The origins of the atomic pocket watch

Posted on May 10, 2013 by Richard Hoptroff

10 May 2013, Greenwich Observatory & Clink Street, London

The story behind the birth of the atomic watch is worth telling.  It began with problems of calibration.  Our workshop's primary clock source is a GPS disciplined oscillator (GPSDO) accurate to around one part per billion, which is used to calibrate our frequency counter, which is in turn used to calibrate the temperature compensated quartz oscillators in our timepieces.

The GPSDO is a pretty unwieldy piece of kit and since our calibration lab is in a windowless room, the whole lot has to get hauled out occasionally to somewhere where it can get a GPS signal.  So I've often wondered if there was a better way.

Earlier this year, I took a visitor to Greenwich.  It's only a short boat ride from our workshops by Thames Clipper, the only part of London's transport network to serve champagne.  At Greenwich, we visit the Royal Observatory, the defining point of our lines of longitude.  I show him various timepieces on display, including Harrison's revolutionary H4 marine chronometer.  

In one corner, happily humming to itself, is a 40-year-old Hewlett Packard HP 5061A rack-mounted atomic clock.  That gets me thinking, "I wonder how cheap they are these days?  Maybe I could get a second-hand one for the calibration lab."

Later that day, I start searching on used test equipment web sites, and, sure enough, in return for £7000 I could have one on the lab shelf.  But it gets more interesting when I extended my search and come across Symmetricom's Chip Scale Atomic Clock (CSAC)  It's an atomic clock module the size of a matchbox.  I immediately began designing a calibration unit around it - something we will get round to making at some point.  It's a nice design, because it's small enough to go inside a fridge or oven for temperature compensation tests.

It wasn't until about a week after this episode that I started thinking back to the Harrison H4 marine chronometer. It suddenly crossed my mind to cut out all the middlemen and put the CSAC directly inside a pocket watch.  It was also very embarrassing - I was in a bar, waiting for my wife.  We were due to go out for a Valentine's Day meal.  There was one of those rare moments when everybody stops talking at the same time, and a room falls silent.  Just as I am struck by the idea and shout, "F**k!"

That was how the idea was born, on a Valentine's Day.  And within ten weeks we had it ticking.


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