave

[BazyleParticipant @bazyle]

When the above mentioned clock was made pennies were pretty valuable given a day's wage could be less than a shilling. I wonder who was providing the coins with such abandon.

[Martin KyteParticipant @martinkyte99762]

The Westmnster clock is regulated as per " .…used at Westminster, where it is easy to put on small weights without disturbing the slow and heavy pendulum at a third of the way from the top.

so presumably on a weight tray. Larger changes are made by as consequence of thunderstorms etc by .. either stopping the scape wheel or letting it trip one beat by lifting a pallet (one of the gravity arms) which alters it by 4 seconds.

The clock has a pendulum of nearly 700lbs of which the bob is approx 4 cwt.The suspension spring is 1/60th inch thick and 3 inches wide with a free part of 5 inches. The pendulum cock and the floor behind is so arranged as to allow a tall man to stand with his head inside the cheeks of the cock so as to look square on at the escapement.

Quotes nd Information taken from Dennison, Clocks Watches and Bells.

regards Martin

As I understand it the addition or subtraction of the small weights was done by the clock keepers to adust the time displayed rather than any permenant alteration of the pendulum period. Basically allowing the clock to be advanced or retarded a little to account for vaguaries of weather etc.

Edited By Martin Kyte on 18/11/2022 18:07:20

[David NobleParticipant @davidnoble71990]

Posted by Michael Gilligan on 18/11/2022 12:34:30:

For David, and anyone else with a serious interest … I recommend starting here:

**LINK**

https://www.gutenberg.org/files/17576/17576-pdf.pdf#page50

… and then maybe check the title page

MichaelG.

Thank you once again Michael, I have downloaded the book and will read it with interest.

I have checked the title and felt a little humble.

David

[Nicholas FarrParticipant @nicholasfarr14254]

Posted by Juddy on 18/11/2022 12:10:36:

its explained here: How Does Big Ben Keep Accurate Time? – YouTube

Hi, where the penny was place in the Juddy's link, is just as I saw it in the program that I watched many years ago, and it had the same two stacks of pennies there as well.

I knew that altering the weight of the bob didn't make any difference, as I also saw a program where they showed large candle holders suspended by chains from the roof in a church. There were several of these and some were bigger and heavier than others, but they all were hanging at the same height, and someone noticed that they were all swinging in total synchrony. I can't remember who it was that was watching them and was puzzled as to why they remained in synchrony, but I think it was someone who realised the fundamental law of pendulums.

Regards Nick.

[Kiwi BlokeParticipant @kiwibloke62605]

Posted by Nicholas Farr on 18/11/2022 22:17:46: … I also saw a program where they showed large candle holders suspended by chains from the roof in a church. There were several of these and some were bigger and heavier than others, but they all were hanging at the same height, and someone noticed that they were all swinging in total synchrony. I can't remember who it was that was watching them and was puzzled as to why they remained in synchrony, but I think it was someone who realised the fundamental law of pendulums.

Are you thinking of Galileo Galilei? Clever bloke. His development of the idea of the helicocentric universe, proposed by Copernicus, was considered heresy, leading to his last few years being spent under house arrest. At least he avoided the torture with which he was threatened. A mere 350 years later, the pope officially pardoned him. So that's alright then…

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Kiwi Bloke, I think you are correct, Wikipedia says he noticed a swinging chandelier that was caused by air currents, which is what was said in the programme that I watched. I still can't remember what programme it was though, as it was quite a few years ago, I just remember them showing these chandeliers swinging in time with each other and the explanation why they did.

Regards Nick.

[Michael GilliganParticipant @michaelgilligan61133]

Nick

May I suggest that you dip-into the ‘Rudimentary Treatise’ at page 35 of the book

… it will probably trigger a few memories of what you have seen and heard.

MichaelG.

[CirclipParticipant @circlip]

And on a smaller scale (?) when the bloke used to come and service the works master clock, he too adjusted the rate using weights on the pendulum. Does mean of course you can alter the timing without stopping the clock. Rescued it when conversion to electronics, it was destined for the tip.

Regards Ian.

[NealebParticipant @nealeb]

Posted by Martin Kyte on 18/11/2022 10:42:08:

Just to be a bit pedantic. Changing the weight can change the period.
Weight is mass x gravitational acceleration m x g.

If g changes either by locating the clock up a mountain or by changes in ground water then the weight changes as well as the period.

I know I’m being picky and postings talking in terms of weight were just loose speaking but I think it’s worth being precise.

regards Martin

An interesting point as it is, to a good approximation, the weight and not the mass that is important. One of the few things I remember from A-level physics was the derivation from first principles of the period of a simple pendulum. If you make the assumption that the swing is over a small arc, then the period is independent of the mass of the pendulum and proportional to sqrt(l/g), where l is length of pendulum (pivot point to CoG) and g the local force of gravity.

So, to regulate the clock, adding additional weights at the CoG is pointless but adding weights higher up the pendulum changes its length…

[noel shelleyParticipant @noelshelley55608]

I well remember an experiment at school using a pendulum to show the rotation of the earth, so the suspension of the pendulum must also have to take the all be it small rotational force ? Noel.

[Martin KyteParticipant @martinkyte99762]

I think we did that one to death here Noel :-

**LINK**

although just now I can't remember what our conclusions were

regards Martin

[Michael GilliganParticipant @michaelgilligan61133]

Foucault

[ no pun intended ]

MichaelG.

Edited By Michael Gilligan on 20/11/2022 14:35:20

[John HaineParticipant @johnhaine32865]

There's an interesting point here about the influence of the atmosphere on a pendulum's rate. It has 3 effects.

One is that the bob has a buoyancy, which reduces it's effective weight though not its mass – or in effect the value of G is reduced. This makes the pendulum go slower with increasing pressure.

Secondly, the bob brings along with it a slug of air in the boundary layer. This in effect increases its mass, but not its weight because it isn't attached. As the pressure increases so does the density so the added mass increases. This effect is sometimes called "accession to inertia" and also makes the pendulum go slower with increasing pressure. For most bob shapes the amount is about equal to the buoyancy effect, and the fractional sensitivity of the rate to pressure variation ends up being the fractional variation in air pressure divided by the ratio of the bob density to air density. Dense bob materials like iron, lead and tungsten are therefore preferred.

Third, increasing pressure increases drag and reduces amplitude, which speeds up the pendulum through reduced circular deviation. Actually it turns out there's an amplitude where these three effects cancel out and the pendulum can be made insensitive to pressure variation.

[Nicholas FarrParticipant @nicholasfarr14254]

Posted by Michael Gilligan on 19/11/2022 08:14:38:

Nick

May I suggest that you dip-into the ‘Rudimentary Treatise’ at page 35 of the book

… it will probably trigger a few memories of what you have seen and heard.

MichaelG.

Hi MichaelG, I have download that from your link, but haven't gotten round to reading any of it yet but should find time in due course.

Regards Nick.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi as MichaelG has mentioned, Foucault, I took this photo below of the one that was in the Science Museum in London ten years ago.

I was hoping to see a really large one back in 2013 in Paris, which I think was in a cathedral, but it had been taken out due to renovations that were going to take three years, and I haven't been to Paris since then.

Regards Nick.

Edited By Nicholas Farr on 20/11/2022 16:11:52

[John HaineParticipant @johnhaine32865]

I don't know if it's the same one Nick, but there is a very impressive one in the Musee des Arts et Metiers which I visited 3 years ago. The Paris equivalent of the Science Museum, and very much worth a visit. Allow an afternoon at least.

[SillyOldDufferModerator @sillyoldduffer]

Clocks are typically mounted in a fixed orientation, so the Foucault effect might cause a clock bob to swing more easily when the earth is aligned with the spring's flat side, because the movement otherwise tends to twist the spring as the planet rotates under the suspension point.

Does the earth's rotation disturb a clock pendulum rigidly suspended with a flat spring? Or is the effect too small to matter on a clock sized pendulum, because the elliptical path is almost a straight line? I notice Foucault pendulums consist of a large heavy bob suspended on a long thin line, so perhaps a cathedral sized enclosure and a massive pendulum is needed to show the apparent movement?

The maths doesn't seem impossible: if 10kg bob is swinging 5 degrees on the end of a 20 metre rod, and the support beam is exactly North South, how much torque is applied to the suspension by the earth's rotation each hour?

Dave

[Michael GilliganParticipant @michaelgilligan61133]

Dave

You might be interested to read this: **LINK**

https://britastro.org/wp-content/plugins/baa-frontend-tweaks/baa-check-file.php?filename=2016/09/JBAA-124-4-Bate.pdf

I assisted Andrew with a couple of his trials, and can confirm that the effect is readily discernible at the pendulum-length that can be configured in a typical parish church.

MichaelG.

Edited By Michael Gilligan on 20/11/2022 20:21:19

[John HaineParticipant @johnhaine32865]

http://leapsecond.com/hsn2006/

**LINK**

This Web page links to a set of articles by Tom van Baak that gives a good discussion of tidal effects of the sun and moon on pendulum rate. I think this subsumes Foucault effects too. Particularly interesting is the article on the Fedchenko clock that shows that it's errors over a given month were essentially only caused by gravity variations. The whole site is well worth exploring, especially the articles on testing General Relativity on a family camping trip.

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