2.08. Clocks

Sundials and obelisks

Ancient Egyptian obelisks, built around 3500 BC, are also among the oldest ghost clocks. The oldest known sundial comes from Egypt. It dates from around 1500 BC.

Candle clocks

The first mention of candle clocks comes from a Chinese poem written in 520 AD. According to this poem, the graduated candle, whose burning rate was measured, was a means of determining the time of night. Similar candles were used in Japan until the early 10th century.

Hourglass

Hourglasses were the first reliable, reusable, reasonably accurate and easily manufactured time measuring devices. From the 15th century onwards, hourglasses were mainly used to tell time at sea. An hourglass consists of two glass spheres connected vertically by a narrow neck that allows a controlled flow of material, usually sand, from the upper sphere to the lower. Hourglasses are still used today. They were also introduced for use in churches, industry and the kitchen.

Candle clocks were another timekeeping device used in the ancient world, from China to England to Mesopotamia. Chronometers were developed in places like India and Tibet and the hourglass (which was widely used throughout Europe) came a little later.

Greek water clocks

An early prototype of an alarm clock was invented by the Greeks around 250 BC. The Greeks built a water clock, called a clepsydra, where rising water told the time and eventually struck a mechanical bird that set off an alarm whistle. The water filled a tank with an hour scale inside and flowed out through a hole in the base of the container. Clepsydras were more useful than sundials – they could be used indoors, at night and even under cloudy skies – but they were not as accurate. Greek water clocks became more accurate around 325 BC and were fitted with a dial with an hour hand, which made reading the clock more accurate and practical.

Monastery clocks and clock towers

The life of the Church, and in particular the monks who called others to prayer, made chronometers a necessity in daily life. The first clockmakers in medieval Europe were Christian monks. The first recorded clock was built by the future Pope Sylvester II around 996. Much more sophisticated clocks and church towers were built by monks later. Peter Lightfoot, a 14th century Glastonbury monk, built one of the earliest surviving clocks, which is still in use at the Science Museum in London.

Mechanical clock

A mechanical clock is a time-measuring instrument that tells the time by an originally entirely mechanical action. It is based on the combination of three functions: a driving weight for the rotary movement, a regulator such as a balance wheel and a display (a graduated scale and hands). The mechanical clock is the successor to the various horologia. The Romans, and before them the Egyptians and Greeks, had already developed time-measuring instruments that demonstrated great astronomical knowledge. The invention of the mechanical clock can be placed around the 1300s and appeared in Western Europe at the end of the 13th century. Originally, it was a weight motor and foliot. A foliot is a vertical pendulum that controls the energy supplied to a wheel by a weight. The motion is then transmitted to gears that drive the movement of the hands. A weight suspended from a rope provides energy to the machine, while a system of rods and foliots periodically interrupts the fall of the weight. By placing the weights at each end of the foliot, the rhythm of the back and forth movements can be adjusted. The low precision of this mechanism, from 1 to 2 hours of deviation per 24 hours, renders the minute hand useless, and it will undergo a long evolution and an important diversification over the centuries.

“Charlemagne’s “Clock

In 806, Charlemagne was presented with a prestigious horologium by an ambassador of the Abbasid Caliph of Baghdad, Haroun ar-Rachid; this clock, whose automata are often described in detail, is sometimes considered a mechanical clock.

Located in the northern part of the Champ-de-Mars (Rome), halfway between the Mausoleum of Augustus and the Pantheon of Agrippa, this monument, inaugurated in 10 BC, was unique in its size in antiquity. It was a gigantic sundial, occupying a large square of 150 by 70 metres, paved with marble and bearing astronomical graduations and bronze inscriptions, some of which have been found.

Eginhard, Charlemagne’s chronicler, described it in the 800s as follows: “A machine which, driven by the motive power of water, marks the hours by an appropriate number of small bronze balls which fall on a brass gong; at the end of each hour, a rider comes out of one of the twelve windows, which is open at first and then closes behind him.”

Later, Pacificus of Verona, who died in 844, is also credited with the invention of a clock, considered the first mechanical clock. His horologium nocturnum was in fact only an observation tube that bore some resemblance to the nocturlabe depicted in one of the astronomical manuscripts in the library of the abbey of Mont-Saint-Michel; this affinity is still found in 20th century encyclopaedic sources.

History of the clock from its origins to the 16th century,

The origins

No mechanical clocks seem to exist before the fourteenth century, but several mentions in manuscript sources reveal some of the early history of the clock.

The Latin word horologium, horologia, derived from the Greek [ὡρα, time and λέγειν, to say], has been used since Roman antiquity to refer to all time-indicating devices, but the use of this word for all time-measuring instruments hides from us the true nature of their mechanisms.

The hydraulic clock

A hydraulic clock is an ancient type of clock, which tells the time by allowing a liquid in a container to flow through a small hole.

From the beginning, in ancient times, the liquid used was water, hence the name water clock. Later, mercury clocks could be found, especially in Arabic and Chinese writings, but this seems to be anecdotal.

The first hydraulic clocks evolved from the simple clepsydra (see the France 2 television game show), to which a more or less sophisticated time indication was added (essentially a graduated scale); over the centuries, we have encountered different types of hydraulic clocks, monumental or not, with sophistications to animate skits, for example, or to strike a precise time.

The water clock or clepsydra comes from the Greek word klepsydra, “thief of water”, as it was used to limit the speaking time of lawyers during trials.

It is thought to have been invented by the Egyptians in the 16th century BC. It is unreliable because the speed of the flow varies according to the temperature and pressure of the water,

From the eleventh to the thirteenth century, documentary sources about water clocks are more numerous, but their interpretations remain ambiguous.

For example, a manuscript mentions that in 1176 a college of church commissioners was established in Sens Cathedral to supervise the clock. In 1198, an ordinance stipulated that the men in charge of the clock during the week risked a fine if they did not wind the mechanism in time. In 1867, G. Juillot, a member of the city’s Archaeological Society, concluded with certainty that the clock was “weighted and stamped”. A. Ungerer, in a 1931 book, makes it a “mechanical clock”, which is even more implausible (according to Gerhard Dohrn-van Rossum, notes 4-52);

According to Jocelin de Brakeland, in 1198, during a fire in Bury St Edmunds Abbey, the monks rushed to the clock to fetch water. There is no ambiguity here, the ‘horologium’ is powered by water, so it is a hydraulic clock whose reservoir was large enough to put out the occasional fire.

The first mechanical clocks

The best historians of time measurement agree today that the origin of mechanical clocks dates back to the end of the 13th century.

The earliest references to clocks with a wheel or pendulum are from the 14th century, including those of Richard of Wallingford (abbot of St Alban), Charles V and the Duke of Burgundy.

At that time, hydraulic clocks were very common in monasteries and cathedrals. They were used to indicate a specific canonical time (liturgical offices dedicated to prayers) for the communities. These increasingly sophisticated machines were equipped with automatic alarm systems about which little is known today.

At the end of the Middle Ages, the first mechanical public clocks made it possible to indicate a uniform time throughout the year and to abandon the use of variable seasonal times. From then on, a division into twenty-four hours of equal duration, known as equinoctial hours, was adopted. Technical improvements soon led to the creation of a new corporation and the birth of a much sought-after profession, that of watchmaker.

The mechanical watch, whose exclusivity lasted for almost five centuries, was used until the end of the 1970s following the invention of the electronic watch.

1271: An imminent invention

In 1941 Lynn Thorndike published a text of great importance for the history of mechanical clocks. This text – probably the earliest record of clocks, dated 13 April 1271 – concerns a commentary by Robert the Englishman, known as Robertus Anglicus, on The Sphere of Sacrobosco, which states in brief:

That a wheel which can turn uniformly over twenty-four hours (to give the equinoctial hours) has not yet been perfected;but that the researches of the clockmakers were going in that direction. Robertus then proposed a wheel driven by a weight (without mentioning the problem of accelerating the movement of this system).

This means that the mechanical clock was still in the research stage at that time. In simple terms, the year 1270 can be considered as the “earliest” date of this invention, which in scientific terms is called “terminus ante quem non”.

“The switch to mechanical time did not translate into language”, it did not lead to a change in terminology: the term horologium was simply retained, as for water clocks.

While the increasingly frequent references to “horologia” in parish registers suggest that a new technology was emerging at the time, the difficulty of interpretation is still at the forefront: are they hydraulic or mechanical “horologia”?

Mechanical clocks

Mechanical clocks began to develop. More than thirty testimonies are quoted from all over Europe at the end of the 13th century. In these texts we find the purchase of iron and weights and the acquisition of expensive clocks in monasteries, cathedrals and princely residences. These prices were often considerable: six marks for the clock of the monastery of Colmar in 1278, thirty pounds for that of Canterbury in 1292, fifty pounds for a simple repair or transformation of the clock of the cathedral of Sens in 1319.

Gradually, the mechanical clock, whose movement is permanently maintained by a motorised weight, replaced the boring water clock that had to be constantly cleaned and filled or emptied. It seems that these early clocks – like the water clocks – were first intended to strike a particular time (e.g. alarm clock) and a little later to strike different times of the day; these clocks were “blind”: they had no dial! The striking mechanism was operated by one or more pins on a wheel of the mechanism.

This transfer of technology from hydraulic to mechanical power enabled the original purpose of the clock to be retained: the striking of a precise hour; this invention has therefore remained practically anonymous. It was not until a little later, in 1336, that an important innovation changed the history of clock-making: striking clocks.

Until the 14th century, time was measured by means of the astronomical dial, the hourglass or the clepsydra. Mechanical clocks built around the middle of this century were driven by a descending weight.

The mechanism of a mechanical clock consists first of all of a driving element, consisting of a weight attached to a string wound on a drum. It ensures the rotational movement of the shaft where the hour wheel is located with its pin to activate a chime; secondly, a gear consisting of the hour wheel and a pinion driving the regulating element and, thirdly, a regulating element consisting of the escapement, i.e. the vertical meeting wheel, acting on the paddle rod which gives its oscillatory movement to the leaf.

1336: the first clock that strikes the hours

It was in Milan, in 1336, that an extraordinary clock was installed on a bell tower in the city. Its particularity was that the twenty-four hours of the day were struck with a number of beats corresponding to the time of day:

“one note on the first, two beats on the second, three on the third and four on the fourth…”.

This admirable way of automatically ringing the hours, useful for all categories of the population, is a real innovation. It was this new and fascinating principle of striking that led to the spread of “ringing” clocks.

1350-1500: diffusion and brief evolution

Thanks to their success with the population, striking clocks spread rapidly; Italian cities were their only home in the first half of the 14th century, and then they spread throughout Europe over the next fifty years.

Over time, new needs and technical advances also made them more complex and miniaturised.

To make the hours visible, the clock has a dial divided into twenty-four hours. This dial originally rotated for a fixed index, but later became static with a single hour hand. As the twenty-four hour division was impractical for reading and especially for counting the hours of striking, the dial was simplified to a twelve hour figure. In the 1400s, the clock was already placed on top of towers, belfries and bell towers, where it could be seen and heard by all.

In addition to the simple indication of the hours, special or prestigious clocks soon indicated other astronomical data: the astronomical clock of Richard of Wallingford between 1327 and 1356 and the astrarium of Giovanni Dondi, around 1360, are the first examples; the astronomical clock of Prague, erected in 1410, will be one of the most beautiful jewels.

Wealthy lords wanted their motor-weight wall clocks in their homes: King Philip the Fair of France from 1314, the Pope in Avignon from 1365 and King Charles V of France in 1377. Technical advances, including the invention of the flat spiral spring in Italy before 1390, encouraged miniaturisation, which gave rise to the first table clocks; a hundred years later, the first watches appeared: from 1490, court officials wore striking clocks on their ceremonial costumes.

In the 15th century, a new industry emerged in Germany, the mechanical clock industry, which reached a decisive stage in its development with the appearance of a mainspring. Technical improvements made it possible to produce outdoor and indoor clocks, table clocks and watches.

In 1504, the first portable watch was invented in Nuremberg, Germany, by Peter Henlein. It was not very accurate.

The first person to wear a watch on his wrist was the French mathematician and philosopher Blaise Pascal (1623-1662). He tied his pocket watch to his wrist with a string. The wristwatch was born.

In the 16th century, the watchmaking profession became more specialised: the processes and the shape of the parts improved; the number of astronomical clocks equipped with automata increased, thanks to the cooperation of mathematicians.

Astronomical clock

The first astronomical clock in Strasbourg was built in the middle of the 14th century; the second, dating from 1575, is the work of the Habrecht brothers. Throughout the 15th century, public clocks were built by craftsmen, mostly of Swiss origin. The emergence of a new craft tradition paved the way for another era, that of the mechanics.

The clock as a symbol of power in the Renaissance

In the 16th century, the production of clocks was the height of technological complexity: owning a clock or a watch was a privilege of the rich. The first production centres appeared in the big cities and in the royal courts. The city of Paris was the first to proclaim the statutes of a corporation of clockmakers in 1544. In Blois, under the reign of Henry II (1547-1559), the presence of the Court led to the development of watchmakers who formed a corporation, which remained very active throughout the 17th century.

The second important centre in the 16th century was in Bavaria, in Nuremberg and Augsburg, where clockmakers specialised in automaton clocks. Germany also began producing watches for the Ottoman Empire. In Geneva and London, the immigration of French Protestant watchmakers fleeing religious conflict led to a flourishing production in the 17th century.

The clock becomes a pendulum

Until the middle of the 17th century, the escapement of the driving force of clocks was regulated by a foliot balance: its irregular back-and-forth movement caused inaccuracies of up to one hour per day. To improve the accuracy of clocks, a balance system had to be found whose stable reciprocating motion ensured the smooth running of the movement.

Leonardo da Vinci had already considered attaching a pendulum to a clock, and Galileo used the pendulum to measure time in his experiments.

In the 16th century, none other than Galileo described the laws of motion of the pendulum and developed a mechanism with a pendulum and a free escapement – an idea simultaneously pursued by Christiaan Huygens (1629-1695), a Dutch mathematician and astronomer whose design eventually led to the first pendulum clock. This development made very accurate timekeeping possible.

The accuracy of the instrument was sufficient to add a second hand (the minute hand) to the dial, which became a “pendulum”. Clocks with a single hand (the hour hand) were however manufactured until the 1780’s. In 1675, Huygens discovered that the use of a spiral spring increased the accuracy and reliability of watches.

Recision watchmaking in the Age of Enlightenment

The search for greater precision was a major feature of the 18th century, although technical limitations remained unresolved, such as the expansion of the metal of the balance wheels as a function of the ambient temperature.

In 1726, the English clockmaker John Harrison proposed a multiple metal balance that compensated for the effects of thermal variations.

High-precision clocks or regulators (intended to indicate a reference time for setting other clocks or watches), became more efficient in the long term and now indicate seconds. They were worthy of the reputation of the watchmakers Jules and Pierre Le Roy, Ferdinand Berthoud (Horloger Mécanicien du Roi et de la Marine), Jean-André Lepaute (watchmaker of the Ecole Royale Militaire in Paris), Abraham Louis Breguet (Marie-Antoinette’s watch), Robert Robin (Louis XVI’s watchmaker) and Antide Janvier (inventor of the mobile sphere and the planetary clock).

In 1735, a prototype of a clock insensitive to swell was developed. The navy needed very reliable instruments to determine the position of ships at sea and therefore also the exact solar time in relation to the meridian of departure. (calculation of longitude). In 1766, the Frenchman Pierre Le Roy invented a reliable precision device at sea, which marked the beginning of modern chronometry.

Antide Janvier, clockmaker to King Louis XVI

A mechanical watchmaker from the Franche-Comté region, specialising in planetary clocks, he presented two to Louis XVI in 1784. The king, who was passionate about clocks and astronomy, had Antide Janvier installed in Versailles and appointed him the king’s clockmaker. In 1789, Janvier made an astronomical clock with great complications, which indicated the tides on all the oceans and ports of the world. In 1790, he undertook the construction of a geographical clock which indicated the real time for each of the (83) French departments created by the new Constituent Assembly. It was Napoleon I who bought it in 1806 for his geographical cabinet in Fontainebleau.

The success of the Comtoises

Despite the many advances in clock-making in the 17th century and the technical prowess of mechanical clockmakers in the Enlightenment, weight clocks were still widely used in Europe. They could be made of iron or brass, fixed to the wall or housed in a tall wooden case. In the 18th century, clocks made in Franche-Comté (les Comtoises) supplanted all French handmade products. They are distinguished by the presence of two mechanisms, one for the movement and the other for the striking. The drive is ensured by two cast iron weights, the regulation is ensured by a long balance (or pendulum). They are often given as wedding gifts and have proven to be accurate and reliable.

Standard time

Sir Sandford Fleming had the idea of creating the standard time system after he missed his train in Ireland. Fleming’s system, which is still in use, defines Greenwich, England (at 0 degrees longitude) as the standard time, and divides the world into 24 time zones, each with a fixed time relative to mean time.

Quartz clock

In 1927, Warren Marrison, a Canadian telecommunications engineer, was looking for reliable frequency standards in Bell’s telephone laboratories. He developed the first quartz clock, a highly accurate clock based on the regular vibrations of a quartz crystal in an electrical circuit.

Big Ben

In 1908, the London-based Westclox Clock Company patented the Big Ben alarm clock. Its special feature is the bell back, which completely surrounds the inside of the case and is an integral part of it. The back of the bell provides an audible alarm.

Battery operated clock

The Warren Clock Company was established in 1912 and produced a new type of battery powered clock. Previously, clocks were either wound or operated by weights.

Self-winding watch

In 1920, British inventor John Harwood created the first self-winding watch and filed a patent in 1923. This unusual and fragile watch had a very short-lived success and life.

Atomic clock

An atomic clock is a clock that uses the permanence and immutability of the frequency of electromagnetic radiation emitted by an electron as it passes from one energy level to another to ensure the accuracy and stability of the oscillating signal it produces. One of its main applications is the maintenance of International Atomic Time (TAI) and the dissemination of Coordinated Universal Time (UTC), which are the reference time scales.

The atomic clock emerged in the twentieth century.

In 1955, Louis Essen and Jack Parry, with the National Physical Laboratory (NPL), produced a prototype atomic clock with a frequency of 9 192 631 830 ± 10 cycles per second, an error of the order of 1 s over 30 years.

In 1967, the 13th General Conference on Weights and Measures decided:

“The second is the exact duration of 9 192 631 770 oscillations (or periods) of the transition between the hyperfine levels of the ground state of the 133Cs atom (atom at rest T=0K)” CGPM

Today, atomic clocks have been transformed into optical clocks and are accurate enough to affect the measurement of time by an altitude difference of 30 centimetres due to the so-called gravitational effect of the theory of general relativity. As a result, caesium may no longer be an accurate enough reference.

Applications

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International Atomic Time is the world reference based on the definition of the atomic second, calculated at the Bureau International des Poids et Mesures in Sèvres by averaging about 500 atomic clocks (in 2016) in more than 70 laboratories around the world. The International Telecommunication Union requires the use of clocks whose variation is limited to one second every 3000 years. Only atomic clocks can meet this requirement.

Atomic clocks are used in satellite positioning systems such as GPS, the Glonass system and the Galileo programme. The latter is more accurate than GPS and offers a guarantee of signal quality, especially for air navigation. If the networks are not synchronised, information is lost or traffic is slowed down.

This need for network synchronisation also applies to the mobile broadband network (4G), but also to the electricity distribution network, in order to connect energy sources with identical frequency and synchronised phases. The development of smart grids further increases the need for clocks.

Banks also need very accurate clocks: the dating of high-frequency transactions must be guaranteed, with microsecond accuracy. They usually use clocks based on special GPS receivers designed to guarantee the date and time of transactions.

Different types of clocks

Since their creation, clocks have become considerably more complex: they can be treated in an order that broadly follows their evolution: weight and folio clocks such as public clocks, monumental clocks, astronomical clocks; pendulum clocks such as comtoises and regulators;

their miniaturisations, for example table clocks, pendulum clocks, mechanical watches; and finally their derivatives: pneumatic and electric clocks, quartz clocks, atomic clocks, etc.

Today, time measurement has reached dizzying accuracies. However, the quest for ultimate precision is a long race that is far from over.

It is not only science that is interested in measurement and precision. Without realising it, surfing the Internet or determining one’s position with a GPS are actions that require measurements of time or distance with very high and ever increasing accuracy. Since the invention of atomic clocks in the mid-twentieth century, the accuracy of time measurement has improved by an order of magnitude every ten years, to the point where optical clocks are now accurate to 18 decimal places.

The past has shown it, the future will confirm it: very high precision measurements undeniably lead to scientific revolutions and technological breakthroughs for our society.