The pendulum on this clock supported at the bottom like that. Goodness knows
how the length is adjusted, because the weight doesn't move freely up and
down the rod, resting on a nut that you can wind up and down a threaded rod.
It looks as if you have to rotate the whole weight to move it up and down
the screw, but if that's how you adjust it, it's very stiff and I'm not
about to try it in case I break something. I just correct it each morning
(and maybe in between if I notice it's wrong) so it remains no more than a
couple of minutes fast.
+/- 5 seconds a day is pretty bloody amazing for a clockwork clock. It's
good even for a quartz clock: the real time clock on my PC loses or gains
more than that, but then gets corrected by scheduled process syncing with
time.windows.com (or whatever) every 24 hours; I think the worst I've seen
it (compared with time.is) is about 90 seconds out.
How long did it take to get it that accurate, given that you have to make a
small adjustment to the pendulum length and then wait ages to see how much
time it has lost over maybe 24 hours to work out how much further you need
to adjust the screw, iteratively homing in on the correct setting. I suppose
if you know the gearing of the clock (pendulum ticks per minute of hand
movement) you can time a few ticks and extrapolate from there - but the more
ticks you count, the less measurement error there will be.
Actually, it's uncanny. When I went to check the pendulum fixing just now, I
found that the clock had stopped literally a minute or so ago, after I'd
forgotten to wind it this morning. What is the chance of it stopping exactly
at the time that I check the clock?
This is a very old full-height grandfather clock that has been in the family
for years. It's named Francis after a distant ancestor who was a
clock-maker. It would have been incredible to have found one of his own
clocks in an antique shop, but my parents did the next best thing and bought
one from a clockmaker who had lived in the same part of the country at about
the same time - which I think was early 19th century. I presume the
mechanism is original - in design, even if some parts have been replaced
like-for-like over the years.
You'd tend to expect a big clock to have a deep sonorous chime, but this one
has a little bell about the pitch of a bell used on a shop counter for
attracting the assistant's attention - a bit weedy, as if its voice hasn't
broken. The chiming mechanism is a bit uneven: I presume it has a wind vane
that crudely regulates the interval between the chimes, and sometimes for no
apparent reason you get a few chimes with a bit of time between them and
then a couple of hurried ones at the end, as if it's saying "Sod it. Twelve
o'clock is a lot of hard work. I'm nearly there, try and get the rest over
quickly." But that just gives it character ;-)
Harrison's H4 was pretty incredible, but in fact the French were close
Can't immediately spot the accuracy of the H4, but this link has an
unreferenced statement suggesting that mechanical marine chronometers
got to 0.1 seconds per day, giving an accuracy of 1 - 2 miles after a
sea voyage of a month.
Here's a better one
0.6 seconds over 100 days, and that was a pendulum clock to Harrison's
design (though not, of course, at sea on a sailing ship).
Yes but it was probably a far more elaborate mechanism with far more checks
and counter checks that a normal clock mechanism. To make something that
keeps consistent time, in a range of temperatures and on board a heaving
ship, was a fantastic achievement. And it must have taken ages to adjust the
clock, little by little, until it was keeping the correct time, even if once
it *had* been set, it remained accurate. I presume it was a rotating balance
wheel as opposed to a pendulum, so it would be a matter of tweaking the
length of the spring. And what would they have used as a reference against
which to measure how far fast or slow the Harrison watch was, while
adjusting it. I suppose for a fixed location, the movement of the stars is
known so you can tell the time fairly accurately using those and compare the
watch against that.
Come to think of it, I wonder why my PC clock is so bad. Quartz watches keep
better time and don't need much adjustment back to correct time. I wonder if
when the PC is on, it uses some other time source than the quartz crystal.
A clock only needs to be as accurate as the gain or loss that you can
tolerate in the interval until it is next synchronised with a master time
Windows defaults to syncing once a week. After I found my PCs drifting by
several minutes, I changed that to once every day. It's a shame that it
requires a registry change, and isn't changeable from a menu built into
As I understand it, the Raspberry Pi doesn't have a battery-backed real time
clock, and has to set itself afresh every time it is booted and can talk to
an NTP server. I presume it resyncs periodically while it is running.
I'm cautious of clocks that have no free-running capability, which depend
entirely on a radio source. My wife bought a Rugby clock/radio years ago and
towards the end it started displaying bizarre times, probably if it lost the
radio signal, because it couldn't keep going (even if with a slight error)
until the next time it got a valid time. Not good when it's the alarm clock
that wakes you to go to work :-(
<snip> >As I understand it, the Raspberry Pi doesn't have a battery-backed real time
Wouldn't all that be down to the OS running on it, so not the RPi
On one ESP32 project I built it makes a WiFi connection, connects to
an NTP server, logs the values of a temperature sensor to an SD card
and then goes to sleep for n time.
On my Arduino projects I have an external RTC.
Cheers, T i m
Some get their time from GPS if they want greater precision.
It only *needs* to be that accurate but there is real clockmakers kudos
in having something that drifts by only a miniscule amount over the
period where it needs to be rewound. 200 day clocks for instance.
Marine chronometers and observatory clocks at the beginning of the last
century represented the peak performance of classic mechanical clocks.
They also included cunning mechanisms that prevent rewinding interfering
with the accuracy and amplitude of the pendulum beat.
The Shortt-Synchronome was the ultimate in precision mechanical clocks -
good enough to detect seasonal variations in the Earth's rotation back
in the 1920's - long before atomic clocks.
That is mainly because the designers CBA to load and trim the crystal
properly unlike the ones in cheap watches which are usually done right.
The original PC design didn't have an RTC either - you had to manually
set time and date every time you booted it (pre internet).
Domestic "atomic clocks" usually do have a local RTC that is disciplined
by the incoming signal to trim it when free running to fractions of a
ppm. Simpler ones just free run at whatever the crystal inside happens
to oscillate at.
My guess at the design of the pendulum that is running fast would be an
invar main pendulum bar with an iron weight that is about 1/10 its
length supported from below. This is approximately tempco = 0ppm.
Trouble is invar used to vary from batch to batch.
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.
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
If it has been copied for cosmetic reasons rather than using the correct
alloys anything could happen.
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)
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