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The Corpus Christi Chronophage Clock

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22nd Sep 2008 | 23:03

I have been admiring the new clock on the corner of Trumpington St and Bene't St. Instead of having hands, the clock has three concentric rings hidden behind its face. The outer ring marks seconds, the middle ring marks minutes, and the inner ring marks hours. Each ring is driven by the next outer ring via a gearing mechanism that makes the rings tick from one mark to the next. It is fairly normal for the second hand's motion to be quantized in this manner, but more unusual for the minute and hour hands to tick like this too.

Each ring sits in front of a circle of 60 blue LEDs, and the face has three circles of 60 slots which would allow the LEDs to shine through if the rings weren't in the way. The position of each disk is indicated by a slot which allows one LED to shine through the disk and the face to be seen by the viewers.

In fact, each ring has 61 slots, one of which is usually aligned with a slot in the face to indicate the time, and 60 of which are normally out of alignment. When a ring ticks from one position to the next, its 60 normally-unaligned slots line up with the face in turn before the normally-aligned slot lines up with the next slot in the face. This has the effect of making it look like the ring has flown around 366° whereas it only moved by 6°

I have made a little animation which shows how this works. It ticks once every 4 seconds and it only has 12 slots on the face and 13 on the disk, so that you can clearly see the mechanism behind the effect. I have drawn the face's slots in the outer circle and the ring's slots in the inner circle. It uses the new-fangled <canvas> HTML element, so you may need to upgrade your browser to view it.

There is a video of the clock where some of its features are described by the designer, John Taylor, who made his fortune from his invention of the kettle thermostat.

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Comments {13}

Matthew

from: emperor
date: 23rd Sep 2008 06:53 (UTC)

That all sounds quite neat. Is it reasonably readable as a clock, or just baffling?

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Tony Finch

from: fanf
date: 23rd Sep 2008 07:53 (UTC)

Yes, it's quickly obvious when you see it that you read it like normal analogue clocks. The slots suggest hands, and the ends of the minute slots are further out than the hour slots, as usual. The LEDs are a little indistinct in daylight, though.

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Arnhem

from: arnhem
date: 23rd Sep 2008 08:14 (UTC)

I was pleased to note that this morning I could read the time as I biked past at speed without having to slow down.

That's much better readability that I was expecting.

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pakennedy

from: pakennedy
date: 23rd Sep 2008 07:52 (UTC)

That's a beautiful piece of engineering. I'm more impressed with the escapement than the dial but the whole thing looks fantastic.

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Tony Finch

from: fanf
date: 23rd Sep 2008 07:58 (UTC)

Yes, but I can't explain the escapement :-)

The really baffling thing is the occasional pauses - the whole thing stops for a second or so. Very disconcerting. The chronophage's blinking and jawing is pretty fearsome too!

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Arnhem

from: arnhem
date: 23rd Sep 2008 08:16 (UTC)

The pauses are deeply weird (but I gather, deliberate).

Anyone want to start a rumour that on leap seconds, the grasshopper takes off and flies right round the clock? </oops>

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Tony Finch

from: fanf
date: 23rd Sep 2008 10:27 (UTC)

There's one at the end of this year :-)

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Pete

from: pjc50
date: 23rd Sep 2008 08:24 (UTC)

It's like a Vernier scale, isn't it?

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Andrew

from: nonameyet
date: 23rd Sep 2008 10:10 (UTC)

Hear the video from just before around 3min30 - yes, he calls then Vernier slits.

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(Deleted comment)

Tony Finch

from: fanf
date: 23rd Sep 2008 10:25 (UTC)

I haven't actually seen it striking the hour yet, except on the video. It looks like what happens is the rings all tick back and forth from NN:59:59 to MM:00:00 which makes it look like the lights whiz round the whole circle back and forth MM times.

I thought it might be nice if the minute ring had 59 slots instead, so that it would produce a whizzing effect in the opposite direction. Sadly the designer thought otherwise.

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gareth_rees

from: gareth_rees
date: 23rd Sep 2008 15:28 (UTC)

Yes, that was what I saw this morning at 11:00:00. The pendulum continued to swing, but the escape wheel jogged back and forth by a second until the hour had finished striking.

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Gerald the cuddly duck

from: gerald_duck
date: 23rd Sep 2008 11:48 (UTC)

Unfortunately, interesting though the mechanism is, the gold ironically makes it look gaudy and cheap.

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Arnhem

from: arnhem
date: 23rd Sep 2008 14:36 (UTC)

I know what you mean; it may improve with time?

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