Christiaan Huygens is credited with the first pendulum clock in 1656 and most history books say pendulum clocks were the most accurate type of clock until the 1930s when they were replaced by quartz and atomic clocks.  But what the history books don’t say is that from the minute the pendulum clock was invented horologists spent the next 260 years improving it.  And that brings us to Mr. William Hamilton Shortt a director of the Synchronome Co Ltd and his patent of the most accurate pendulum clock ever designed in 1921.

The ultimate pendulum clock.

Mr. William Hamilton ShorttThe best made pendulums were encased in low pressure vacuum tanks to get the absolute minimum air resistance, temperature change and air buoyancy of the pendulum.  And yes, I do mean buoyancy.  At this extreme accuracy pendulums are affected by air pressure, not just air resistance.  There is nothing more accurate in frequency than a free swinging pendulum in an ideal world.  Putting a pendulum in a vacuum almost gets you there except for one last problem: clocks have hands.

Something has to turn those hands or the clock is useless.  When a pendulum turns a hand, or does work of any kind really, you introduce error.  There is no way to avoid it.  That beautiful grandfather clock you saw in your grandparent’s house may be a work of art but the tick-tock sound you hear is the little toothed wheel, called an escapement.  High end clocks will have wonderfully made escapements but unfortunately, when a pendulum is connected to anything doing work, the accuracy of the pendulum is going to suffer.  In 1921 Mr. Shortt patented a solution.

The master/slave solution.

Using electricity Mr. Shortt designed a two pendulum solution: a master and a slave.  The master pendulum, the one in the vacuum tank, does one thing only, it swings.  That’s it.  The slave pendulum does two things: it triggers the movement of the clock hands and, when required, it occasionally gives a tiny nudge to the master pendulum to keep both of them in sync and to overcome the very small amount of friction left in the vacuum tank.  It was a brilliant design and it’s estimated about 100 clocks were built and installed in astronomical observatories around the world.

So, just how accurate is it?

Mind numbingly accurate.  At the time, it was believed the clock was accurate to one millisecond a day.  A fabulous number but just a calculated guess because, after all, when you invent the world’s most accurate clock what other clock are you going to test it against!  In 1985 Mr. Pierre H. Boucheron published an article in The Bulletin of the National Association of Watch and Clock Collectors (Vol. XXVII, No. 2, Whole number 235 (April, 1985): 165-173, entitled “Just How Good Was the Shortt Clock?”  Using optical equipment that wouldn’t disturb the pendulum he compared the Shortt clock in the US Naval Observatory with an atomic clock for 8500 hours of measurement.  The results were astounding.  The Shortt clock was accurate to 200 microseconds per day, equivalent to an error rate of one second in 12 years!  What’s more, the clock, called Short #41, was “seeing tides”.  That is, the pendulum was moving slightly ahead and slightly behind depending on the pull of the Sun and Moon on the pendulum.  Only the most accurate pendulum clocks are capable of that accuracy.

Shortt Clock

For those of you with Adobe Shockwave installed on your computers there is a sensational Shockwave animation of the Shortt clock available but if you visit the site and you don’t have Shockwave installed you’ll be prompted to download and install it.  You may also be prompted to install a registry tool called registry mechanic which is unneeded so uncheck that install box.  Many people won’t install Shockwave because of the annoying ads that use it.  I know because I’m one of those people but this animation is worth installing Shockwave for.  It allows you to speed up or stop the clock, display the wires or hide them and other options.  After a little study you’ll see the beauty of the Shortt clock design.

Horology, the study of time and time measurement, is a fascinating subject and I hope I’ve sparked your interest in digging deeper into it.

Author’s note – no self referring links, no affiliate links and no animals were used in the writing of this article.