Pendulum clock runs *fast* in hot weather

[harry]

Who cares? Nobody actually needs a clock to be that accurate. Plus or minus five minutes is good enough for my needs

[T i m]

Those interested in such things, especially horologists?

Do they?

Ah, then that makes you 'a nobody' (funny how the Fanatical Brexiteers can only see everything from *their* POV).

5 minutes is a very long time if you miss a train, flight, the beginning of the film, interview, auction or the launch.

Sorry, just remembered ... you are a Fanatic Brexiteer so don't like standards ... probably want us to launch a new post Brexit 'Little England only meantime'? 1440 +- 5 minutes / day?

Cheers, T i m

[newshound]

Anyone who is interested in science?

[newshound]

Fascinating. This states that variations in atmospheric pressure are enough to affect high accuracy pendulum clocks.

[dennis]

Not exactly high accuracy if the pressure affects it. Maybe a suitable bellows mechanism is need in the pendulum?

[newshound]

I'm still interested in the physical mechanism. The thread is about why someone's clock runs fast (rather than slow) in hot weather, and there have been suggestions that it might be the fact that atmospheric pressure is consistently high (rather than that the temperature is hot) which is the explanation. But high atmospheric pressure would cause more drag on the pendulum, and thus should slow the clock down.

To my mind one of the more plausible explanations is that the C of G of the metal "bob" might go up, shortening the period. But reduced drag in lubricants is another possibility.

[dennis]

Well I don't believe anyone has mentioned the special thermally compensated pendulums that consist of several lengths of different wire running up and down the pendulum so its centre of mass stays in same place when the temperature rises so its probably the length of the pendulum changing.

If it has been copied for cosmetic reasons rather than using the correct alloys anything could happen.

[Thomas Prufer]

But, but...

CoG would be a first-order effect, and drag and viscosity of air or lubricant an second order-effect?

Thomas Prufer

[The Natural Philosopher]

Well maybe

"The difference between this true period and the period for small swings (1) above is called the circular error. In the case of a typical grandfather clock whose pendulum has a swing of 6° and thus an amplitude of 3° (0.05 radians), the difference between the true period and the small angle approximation (1) amounts to about 15 seconds per day. "

Thank you wiki. So amplitude of swing is definitley in te second order,m raytetr than the 3,4, 5th order effects

15 seconds per day is around one in 5000 or 200 parts per million.

Now what does the change in pendulum length give us..

let's go from say 20 degrees C to 30 degrees C on a steel pendulum shaft.

That is around 110-125 parts per million

Oh dear. Its in the same ballpark as amplitude.

Note that lead is around 300ppm over the same temperature range, which suggests that a long lead bob of around 30% of the length of the steel pendulum BOTTOM mounted should give a decent sort of temeperature compensation.

Where does this leave us? well in the same shit we were in already. At least two possible effects - thermal expnasion and frictional reduction leading to larger amplitudes are in theiry capable of altring te clock times. In unpredictable directions - since most pendulum clocks are thermally compensated, it maye be that they are OVER compensated to allow better accuracy over more normal temperature swings.

[NY]

In the case of my clock, the pendulum is about 1.2 m long, made of rust-coloured rod about 2 mm diameter which is almost straight but very slightly wiggly in places. The pendulum bob is a brass disc about 10 cm diameter, supported at the bottom so thermal expansion *may* move its C of G closer to the pivot point, compensating for expansion in the length of the rod. I say "may" because the bob seems to be attached firmly to the rod, rather than resting on it, so the centre of the bob may not be able to rise as it expands - it may be better to regard the bob as being attached at its centre rather than its bottom.

These show the mechanism (side-on) with the rod attached to a torsion strip of flexible metal and the attachment to the escapement, and the pendulum bob. It took several attempts to get the pendulum bob in the right place, not obscured by the winding chain, because my camera seemed to have a long delay between pressing the shutter button and the shutter/flash firing. Not sure what's going on there because the SLR (which I used for the photo of the bob) should fire instantaneously.

The period of the pendulum is 2.0 seconds (averaged over about 10 cycles), made up of 1.16 sec to the opposite side and 0.84 sec to return to the start - so the tick and the tock are not quite evenly spaced.

[newshound]

The OP specifically said it was *not* a compensated pendulum. Apparently it is a wooden rod with a metal (brass, lead?) fixed at the bottom (so that the CofG may move upwards with increasing temperature)

[NY]

Metal, not wooden, rod. Looks like iron or brass, but dark, not silvery (iron) or yellow (brass), so very tarnished with age.

[newshound]

That's an interesting point. ISTR that you normally try to adjust the mechanism so that the swing is symmetrical either by tilting the case, or adjusting the whole mechanism within it. Perhaps some simple thermal expansion effect shows up if there is assymetry.

[whisky-dave]

Might be some clue here.

or you live in an alternatoive universe

[newshound]

Sorry, my mistake; but another poster *does* have a wooden one (and this does have a surprisingly low expansion coefficient)

[newshound]

That's a pretty light-weight link. If the OP claims to observe the opposite effect, I am inclined to believe them.

[whisky-dave]

His name wasn't Pinokio was it ;-)

[Jim Ericsson]

I was going to flippantly say the pendulum was made of Zirconium tungstate a material with a negative thermal coefficient, but this picture gives me another idea.

The picture clearly shows that the pendulum has rubbed against the wooden case, this would slow it down. Perhaps the clock was adjusted to take this rubbing into account when cool but at higher temperatures the rubbing stops or is lessened by movement of the pendulum due to thermal expansion?

[NY]

I think the arc-shaped scratch on the back wall of the clock, roughly level with the nut at the bottom of the pendulum, is from times when the pendulum has not been set in motion truly parallel to the back wall and has scraped until it has started to swing true. I've not seen any evidence of it happening all the time, only as an initial start condition.

Oh, by the way, I solved the problem with the camera delaying taking flash photos. I'd somehow set the flash to red-eye, where it flashes a dim light on the camera a few times to contract the subject's pupils before firing the main flash and taking the photo. Without my reading glasses on, the symbol in the display looked like rear-curtain flash rather the one for red-eye. Too much technology...

[Dave W]

You guessed that the bob might be adjusted by a nut on a screwed rod, but I can't see any trace of a thread on the rod. Also you said you don't want to turn the bob, so how can you see the back and say that it's fixed at its bottom?

It looks as if the whole pendulun might be removed by unhooking at its top for examination.

How much does it gain in hot weather compared to cold?