John Harrison and the Marine Chronometer
How Accurate Timekeeping Solved the Longitude Problem
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The book covers how Harrison's chronometers worked, including his use of the bimetallic strip to compensate for thermal expansion. It traces the history of longitude determination from ancient methods to the competing lunar-distance approach favored by the Board of Longitude. The author explains how Harrison's H4 chronometer finally won the prize money after years of disputes with the board.
This detailed account of maritime timekeeping will interest anyone fascinated by mechanical engineering, navigation history, or the story of how one man's persistence solved a problem that had stumped humanity for millennia.
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Overview
John Harrison, an English joiner and clockmaker born on 3 April 1693, invented the marine chronometer, a device that finally solved the problem of calculating longitude at sea. His invention transformed navigation and made long-distance travel much safer. After the Scilly naval disaster of 1707, the British Parliament offered rewards of up to £20,000 under the 1714 Longitude Act. Harrison never received the full reward due to political rivalries, though he presented his first design in 1730 and spent years refining it. He gained support from the Longitude Board and worked on what were called sea watches. He died in 1776, having achieved recognition and a reward from Parliament.
Early life
John Harrison was born in Foulby, Yorkshire, the first of five children, and his stepfather worked as a carpenter at Nostell Priory. The family later moved to Barrow upon Humber in Lincolnshire, where Harrison followed his father’s trade, building and repairing clocks in his spare time. He built his first longcase clock in 1713, made entirely of wood, and three of these early clocks survive. One is at Nostell Priory, bearing the inscription "John Harrison Barrow." In 1718, he married Elizabeth Barret, and after her death in 1726, he married Elizabeth Scott. During the early 1720s, he was commissioned to make a turret clock at Brocklesby Hall, which still functions today. He also developed the gridiron pendulum and the grasshopper escapement, inventions that improved timekeeping accuracy. Harrison received support from George Graham, introduced to him by Edmond Halley, who championed his work.
Longitude problem
Longitude measures position east or west of the prime meridian, from 0° to ±180°. Accurate navigation was essential for growing global trade, but ships often wrecked due to dead reckoning errors. Many proposed methods compared local time with reference places like Greenwich or Paris. One idea, based on Gemma Frisius's work, relied on astronomical observations and accurate timekeeping, but precise timekeeping over long voyages proved difficult. John Harrison aimed to build a reliable clock maintaining reference time without constant adjustment, facing challenges from temperature, humidity, salt air, and sea motion. Scientists like Isaac Newton and Christiaan Huygens doubted such a device was possible. Huygens tested pendulum and spiral spring clocks, but they gave inconsistent results. Newton said, "a good watch may serve to keep a reckoning at sea for some days... but when longitude at sea is lost, it cannot be found again by any watch."
First three marine timekeepers
In the 1720s, English clockmaker Henry Sully invented a marine clock to solve the longitude problem, using a large vertical balance wheel and frictional Debaufre-type escapement; his design avoided temperature error by controlling oscillations with a weight on a pivoted lever. Though his clocks worked only in calm weather, they were among the first serious attempts at sea timekeeping. In 1730, John Harrison designed a competing marine clock to compete for the Longitude prize, presenting his ideas to Edmond Halley and then to George Graham, who loaned him money. Harrison built H1, which he demonstrated to the Royal Society; it was the first proposal considered worthy of a sea trial. In 1736, Harrison sailed to Lisbon on HMS Centurion and returned on HMS Orford, where his clock performed well on the return voyage. The Board granted him £500 for further development, and by 1737 he had moved to London, where he began work on H2. After three years of building and two of testing, he abandoned H2 when he found a flaw in the bar balance design; this led him to use circular balances in H3. He spent seventeen years on H3, though it didn't perform as intended due to his incomplete understanding of spring physics. Still, the machine yielded important innovations: the bimetallic strip and the caged roller bearing.
Longitude watches
After decades of experimenting with different approaches, John Harrison was surprised to find that watches made by Thomas Mudge, who had succeeded William Graham, kept time just as accurately as his large sea clocks. It may have been because Mudge, starting in the 1740s, had access to a new kind of steel called "Huntsman" or "Crucible" steel, first made by Benjamin Huntsman around that time. This material allowed for stronger pinions and, more importantly, a more durable and polished cylinder escapement. That’s when Harrison realized a regular watch could be accurate enough for navigation and might work better than his previous designs. He then reimagined the watch as a precise timekeeping tool, building it on solid scientific principles.
"Jefferys" watch
In the early 1750s, John Harrison made a precision watch for himself, one that was crafted by the watchmaker John Jefferys around 1752–1753. This watch used a new kind of escapement and included temperature compensation, as well as the first miniature going train fusee designed by Harrison. It performed so well that he used its design as the basis for two new timekeepers: a large one and a smaller one with a similar pattern. Only the larger model, known as No. 1 or "H4," was completed. Finished in 1759, it looked like an oversized pocket watch, engraved with Harrison’s signature, marked Number 1, and dated that year.
H4
John Harrison's H4, built in silver cases about 5.2 inches across and powered by a steel spring inside a brass barrel running for 30 hours, used a modified verge escapement with diamond pallets and a balance wheel nearly 2.2 inches wide. Tested on a voyage from Portsmouth to Kingston, Jamaica, aboard the HMS Deptford departing 18 November 1761, it was found to be only 5 seconds slow—about one nautical mile off in longitude. Harrison waited for his prize of £20,000, but the Board of Longitude doubted the accuracy was more than luck and demanded another trial. The second test took place during a voyage to Bridgetown, Barbados, while another method for measuring longitude, the Method of Lunar Distances, was also being tested.
Death and memorials
John Harrison passed away on 24 March 1776, just short of his eighty-third birthday, and was laid to rest in St John’s Churchyard, Hampstead, alongside his second wife Elizabeth and son William. His tomb was later restored in 1879 by the Worshipful Company of Clockmakers, despite Harrison never having joined the group. A blue plaque marks his final home at 12 Red Lion Square in Holborn, while a memorial tablet was dedicated in Westminster Abbey on 24 March 2006. The Corpus Clock in Cambridge, unveiled in 2008, incorporates his grasshopper escapement as a tribute to his legacy. In 2014, Northern Rail honored him by naming diesel railcar 153316 the John ‘Longitude’ Harrison. Google commemorated his 325th birthday with a doodle on 3 April 2018, and in February 2020, a bronze statue by Marcus Cornish was unveiled in Barrow upon Humber.
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Overview
A marine chronometer is a precise timepiece used on ships to figure out a vessel’s location at sea, specifically its longitude. Sailors compared the time shown by the chronometer—set to Greenwich Mean Time—with the time they observed from the sun or stars at their current position. This difference in time told them how far east or west they were. The first real marine chronometer was built by one man, John Harrison, who spent thirty-one years perfecting it. In the early 1700s, this invention changed navigation forever. The word "chronometer" comes from Greek, meaning “time” and “measure.” One of the earliest descriptions of such a device appeared in a 1713 book called Physico-Theology by William Derham. Today, the term is also used for watches that meet strict timing standards.
History
To navigate at sea, sailors needed latitude, longitude, and altitude—but altitude could be ignored at sea level. By the mid-1750s, determining longitude remained a难题 because of the difficulty in keeping accurate time on a rolling ship. Navigators could find latitude by measuring the sun's angle at noon or Polaris in the Northern Hemisphere, but for longitude, they needed a reliable time standard. The Dutch scientist Gemma Frisius first proposed using a chronometer for this in 1530. A marine chronometer measures time from a fixed location; comparing that to local time lets sailors calculate their longitude using Earth's rotation. The lunar distances method was also developed around the same time, though it required complex observations. Before John Harrison's work, pendulum clocks were the best timekeepers, but they failed at sea due to motion and changing gravity.
First examples
In 1673, Christiaan Huygens, inspired by his pendulum clock invention, built the first marine chronometer in France under Jean-Baptiste Colbert’s sponsorship. By 1675, he had created a spring-driven model, but it was imprecise at sea. His attempt to patent it in England led to a rivalry with Robert Hooke, who also delivered two devices to Charles II that failed to work. The term “chronometer” first appeared in print in 1684, and further theoretical work followed in 1713 by William Derham. In 1714, the British government offered a prize for solving the longitude problem. Jeremy Thacker and Henry Sully began building working models, but neither succeeded. John Harrison, a Yorkshire carpenter, eventually solved the issue with his H4 design in 1761, using a balance wheel and temperature-compensated spring, leading to the first successful sea trials in 1767.
Further development
In 1748, Pierre Le Roy invented the detent escapement used in modern chronometers, and by 1766 had built a revolutionary timekeeper using that escapement, a temperature-compensated balance, and an isochronous balance spring. Though Harrison had shown the possibility of a sea-worthy chronometer, Rupert Gould considered Le Roy's work the foundation of the modern design. Later, Ferdinand Berthoud in France and Thomas Mudge in Britain also made successful marine timekeepers. Then, in 1780, Thomas Earnshaw and John Arnold simplified the design with a detached "spring detent" escapement, moved temperature compensation to the balance, and improved spring manufacturing—advances that defined marine chronometers until the electronic era. Beginning in 1820, the Royal Observatory in Greenwich ran trials to test chronometers, later continued by George Biddell Airy, until World War I. Ships checked their timepieces using a time ball at Greenwich, and chronometers were routinely tested during voyages with lunar or solar observations.
Rationalizing production methods
While watchmaking shifted toward industrial methods in the 19th century, chronometer production held on to traditional craftsmanship much longer, dominated by British and Swiss makers. Around the turn of the 20th century, companies like Ulysse Nardin began using interchangeable parts, but it wasn't until World War II that Hamilton Watch Company perfected mass production. From 1942 onward, Hamilton produced thousands of its Model 21 and Model 22 chronometers for the U.S. military and Allied forces. In Germany, a collaboration between Wempe Chronometerwerke and A. Lange & Söhne developed the Drei-Pfeiler Werk Einheitschronometer, a unified design driven by naval and aviation ministries. Production started in 1942, with all parts made in Germany and fully interchangeable. Modifications due to material shortages were made during the war, and German manufacturers shared work voluntarily or compulsorily to speed up output. After the war, production continued in both West and East Germany, and later in the Soviet Union, which set up a factory in Moscow in 1949 to produce the MX6 chronometer using German designs. From 1952 until 1997, the Soviets produced MX6s with their own altered components. The German Einheitschronometer became the most widely produced mechanical marine timekeeper, with about 58,000 units made in total—less than 3,000 during the war, around 5,000 afterward, and about 50,000 in the Soviet Union. Hamilton's Model 21 saw production of about 13,000 units during and after the war. Still, traditional chronometers never vanished from the market, with companies like Thomas Mercer continuing their production.
Historical significance
Marine chronometers were the most precise portable timekeepers ever built, matching only observatory clocks in accuracy. They allowed ships to find their longitude at sea, giving nations a major naval edge. The Royal Navy's dominance and the rise of the British Empire depended heavily on this technology, while rivals like the French were often defeated due to poor navigation. Even after World War I, the Royal Observatory continued rating chronometers, and by 1937, the British military had its own workshop for repairs. By the 1960s, electric models replaced mechanical ones, and in 1985, the British Ministry of Defence sought bids to dispose of their Hamilton Model 21s. The U.S. Navy kept theirs as backups until GPS became reliable in 1988. By the end of the century, only a few makers like Russia's First Moscow Watch Factory, Wempe in Germany, and Mercer in England still produced them on special order. A complete collection, including Harrison's H1 to H4, is at the Royal Observatory in Greenwich, London.
Characteristics
The marine chronometer's heart was a balance wheel regulated by a spring, but most springs changed elasticity with temperature. Makers used bi-metallic strips to adjust the balance's oscillation, and later, nickel-steel alloy called Elinvar, invented by Charles Édouard Guillaume, who won the 1920 Nobel Prize in physics. The escapement recorded the balance's motion and supplied tiny amounts of energy to overcome friction; spring detent and pivoted detent designs were most common. These used minimal lubrication, with gold components reducing slide friction. Jewel bearings of ruby or sapphire decreased pivot wear, while diamond cap stones protected the lower staff pivot. A marine chronometer was kept in a gimballed box below deck to stay level during ship motion. It included a maintaining power and a power reserve indicator. These mechanisms made mechanical marine chronometers accurate to half a second per day.
Chronometer rating
In strictly horological terms, "rating" a chronometer means that before it went into service, its average daily gain or loss was observed and recorded on a certificate. This rate was used in the field to correct the time shown by the instrument. Even the best-made chronometer with the finest temperature compensation still had two types of error: random and consistent. The quality of design and manufacture kept random errors small. Consistent errors could theoretically be fixed by adjustment, but in practice, they couldn’t be eliminated completely, so rating became necessary. The rate would also change while the instrument was in use—due to things like thickening oil—so on long voyages, its accuracy had to be periodically checked against time determined by astronomical observations.
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Overview
The history of longitude is the story of how sailors and scientists spent centuries trying to figure out exactly where they were on Earth's surface, east or west—something crucial for safe sea travel. While latitude could be found using the sun or stars, longitude required a more complex solution. The problem came down to time: to know your longitude, you had to compare local time with time at a fixed reference point, like Greenwich. That meant building clocks that could keep accurate time even on a rocking ship. Astronomers once tried predicting events like eclipses, but it was John Harrison who finally cracked the code in 1773, inventing a marine chronometer so precise it made sea navigation possible. Later, tools like the telegraph and radio helped synchronize time across great distances. Today, satellite navigation solves the longitude problem with centimeter-level accuracy.
Longitude before the telescope
In the 3rd century BC, Eratosthenes laid groundwork for global mapping with his latitude and longitude system, using Alexandria and Rhodes as prime meridian. By the 2nd century BC, Hipparchus refined this approach, introducing a 360-degree grid and proposing longitude be determined by comparing local times of lunar eclipses—though method was limited by lack of accurate timekeeping. Ptolemy, in the 2nd century AD, built on earlier work using estimated distances and directions to create maps with curved parallels that reduced distortion, yet his data contained systematic errors, especially in longitude calculations. The Hindu astronomers described similar method in the Sûrya Siddhânta, referencing prime meridian through Avantī. Islamic scholars, beginning in 9th century, expanded on Ptolemy's geography; al-Battānī in 901 used simultaneous eclipse observations to determine longitude differences with less than one degree of error, while al-Bīrūnī developed early triangulation method that also approached modern accuracy. In Europe, knowledge of Ptolemy faded until later medieval period, when progress slowly resumed.
Telescopes and clocks
In 1608, a patent for a refracting telescope was submitted in the Netherlands. Galileo picked up the idea the next year, making his first telescope and beginning groundbreaking astronomical observations, including Jupiter's moons and Venus's phases. Over the following fifty years, advances in optics and precise adjustments turned telescopes into accurate measuring tools that expanded the range of events usable for determining longitude. At the same time, Christiaan Huygens patented the pendulum clock in 1657, improving timekeeping accuracy by about thirty times—best pendulum clocks lost only about ten seconds per day. Huygens intended his clocks for sea navigation, but they couldn't handle ship motion well, so other methods were needed. His work also introduced the balance spring, allowing portable timepieces to be made, though it was John Harrison's later innovations that finally produced marine chronometers accurate enough for this purpose.
Methods of determining longitude
The development of the telescope and accurate clocks opened up new ways to figure out longitude. Almost all methods relied on comparing local time at two different places, using a shared moment in time as a reference. Since Earth spins 360 degrees in 24 hours, every hour of difference equals 15 degrees of longitude. Determining local noon—when the Sun is highest—was tricky because it moves nearly horizontally at that moment. People often used the midpoint between two sightings of the Sun at the same altitude, or the midpoint between sunrise and sunset if the horizon was clear. At night, they measured star positions to find local time. One way to get absolute time was through lunar eclipses, which were widely observed and recorded.
Lunar distances
Lunars, or lunar distances, were an early method for calculating longitude, first made practical by Regiomontanus in his 1474 Ephemerides Astronomicae. Amerigo Vespucci used this technique in 1499 while exploring South America, making calculations on August 23 and September 15. The approach was later published by Johannes Werner in 1514 and discussed by Petrus Apianus in 1524. It relied on measuring the Moon's movement relative to stars, since the Moon travels just over half a degree per hour. A small error in that angle—just two minutes of arc—could cause a full degree of longitude difference, or sixty nautical miles at the equator. The method needed precise tables and instruments, which weren't accurate enough in the early 16th century. Vespucci's first attempt placed him near the correct longitude, but his second was off due to faulty ephemerides from Regiomontanus. Over time, better tools and observations improved accuracy. By the 1760s, the UK began publishing the Nautical Almanac and the U.S. started the American Ephemeris and Nautical Almanac in 1852, both including lunar distance data.
Moon culminations
Moon culminations were a method of determining longitude that worked similarly to lunar distances but were generally simpler to calculate. The observer recorded the time of the moon's highest point in the sky and compared it with a reference time from an ephemeris table, adjusting for things like refraction and other errors. This technique was developed and established by Nathaniel Pigott around 1786. Because a culmination only happened about once a day, it was often combined with other astronomical observations to improve accuracy.
Satellites of Jupiter
In 1610, Galileo discovered Jupiter's four largest moons—Io, Europa, Ganymede, and Callisto—and proposed using their regular movements to tell time anywhere on Earth, solving the longitude problem. He sought Spanish support in 1616 but was unsuccessful, later trying Holland after being tried for heresy and sentenced to house arrest. His approach required a telescope, and he suggested a device called the celatone to keep observers steady on rolling ships. A jovilabe was also proposed as an analogue computer computing time from the moons' positions. Though never used at sea, this method worked well on land. In 1671 and 1672, Jean Picard and Giovanni Domenico Cassini used Jupiter's moons to measure the longitude of Tycho Brahe's old observatory on Hven, arriving at a value slightly higher than today's accepted figure. Later, in the 1740s, Jupiter's moons helped the French Académie des Sciences produce a more accurate map of France.
Appulses, occultations, transits, and eclipses
Several methods for determining longitude relied on the movements of the Moon as it passed in front of stars or planets. An appulse is when the Moon and a star come closest together in the sky, while an occultation happens when the Moon blocks the view of a star or planet—essentially a type of eclipse. The times of these events could be used just like lunar eclipses to mark absolute time. Edmond Halley described using this method in 1680 to find the longitude of Balasore, India, observing the star Aldebaran, with an error of just over half a degree. He later published a more detailed account in 1717. A longitude determination using the occultation of Jupiter was reported by James Pound in 1714. The transit of Venus in 1769 offered a chance to calculate longitude for over 100 seaports around the world.
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Overview
Longitude by chronometer is a method of finding longitude at sea using a marine chronometer developed by John Harrison in the first half of the eighteenth century. It relies on knowing exact time to calculate where a celestial body appears in the sky, helping determine a navigator's position line. A sextant measures the angle between horizon and sun or star, corrected using tables to get true altitude. From this, along with assumed latitude, a formula calculates true hour angle and corrects longitude. Multiple sights taken at different times allow for a fix—where lines cross—and pinpoint exact location. The method requires accurate timekeeping, which Harrison's H-4 finally provided. The reference longitude used globally, called the Prime Meridian, runs through Greenwich, England, and is now known as Coordinated Universal Time, or UTC.
Noon sight for Longitude
A noon sight gives you latitude, but not longitude. You can’t figure out longitude accurately at noon because the sun’s altitude changes very slowly at its highest point, making it hard to pin down the exact time. That’s why sailors needed another method. By taking a reading just before local noon—say, between 15 and 30 minutes early—and then watching the sun drop back through the same angle later, you can calculate an average time. This technique, called a meridian altitude, lets you determine longitude within about two nautical miles, which is good enough for safe navigation.
Corrections to the process
The Earth’s orbit around the Sun isn’t perfectly circular, which means the Sun's apparent movement varies throughout the year. This causes local noon to happen at different times compared to 12:00 UTC, sometimes a few minutes early or late. The Nautical Almanac includes a correction called the Equation of Time for each day, and navigators must apply this to their readings. When combined with time-averaged sightings of the Sun at local apparent noon, this method can achieve accuracy within one nautical mile. Without averaging, the shape of the Sun’s path across the sky makes it hard to pinpoint exact noon time, reducing precision. Other techniques using the Nautical Almanac and sight reduction tables also help navigators determine longitude within a similar margin of error.
Time sight
Time sight is a method for finding longitude using celestial observations and a chronometer, and it works alongside other techniques like the noon sight or latitude by Polaris to pinpoint a ship’s location. It involves solving the navigational triangle to calculate the local hour angle, LHA, using known values of altitude, latitude, and declination. The formula for this calculation is: cos(LHA) = (sin(Ho) − sin(δ) · sin(B)) / (cos(δ) · cos(B)). This gives the meridian angle, which is then compared with the Greenwich hour angle to determine longitude. Though it only calculates a longitude at a given assumed latitude, it still produces a position line, and the observer’s actual location lies somewhere along that line.
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Overview
The British government offered prizes to solve the longitude problem, starting with the Longitude Act 1714, formally known as 13 Ann. c. 14, which created the Board of Longitude to oversee the rewards. This wasn’t the first attempt—Philip II of Spain had offered a prize in 1567, and later Philip III of Spain promised 6,000 ducats and a pension in 1598. The States General of the Netherlands also offered 10,000 florins soon after. In 1675, Robert Hooke applied for a £1,000 reward in England for his spring-regulated watch, but although large prizes were never claimed, smaller awards were given for important progress.
Background
The problem of measuring longitude came to a head as sea travel grew more common, because finding latitude was easy—using the sun's height at noon and a table of its daily position. But for longitude, sailors had to guess their way across the ocean using dead reckoning, which often led to shipwrecks. An accurate method was also needed to find magnetic declination—the difference between magnetic north and true north, sometimes up to ten degrees in key trade areas. The Longitude Act of 1714 offered a reward for solving this issue at sea. Before that, countries like Portugal, Spain, and the Netherlands had already offered prizes as early as 1598. Solutions were sought in three ways: land-based methods using Jupiter's moons, charts, or timekeeping devices. Scholars believe it was the promise of economic gain and power from ocean exploration—not science alone—that drove the Act's passage and the large reward for a working solution.
Establishing the prizes
In the early 1700s, maritime disasters like the 1707 wreck of a naval squadron on the Isles of Scilly highlighted the urgent need for accurate longitude determination. Mathematician Thomas Axe had already left £1,000 in his will for research into finding "true longitude," and annual payments for map corrections. In 1713, Parliament passed the Longitude Act after hearing proposals from William Whiston and Humphrey Ditton, establishing three prizes based on accuracy levels: £10,000 for within one degree, £15,000 for 40 minutes, and £20,000 for 30 minutes. These standards matched those set by Whiston and Ditton and recommended by Isaac Newton and Edmund Halley. Methods had to be tested on ocean voyages, and contenders were required to prove their accuracy using known landmarks. A Board of Longitude was created to judge submissions and offer up to £2,000 in advance for promising projects. Though no one won the full £20,000 prize, John Harrison received a total of £23,065. The Board remained active for over a century, disbursing more than £100,000 before its dissolution in 1828.
Notable recipients
The Longitude Act offered a large reward for solving the longitude problem, and some recipients like Euler and Mayer said publicly that the money wasn’t what drove them—instead, they valued the progress in navigation and mapmaking. Others, such as Kendall and Harrison, had to ask the Board of Longitude and government officials for fair pay. Some submitted wild or unworkable ideas, a few of which are kept at Harvard’s Houghton Library. The Board did not give £20,000 all at once, but they did give smaller amounts to people who improved instruments or published star charts and atlases.
John Harrison's contested reward
John Harrison, who was just twenty-one when the Longitude Act was passed, spent forty-five years perfecting his sea timekeepers, including the H4 watch. He first received £250 in 1737 to improve his H1, leading to H2, and later £2,000 between 1741 and 1755 for H2 and H3. From 1760 to 1765, he was paid £2,865 for the construction and ocean trials of H4. Though H4 exceeded the accuracy needed for the top reward, Harrison only received £7,500 after revealing his method and proving the device could be replicated. He made one instead of two additional H4s, and after appeals to the Board of Longitude went unanswered, he and his family petitioned King George III. Parliament granted him £8,750 in 1773, bringing his total reward to £23,065 over thirty-six years.
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Overview
In the 1750s, John Harrison, the brilliant clockmaker who was trying to solve the longitude problem at sea, invented a key component for his third marine chronometer, H3, in 1759. That device, known as a bimetallic strip, was made of two different metals that expanded at different rates when heated. When the temperature changed, the strip would bend one way or the other, turning heat into movement. Harrison used this to keep his timekeeper accurate despite the varying conditions on a ship. His invention is remembered today in a memorial dedicated to him in Westminster Abbey, England.
Characteristics
A bimetallic strip is made of two different metals, like steel and copper, bonded together so they expand at different rates when heated. When the strip heats up, it bends one way; when it cools, it curves the opposite direction. The metal with the higher expansion coefficient lies on the outside of the curve when heated and inside when cooled. These strips can be used flat or coiled, with the latter offering greater sensitivity due to its increased length. French physicist Yvon Villarceau derived a formula in 1863 to calculate how the radius of curvature changes with temperature, which helped improve clock accuracy. The calculation involves the materials’ thermal expansion coefficients, thicknesses, and elastic moduli, with a simplified version used when those properties are similar for both metals.
History
John Harrison, the eighteenth-century clockmaker widely credited with inventing the bimetallic strip, first used it in his third marine chronometer, H3, in 1759. He created it to counteract temperature changes that affected the balance spring. This early version joined two metal strips with rivets, but he later developed a method of fusing molten brass directly onto a steel base. A strip made this way was fitted into his final timekeeper, H5. Harrison’s innovation is honored in a memorial to him in Westminster Abbey, England.
Manufacturing
Bimetallic strips are made by joining two different metals together, and John Harrison used a method called electroplating, where one metal is cast onto another—specifically, brass on steel. The most common modern way to make these strips is roll bonding, where metal strips are pressed together under high heat and pressure. Other techniques include explosive welding, brazing, or soldering. Once the metals are joined, the strip often goes through annealing, a heat treatment that removes internal stresses. It’s then rolled or drawn to reach the exact thickness needed. Getting the layer thickness just right is essential because it controls how sensitive and curved the strip becomes. Today’s technology can produce very thin strips, down to 0.1 millimeter or less, which are used in small devices like thermostats and circuit breakers.
Clocks
Mechanical clocks are affected by temperature changes because every part has tiny tolerances that can throw off timekeeping. One solution used in some timepieces is the bimetallic strip, which compensates for these fluctuations. The most common use of this method is in the circular rim of the balance wheel. It works by moving a weight radially, changing the momentum of inertia. As temperature rises and the controlling spring grows weaker, the balance wheel shrinks in diameter to keep the oscillation period — and thus timekeeping — constant. Today, this system is no longer used because low temperature coefficient alloys like nivarox and parachrom have replaced it.
Thermostats
In heating and cooling systems, thermostats use bimetallic strips that bend with temperature changes. One end is fixed and connected to a power source, while the other moves to touch an electrical contact. Adjustable models use a knob or lever to set the desired temperature, known as the set point. Some thermostats use mercury switches with angled units to adjust the set point. Rising temperature can either open or close circuits—like in heaters or refrigerators. Contacts might directly control power, like in irons, or work through relays or valves for gas or oil. Older gas heaters used thermocouples heated by pilot flames for power, while modern units use reduced household electricity to power electronic ignitors, either resistance heaters or spark generators.
Thermometers
A bimetallic strip is used in many everyday thermometers, like those found on patios or in ovens, where it helps convert heat into a readable dial reading. In its most common form, the strip is coiled into a helix, turning the straight expansion of metals into circular motion. One end of the coil stays fixed to the device’s housing, while the other moves a needle across a circular scale. This same principle appears in recording thermometers too. Breguet's version uses a tri-metallic helix for even greater accuracy.
Heat engine
Heat engines aren’t very efficient, and when bimetallic strips are used in them, their efficiency drops even further because there’s no chamber to contain the heat. The strips also can't generate much force in their movements. To achieve noticeable bending, both metal layers have to be thin, which limits their strength. That’s why bimetallic strips are mostly used in simple toys built just to show how the principle might drive a heat engine.
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Overview
A clock measures and displays time, one of humanity’s oldest inventions, created to track intervals shorter than days, months, or years. Early devices included sundials, hourglasses, and water clocks, with the verge escapement in 1300 enabling mechanical clocks in Europe. By the 15th century, spring-driven clocks appeared, and clockmaking flourished. Christiaan Huygens invented the pendulum clock in 1656, improving accuracy. Navigation demanded precise timekeeping, driving further advances. Modern clocks use harmonic oscillators—pendulums, balance wheels, quartz crystals, or atomic vibrations—to keep time. Some display time with hands, others digitally, using either 12- or 24-hour notation. Electronic clocks emerged in the 20th century, and speaking clocks audibly announce the time for the blind or over phones.
Etymology
The word clock comes from the medieval Latin clocca, meaning 'bell,' and shares roots with similar words across Europe. These timekeeping devices made their way to England from the Low Countries, where the local term for bell was Klocke in Middle Low German and Middle Dutch. In Middle English, the word was spelled clokke, while Old North French used cloque and Middle Dutch had clocke— all referring to that familiar sound.
Sundials
The Sun’s position in the sky shifts throughout each day as Earth turns, and so do the shadows cast by fixed objects. A sundial uses this movement to tell time, showing the hour by where a shadow falls on a marked surface—whether flat, vertical, or angled. These devices were common in ancient times and, when built well and adjusted for latitude, could show local solar time within a minute or two. People kept using sundials to check the accuracy of clocks right up until the 1830s, when trains and telegraphs led to standardized time zones between cities.
Devices that measure duration, elapsed time and intervals
Many tools exist to track the passing of time without relying on clocks that show hours or minutes. These devices measure duration or intervals by using a steady, predictable process. Examples include candle clocks, incense clocks, and the hourglass. Both candle and incense clocks work on the same idea: a consistent rate of resource use gives a reliable estimate of elapsed time. In an hourglass, sand flows through a small opening at a steady pace, marking a set amount of time. The material isn't used up—it's recycled. Each of these instruments offers a way to know how long something takes, without needing a reference like noon or midnight.
Water clocks
Water clocks, alongside sundials, are among the oldest timekeeping tools, with only tally sticks possibly older. The simplest type, a bowl-shaped outflow device, existed in Babylon and Egypt around 16th century BC. Evidence also points to early use in India and China, though dates are less certain; some writers claim water clocks as early as 4000 BC. In the 1st century BC, the Macedonian astronomer Andronicus of Cyrrhus oversaw construction of the Tower of the Winds in Athens, which housed a large clepsydra along with sundials. Greek and Roman innovations were passed through Byzantine and Islamic times, eventually reaching Europe. Independently, China developed advanced water clocks by 725 AD, sharing designs with Korea and Japan. These early timepieces weren't precise by modern standards but were the most accurate available for millennia. In 797 or possibly 801, the Abbasid caliph Harun al-Rashid gave Charlemagne an elaborate water clock along with an Asian elephant named Abul-Abbas.
Mechanical water clocks
The first known geared clock was invented by Archimedes in the 3rd century BC; it was an astronomical clock that also functioned as a cuckoo clock, with birds singing and moving hourly, regulated by floats in a water container and siphons. A later Greek clock in Gaza, described by Procopius, may have been a Meteoroskopeion, showing celestial events and time with automations like 12 doors opening hourly, each depicting a labor of Hercules, and lamps appearing at night. In China, the Tang dynasty monk Yi Xing and official Liang Lingzan created the first clockwork escapement in 723 or 725, which Su Song later used in his 1088 astronomical clock tower in Kaifeng, combining hydraulics and mechanics, running on either water or mercury. Su Song wrote that the heavens move without ceasing, just as water flows evenly, allowing for accurate comparison between celestial and mechanical motion. His work built on Zhang Sixun's earlier use of mercury in an astronomical clock from 976 AD. The Chinese escapement eventually spread westward, influencing Western clockmaking. In the 12th century, Al-Jazari, who worked for the Artuqid king Nasir al-Din in Mesopotamia, built various elaborate clocks including elephant and castle designs, some of which have been reconstructed.
Fully mechanical
In 1176, Sens Cathedral in France installed an ‘horologe’, but the mechanism is unknown. By 1198, monks at St Edmundsbury ran to their clock during a fire, showing it had a water reservoir. The word “clock” comes from Old Irish clocc, meaning bell, and reflects early mechanical clocks that made sound. Between 1280 and 1320, references to clocks increased in church records, pointing to new mechanisms using falling weights instead of water. These used an escapement—likely derived from bell-ringing devices—which marked the start of true mechanical clocks. They served two purposes: signaling times for services and modeling the solar system. In 1283, a large clock was installed at Dunstable Priory; in 1292, Canterbury Cathedral got a ‘great horloge’. By 1322, a new clock was installed in Norwich, featuring a two-meter dial, automata, and bells, requiring two full-time clockkeepers for two years.
Astronomical
In 1092, Su Song, a Chinese polymath, created the 'Cosmic Engine,' an elaborate water clock that stood about ten metres high and used falling water and liquid mercury to power an armillary sphere for calculating astronomical problems. In Europe, Richard of Wallingford built a clock in Albans by 1336, while Giovanni de Dondi constructed the Astrarium in Padua between 1348 and 1364. The Astrarium had seven faces and 107 gears, showing the positions of the Sun, Moon, and five known planets, along with religious calendars and eclipse predictions. Wallingford’s clock featured a large astrolabe dial and indicators for tides and the hour. These mechanical marvels, though likely adjusted daily for accuracy, demonstrated how astronomical knowledge drove clockmaking in both China and Europe.
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Overview
An hourglass is a device used to measure time, made of two glass bulbs connected by a narrow neck, through which sand flows from the top to the bottom due to gravity. The amount of time it measures depends on the size and coarseness of the sand and the width of the neck. These devices have been symbols of time’s passage in art, appearing on monuments and tombstones from ancient times to today. A winged hourglass is often used to represent the Latin phrase "tempus fugit," meaning "time flies."
Middle Ages
The hourglass first appeared in Europe during the Late Middle Ages, with the earliest clear record being a 1338 fresco by Ambrogio Lorenzetti. By the 14th century, ships used marine sandglasses, as documented in logbooks and ship inventories. A specific reference from c. 1345 mentions Thomas de Stetesham, clerk of King Edward III's ship La George, paying for glass horologes in Flanders. These devices were reliable at sea because they weren't affected by a ship's motion or temperature like clepsydras were. Though not precise enough to determine longitude, they helped measure speed with a chip log. On land, hourglasses became common for everyday tasks like timing sermons or cooking. Their designs grew smaller and more discreet. After 1500, mechanical clocks began to replace them, but the hourglass didn't vanish entirely. It remained popular for its design. John Harrison wouldn't invent a marine chronometer accurate enough to surpass the hourglass until the 18th century, when his 1761 model tracked a journey from England to Jamaica within five seconds.
Design
The hourglass design we know today didn’t come about suddenly, and there’s little written explanation for why it ended up looking the way it does. The glass bulbs were always ampoule-shaped, but early versions used two separate pieces joined by a cord wrapped at the connection point and sealed with wax to keep the sand flowing. It wasn’t until 1760 that both bulbs were blown together, which helped prevent moisture from entering and regulated internal pressure that otherwise affected how quickly the sand would pass through.
Material
Some early hourglasses used silica sand to measure time, but many didn’t use sand at all. Most bulbs were filled with powdered marble, tin or lead oxides, or crushed, burnt eggshell. People tested different textures of granular material to find what gave the most consistent flow. Eventually, it was discovered that for ideal timing, the size of the grains needed to be in a specific ratio to the neck of the bulb—more than 1/12 but no more than 1/2 the width of the neck.
Practical uses
Hourglasses were an early reliable way to measure time, with steady sand flow regardless of upper chamber height, working well in cold weather. From the 15th century, they were used widely—on ships, in churches, industry, and cooking. During Ferdinand Magellan's global voyage approved by King Charles I of Spain, 18 hourglasses from Barcelona were carried aboard, with a ship's page responsible for turning them to keep the ship's log. Noon served as navigation reference point since sun would be directly overhead. Some hourglasses were grouped together in frames, each set for different time—like a four-way Italian sandglass from the 17th century kept in the Science Museum in London, measuring quarter, half, three-quarters, and full hours, used in churches to time sermons.
Modern practical uses
Even though hourglasses aren’t used much anymore for telling time, certain places still keep them around. In Australia, both houses of Parliament use three of them to manage specific procedures like voting. Sand timers also show up in some tabletop games, such as Pictionary and Boggle, where they help enforce time limits during rounds.
Symbolic uses
The hourglass stands apart from other timepieces because it visually shows the present moment as existing between past and future, making it a powerful symbol for the concept of time itself. Often paired with wings, it represents how human life is brief, with the "sands of time" running out for everyone. This imagery was used on pirate flags to inspire fear, and in England, hourglasses appeared in coffins and on gravestones for centuries. Alchemists also associated the hourglass with the passage of time. The former Metropolitan Borough of Greenwich in London included an hourglass in its coat of arms, symbolizing its role in establishing Greenwich Mean Time, and the current Royal Borough of Greenwich uses two hourglasses to represent that same legacy.
Modern symbolic uses
The hourglass, once a practical timekeeping tool, still stands as a powerful symbol today. In the American soap opera Days of Our Lives, which has aired since 1965, an hourglass appears in the opening credits with narrator Macdonald Carey saying, "Like sands through the hourglass, so are the days of our lives." Modern computers use the hourglass icon to show when a program is busy and not accepting input. If the icon stays, it often means the program is stuck or waiting for something. The Unicode standard includes the HOURGLASS symbol at U+231B. In the 21st century, an hourglass has also become part of the extinction symbol, representing time running out for endangered species and climate change efforts.
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