Shortt's Free Pendulum: The Clock That Checked the Earth's Rotation

For more than two and a half centuries after the pendulum entered clockmaking, the best way to make a clock more accurate was to make its pendulum less bothered. Every tooth of the escapement that catches it, every turn of the gears it has to drive, and every gust of air it has to push aside nudges the swing away from its natural rhythm. In the early 1920s a British railway engineer named William Hamilton Shortt carried that idea about as far as a mechanical clock could go. His solution was to give the clock two pendulums: one to do all the work, and one to do nothing but keep time.

A railway engineer's sideline

Shortt (1881–1971) was born in Wimbledon and began his career as a railway engineer in 1902. He served as a captain in the Royal Engineers during the First World War and later held the post of Western Divisional Engineer for the Southern Railway, according to the note that accompanies his papers at the National Maritime Museum in Greenwich. Alongside that career, he set out to build a better clock.

He worked on the problem with the horologist Frank Hope-Jones, and the clocks were built by the Synchronome Company of Clerkenwell Road, London. The idea of a "free" pendulum, one that drives nothing and is touched as little as possible, was not new; the Greenwich archive credits R. J. Rudd with first conceiving it. What Shortt achieved was a practical, reliable version. His clock was introduced in 1921, and National Museums Scotland holds the pendulum of the prototype, known as Shortt No. 0, which was made in 1922 and put to work at the Royal Observatory, Edinburgh, that same year.

Two pendulums, one job each

The genius of the design lies in a division of labor. Royal Museums Greenwich preserves Shortt Free Pendulum System No. 3 (made about 1924) along with its many parts, and its catalogue description explains how the pair cooperate.

The master, or free pendulum, hangs inside a sealed copper tank with a thick glass base and a heavy glass dome on top, through which an observer can check its thermometer and pressure gauges. The tank is partly evacuated, reducing the air the bob must push aside. The master has no escapement and turns no wheels. Its only job is to swing.

The slave, in a mahogany case, is essentially a Synchronome electric clock with a pendulum of its own. It does the practical work: it counts, carries the dials, and supplies the energy that keeps both pendulums going. The heart of a Synchronome clock is a "gravity arm," a small weighted lever that is lifted and then allowed to fall, giving a measured push to a pendulum. In the Shortt system that gravity arm drops every 30 seconds.

Here is the clever part. Every 30 seconds the slave also releases a second gravity arm poised above the master pendulum. A tiny semicircular jewel on the end of that arm drops onto a light wheel fixed to the master's rod, and as the pendulum swings away the jewel passes over the wheel's edge, delivering a gentle push at the centre of the swing. Once the arm has done its work it is reset, and that resetting sends an electrical pulse back to the slave. For the rest of the half-minute, the master swings undisturbed.

A slave pendulum, however well made, will wander, so Shortt needed a way to make it follow the master's lead. His answer was the "hit and miss" synchroniser. The slave is deliberately set to run slightly slow; the Greenwich record puts it at about six seconds a day behind the master. Each electrical pulse from the master briefly pulls a small armature into position. If the slave is in step, its pendulum passes without contact, a "miss." If it has fallen behind, a spring on the pendulum catches the armature and the slave gets a small nudge forward, a "hit." Rated this way, the synchroniser normally engages on about every other pulse, and the slave never strays far from the master's beat.

The beauty of the arrangement is that the master receives the same small, regular push no matter what the slave is doing. In modern terms it is a feedback loop: a working oscillator continually corrected against a steadier reference.

Testing the turning Earth

At Greenwich, Shortt No. 3 replaced the observatory's Riefler regulator as the Sidereal Standard Clock, the master reference for star time, on 1 January 1925. It held that post until 1938, when the observatory stopped relying on any single clock and began taking the mean of many.

How good were they? A NIST history of time measurement puts the Shortt clock's uncertainty at a few seconds per year, about one part in ten million, describing it as the practical limit of mechanical clocks and a reference for laboratories. That raised a startling question: was the Earth itself a perfect clock?

The second was then defined by the Earth's rotation, and observatory clocks were checked against the stars. In January 1929 J. Jackson of the Royal Observatory published a paper in the Monthly Notices of the Royal Astronomical Society titled "Shortt Clocks and the Earth's Rotation." He analysed two Greenwich Shortts, No. 3 (installed in November 1924) and No. 11 (May 1926). Between March 1927 and December 1928, No. 3's departures from its calculated rate stayed within roughly a quarter of a second either way, and the two clocks agreed closely enough for him to conclude that the Earth's rotation in 1928 had not differed from 1927's by more than about a second. He tracked the clocks' own flaws too, noting that No. 3's tank pressure had apparently slipped from about 36 to 34 millimetres of mercury. A clock had become precise enough to test the Earth's steadiness.

A worldwide standard, briefly

According to Museums Victoria in Australia, about 100 Shortt clocks were made. Their own example, Shortt No. 5, was installed at Melbourne Observatory in 1925 to run its time service. It served there until 1944, then passed to the Postmaster-General's research laboratories, moved to South Australia in 1955, and stayed in use until 1968.

The free pendulum's reign as a laboratory standard, though, was short. In 1927 Joseph Horton and Warren Marrison of Bell Laboratories built the first quartz crystal clock, and by the 1940s quartz had replaced the Shortt as the primary standard in laboratories. Shortt was honored for his work, receiving the Gold Medal of the British Horological Institute in 1941 and the Tompion Gold Medal of the Worshipful Company of Clockmakers in 1954.

The surviving clocks are museum pieces now, but they mark a remarkable moment: when a pendulum swinging in thin air became steady enough to examine the turning planet that had always set the standard for time.