A Pendulum clock: Accurate Timekeeping for 270 years.
In this article, we will look at the history and evolution of probably the most important invention in Horology, the Pendulum Clock.
A pendulum clock is a clock that uses a pendulum, which is a swinging weight, as its timekeeping element. The advantage of a pendulum for timekeeping is that it is a harmonic oscillator: it swings back and forth in a precise time interval dependent on its length and resists swinging at other rates.
From its invention in 1656 by Christiaan Huygens until the 1930s, the pendulum clock was the world’s most precise timekeeper, accounting for its widespread use.
Throughout the 18th and 19th centuries pendulum clocks in homes, factories, offices, and railroad stations served as primary time standards for scheduling daily life, work shifts, and public transportation. Their greater accuracy allowed the faster pace of life which was necessary for the Industrial Revolution.
The home pendulum clock was replaced by cheaper synchronous electric clocks in the 1930s and ’40s, and pendulum clocks are now kept mostly for their decorative and antique value.
A Pendulum clock must be stationary to operate; any motion or accelerations will affect the motion of the pendulum, causing inaccuracies, so other timepieces must be used in portable timepieces.
History of the Pendulum Clock
The first pendulum clock was invented in 1656 by Dutch scientist and inventor Christiaan Huygens and patented the following year. Huygens contracted the construction of his clock designs to clockmaker Salomon Coster, who actually built the clock. Huygens was inspired by investigations of pendulums by Galileo Galilei beginning around 1602.
Galileo had discovered the key property that makes pendulums useful timekeepers: isochronism, which means that the period of swing of a pendulum is approximately the same for different sized swings. Galileo had the idea for a pendulum clock in 1637, which was partly constructed by his son in 1649, but neither lived to finish it.
The introduction of the pendulum, the first harmonic oscillator used in timekeeping, increased the accuracy of clocks enormously, from about 15 minutes per day to 15 seconds per day leading to their rapid spread as existing ‘verge and foliot’ clocks were retrofitted with pendulums.
These early clocks, due to their verge escapements, had wide pendulum swings of 80–100°. In his 1673 analysis of pendulums, Horologium Oscillatorium, Huygens showed that wide swings made the pendulum inaccurate, causing its period, and thus the rate of the clock, to vary with unavoidable variations in the driving force provided by the movement.
Introduction of the Anchor Escapement and the Royal Pendulum
Clockmakers’ realization that only pendulums with small swings of a few degrees are isochronous
motivated the invention of the anchor escapement by Robert Hooke around 1658, which reduced the pendulum’s swing to 4–6°.
So the anchor became the standard escapement used in pendulum clocks. In addition to increased accuracy, the anchor’s narrow pendulum swing allowed the clock’s case to accommodate longer, slower pendulums, which needed less power and caused less wear on the movement.
The seconds pendulum (also called the Royal pendulum), 0.994 m (39.1 in) long, in which the time period is two seconds, became widely used in quality clocks. The long narrow clocks built around these pendulums, first made by William Clement around 1680, became known as grandfather clocks. The increased accuracy resulting from these developments caused the minute hand, previously rare, to be added to clock faces beginning around 1690.
Improvements to the pendulum clock.
The 18th and 19th-century wave of horological innovation that followed the invention of the pendulum brought many improvements to pendulum clocks. The deadbeat escapement invented in 1675 by Richard Towneley and popularized by George Graham around 1715 in his precision “regulator” clocks gradually replaced the anchor escapement and is now used in most modern pendulum clocks. The observation that pendulum clocks slowed down in summer brought the realization that thermal expansion and contraction of the pendulum rod with changes in temperature were a source of error.
This was solved by the invention of temperature-compensated pendulums; the mercury pendulum by George Graham in 1721 and the gridiron pendulum by John Harrison in 1726. With these improvements, by the mid-18th century precision pendulum clocks achieved accuracies of a few seconds per week.
Until the 19th century, clocks were handmade by individual craftsmen and were very expensive. The rich ornamentation of pendulum clocks of this period indicates their value as status symbols of the wealthy. The clockmakers of each country and region in Europe developed their own distinctive styles. By the 19th century, factory production of clock parts gradually made pendulum clocks affordable by middle-class families.
The introduction of the more accurate regulators.
During the Industrial Revolution, daily life was organized around the home pendulum clock. More accurate pendulum clocks, called regulators, were installed in places of business and railroad stations and used to schedule work and set other clocks. The need for extremely accurate timekeeping in celestial navigation to determine longitude drove the development of the most accurate pendulum clocks, called astronomical regulators.
These precision instruments, installed in naval observatories and kept accurate within a second by observation of star transits overhead, were used to set marine chronometers on naval and commercial vessels.
Beginning in the 19th century, astronomical regulators in naval observatories served as primary standards for national time distribution services that distributed time signals over telegraph wires. From 1909, the US National Bureau of Standards (now NIST) based the US time standard on Riefler pendulum clocks, accurate to about 10 milliseconds per day. In 1929 it switched to the Shortt-Synchronome free pendulum clock before phasing in quartz standards in the 1930s. With an error of around one second per year, the Shortt was the most accurate commercially produced pendulum clock.
Pendulum clocks remained the world standard for accurate timekeeping for 270 years, until the invention of the quartz movement in 1927, and were used as time standards through World War 2. The French Time Service used pendulum clocks as part of their ensemble of standard clocks until 1954. The home pendulum clock began to be replaced as a domestic timekeeper during the 1930s and 1940s by the synchronous electric clock, which kept more accurate time because it was synchronized to the oscillation of the electric power grid.
The most accurate experimental pendulum clock ever made maybe the Littlemore Clock built by Edward T. Hall in the 1990s (donated in 2003 to the National Watch and Clock Museum, Columbia, Pennsylvania, USA).