Pendulum clock

A pendulum clock is a clock that uses a swinging weight called a pendulum as its timekeeping element. The pendulum is an approximate harmonic oscillator. It swings in a time interval that depends on its length, and it resists swinging at other rates.

The pendulum clock was invented in 1656 by Christiaan Huygens, who was inspired by Galileo Galilei. Until the 1930s, it was the world's most precise type of timekeeper.

Throughout the 18th and 19th centuries, pendulum clocks were found in homes, factories, offices, and railroad stations. They served as the main time standards for scheduling daily life, work shifts, and public transportation. Their accuracy helped people keep up with the fast pace of life during the Industrial Revolution.

The home pendulum clock was replaced by cheaper synchronous electric clocks in the 1930s and 1940s. Today, pendulum clocks are kept mostly for their decorative and antique value.

Because a pendulum clock must stay still so that its swinging is not disturbed, other mechanisms are used in portable timepieces.

History

The first pendulum clock, invented by Christiaan Huygens in 1656

The pendulum clock was invented on 25 December 1656 by Dutch scientist and inventor Christiaan Huygens. The invention was patented the following year, and Huygens described it in his manuscript Horologium (1658). He had Salomon Coster build his clock. Huygens was inspired by Galileo Galilei, who had studied pendulums around 1602 and found that they swing with a period that does not depend on the size of the swing. In 1637, Galileo described to his son, Vincenzo, a way to keep a pendulum swinging. Before pendulums were used, clocks were accurate to about 15 minutes per day. The pendulum, the first harmonic oscillator used in timekeeping, improved accuracy to 15 seconds per day, which led to their rapid spread. Many existing "verge and foliot" clocks were fitted with pendulums, and by 1659 pendulum clocks were made by the French clockmaker Nicolaus Hanet and in England by Ahasuerus Fromanteel. Some of the most accurate pendulum clocks were the Riefler regulator clock, which served as the US time standard from 1909 to 1929, and the Shortt-Synchronome clock, which was the most accurate pendulum clock ever manufactured when it was created in the 1920s.

Early clocks with verge escapements had wide pendulum swings, and in his 1673 analysis Horologium Oscillatorium, Huygens showed that wide swings made the pendulum inaccurate because the movement of the clock varied its driving force. Clockmakers then found that only pendulums with small swings are isochronous, which led to the anchor escapement created by Robert Hooke around 1658. This became the standard escapement in pendulum clocks. Later, the seconds pendulum became widely used in quality clocks, and the tall, narrow freestanding clocks built around these pendulums became known as grandfather clocks. The 18th and 19th century wave of horological innovation brought many improvements, including the deadbeat escapement invented by Richard Towneley and popularized by George Graham, and temperature-compensated pendulums such as the mercury pendulum and the gridiron pendulum by John Harrison, which solved errors from thermal expansion. Until the 19th century, clocks were handmade and expensive, but factory production later made pendulum clocks affordable for middle-class families, and clockmakers in each region developed their own styles. During the Industrial Revolution, accurate pendulum timekeeping was needed for schedules and trains, and the most accurate pendulum clocks, called astronomical regulators, were used in naval observatories and for celestial navigation to find longitude at sea. From 1909, the US NIST based its time standard on Riefler clocks, switching in 1929 to the Shortt-Synchronome free pendulum clock before quartz standards took over in the 1930s. Pendulum clocks were the world standard for 270 years, until the quartz clock was invented in 1927, and they were used as time standards through World War II. The home pendulum clock was later replaced by the electric clock, which is synchronized to the electric power grid. Today, the most accurate pendulum clock ever to go into production is the AChF-3 isochronous clock, and the largest pendulum clocks were built in Geneva (1972) and Gdańsk (2016).

Mechanism

A pendulum clock's movement has five main parts. The first is a power source, either a weight on a cord or chain that turns a pulley or sprocket, or a mainspring. A gear train (wheel train) speeds up this power so the pendulum can use it, and the gear ratios slow it down again to turn the hands. An escapement gives the pendulum timed pushes and releases the gears one step at each swing, which makes the "ticking" sound. The pendulum, a weight on a rod, keeps the time. The last part is a dial that shows how much time has passed, usually a traditional clock face with hands.

Extra features besides telling time are called complications. Fancier pendulum clocks may have a striking train that rings a bell or gong every hour, and some strike every quarter hour and play melodies, usually the Westminster quarters. Other complications include calendar dials, a moon phase dial, an equation of time dial that shows the difference between clock time and sun time, and a repeater that chimes the hour when triggered by hand. In electromechanical Master clocks, a solenoid uses magnetic force to push the pendulum, and a switch or photodetector senses when to give the push in place of an escapement. Quartz pendulum clocks are not true pendulum clocks, because a quartz crystal controls the timekeeping and the swinging pendulum is only decoration.

Gravity-swing pendulum

Most pendulum clocks have a metal bob hanging from a rod made of wood or metal. The bob is usually shaped like a lens to reduce drag. Wood was often used for the rods in good clocks because it changes length less than metal when temperatures change, as explained by the coefficient of thermal expansion. An escapement keeps the pendulum swinging by giving it a small push with each swing. Most quality clocks, including all grandfather clocks, have a "seconds pendulum," where each swing takes one second. Mantel clocks often have a shorter half-second pendulum. Only a few tower clocks use longer pendulums, such as the two-second pendulum in the Great Clock of Westminster, which houses Big Ben.

The time for one full swing depends on the pendulum's length and the pull of gravity. Clocks are adjusted by moving the bob up or down. Heat makes the rod longer, which slows the clock, so inventors found ways to correct this. The mercury pendulum, invented by Graham in 1721, used liquid metal that rises as the rod grows. The gridiron pendulum, invented by John Harrison around 1726, combined rods of zinc or brass with rods of steel so their changes in length cancel each other out. Later, very precise clocks used Invar or fused silica, which need little correction. Air also slows the pendulum, so some precision regulator clocks were run in low-pressure chambers. Clocks must also be level, and a spirit level or watch timing machine can help check this. Gravity changes with latitude, and it is stronger near the poles than at the equator because Earth is oblate. For this reason, regulators used for celestial navigation had to be adjusted when moved to a different latitude.

Torsion pendulum

Main article: Torsion pendulum clock

A torsion pendulum, also called a torsion-spring pendulum, is a wheel-like weight, usually with four balls on crossed arms, hung from a thin strip of spring steel. It controls the timing in torsion pendulum clocks. Turning the weight winds and unwinds the spring. Its main advantage is that it uses very little energy. Each swing takes 12 to 15 seconds, compared with 0.5 to 2 seconds for a gravity pendulum, so some clocks need winding only every 30 days, or even once a year or less often. Because the spring's pull depends on its elasticity, which changes with temperature, temperature affects it more than a gravity pendulum. The most accurate torsion clocks use a special spring made of elinvar, which changes very little with temperature.

A clock that needs winding only once a year is sometimes called a "400-Day clock" or "anniversary clock," and it is sometimes given as a wedding gift. Some "perpetual" clocks never need winding, because their mainspring is kept wound by changes in air temperature and pressure through a bellows. The Atmos clock is one example, and it uses a torsion pendulum with a long swing time of 60 seconds.

Escapement

Main article: Escapement

The escapement is a mechanical linkage that turns the force from the clock's wheel train into small pushes that keep the pendulum swinging. It makes the "ticking" sound. A wheel with pointed teeth, called the escape wheel, is turned by the wheel train and rests against surfaces called pallets. With each swing, a pallet releases one tooth, and the wheel moves forward a fixed amount until another tooth catches. This lets the hands move forward at a steady rate controlled by the pendulum. Although the escapement is necessary, its force disturbs the pendulum's natural motion, and in precision clocks this was often what limited accuracy.

Over time, different escapements were tried to solve this problem. The anchor escapement was the standard until the 1800s, when the deadbeat escapement took over in precision clocks, and it is used in almost all pendulum clocks today. The remontoire, a small spring rewound at intervals, was used in a few precision clocks, while gravity escapements were used in tower clocks. By the end of the 19th century, specialized escapements were used in the most accurate clocks. Electromagnetic escapements used a phototube or switch to power an electromagnet instead of a mechanical linkage. In 1923, W.H. Shortt and Frank Hope-Jones developed the Shortt-Synchronome clock, which was accurate to better than one second per year.

Time indication

The clock shows the time with a traditional dial that has moving hour and minute hands. Many clocks also have a small third hand on a smaller dial that shows seconds. To set the time, you open the glass cover over the face and push the minute hand around the dial by hand. The hour hand moves with the minute hand, so setting the minute hand also sets the hour hand.

Maintenance and Repair

Pendulum clocks last a long time and do not need much maintenance, which is one reason they are popular.

Like any machine with moving parts, they need regular cleaning and lubrication. Special thin oils called lubricants have been made for clocks, and one of the most widely used is a synthetic oil. Springs and pins may wear out or break over time and will need to be replaced.

Styles

Pendulum clocks were more than just useful timekeepers. Because they were expensive, they were status symbols that showed the wealth and culture of their owners. They developed in many traditional styles, which differed by country, by time period, and by how the clock was meant to be used. Case styles often matched the furniture styles popular at the time. Experts can often tell when an antique clock was made, within a few decades, from small differences in its case and face. These are some of the different styles of pendulum clocks: