Pendula

I've been thinking about pendulums (pendula)? and I seem to remember a TV program about Big Ben. The engineers were adding old pennies to the pendulum to correct the time. My limited understanding can't make any sense of this as the weight doesn't change the period.

Two things may be happening here, either I'm remembering the program badly or my understanding of pendulums is wrong.

David

Hi David, I've seen much the same programme, and yes, they did add or remove pennies to keep the clock in time. I don't understand it truly myself, but as you say the period remains the same, but I think it's the stroke that is altered. On My carriage clock, the length of the pendulum is altered by the weight being let up or down with a nut on a screw thread.

Regards Nick.

It depends where the pennies are added. The effective length of the pendulum will be changed, by moving the C of G of the pendulum.

I'm back into pendulums too, having fitted a new one to my experiment clock after a long delay. Another story!

Putting pennies on top of a bob isn't done to alter the weight - weight doesn't effect period. The timing of a pendulum depends on the rod's length, and the rod ends at the centre of mass inside the bob, not at the top of the bob. Adding pennies to the top of the bob lifts its centre of mass, effectively shortening the rod so the pendulum swings faster. Adding pennies to a shelf suspended underneath the bob would drop its centre of mass and the pendulum would slow down.

I guess old pennies are used because they're flat discs of a stable metal of about the right size and weight, that are unlikely to fall off! They should send it back, buying a new clock I'd expect it to come with a set of special weights lovingly finished by time-served master-craftsmen, costing only 50 Guineas each...

Dave

Mass independent period assumes a point mass, for a real pendulum the bob is an extended object. Adding mass to the top changes the centre of mass and thus the effective length of the pendulum and thus it’s period.

regards Martin

A pendulum bob suspended from the bottom will expand upwards with increasing temperature and the period will shorten. To compensate the bob must be suspended from the middle ( assuming it’s symmetrical about that point). Small variations in the suspension point can also compensate for the expansion of the rod.

regards Martin

They by-passed the master craftsmen in favour of mass produced items that were legally required to be identical and only cost a penny each. And the man adjusting the clock already had spares in his pocket.

The modern equivalent would be strapping your phone to the part, and having the specially written app tell you how much to adjust it. That's unlikely to work in 5 years time; it certainly won't in 160.

This place is truly amazing ! A subject that I had never considered has been explained and understood ! Thank you Gentlemen for your time !

Does anybody remember in the year of the BHI anniversary, I think at Olimpia a clock that ONLY had 2 hands NO obvious works yet told the right time ! I have my own theory as to how it worked, but does anyone know for sure how it was done ? Noel.

Edited By noel shelley on 18/11/2022 09:45:24

Brilliant, and simple too ( once you know)! Thank you.

David

" And the man adjusting the clock already had spares in his pocket."

Back when the clock was new a penny would buy two pints of beer, so I doubt that!

that's a £10 note today. £5 if you drink in Wetherspoons. I doubt that a Victorian clock mender, working on the most prominent clock in the country, wasn't able to afford a couple of beers on the way home. I'd be surprised if he didn't have them!

Hi, I the program that I saw about the Westminster clock, (Big Ben) the pennies used, were always kept with the clock and had been there for many years, and I was aware they used pennies back in the early sixties.

The program that I saw was several years before the recent major overhaul of the clock and tower.

Regards Nick.

Edited By Nicholas Farr on 18/11/2022 10:07:48

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

So, do you think the original engineers thought that they were adding pennies to alter the weight or the length of the pendulum?

David

p.s. sorry for the smiley

Edited By David Noble on 18/11/2022 10:47:23

It was on Blue Peter with John Noakes and was also an article in a Blue Peter Annual.

Martin C

I'm sure he could afford a couple of beers but I doubt he'd use his own money to trim the clock! A few discs of miscellaneous scrap would be just as good.

I'm sure they knew they were changing the centre of mass and thus effective length.

regards Martin

I think Synchronomes and/or Pulsynetic clocks had a little tray on top of the pendulum bob so you could add little weights, so many grammes per second per day

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

As an interesting aside, it's only recently the speed of gravity has been confirmed. Though most thought gravity travelled at the speed of light, it's very difficult to measure and the point was unproven, leaving open the possibility that gravity was instantaneous.

Pendulums reveal that gravity varies around the planet and a very good pendulum can detect gravity waves, but a pendulum can't be used to measure the speed of gravity. For that you need see two Neutron Stars to collide. No problem arranging that then!

Dave