Verge escapement

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Verge watch escapement.png
Verge escapement and balance wheel from an early pocketwatch
Verge and foliot from De Vick's clock.svg
Verge and foliot escapement from De Vick tower clock, built in Paris, 1379, by Henri de Vick

The verge (or crown wheel) escapement is the earliest known type of mechanical escapement, the mechanism in a mechanical clock that controls its rate by allowing the gear train to advance at regular intervals or 'ticks'. Verge escapements were used from the late 13th century until the mid 19th century in clocks and pocketwatches. The name verge comes from the Latin virga, meaning stick or rod. [1]

Contents

Its invention is important in the history of technology, because it made possible the development of all-mechanical clocks. This caused a shift from measuring time by continuous processes, such as the flow of liquid in water clocks, to repetitive, oscillatory processes, such as the swing of pendulums, which had the potential to be more accurate. [2] [3] Oscillating timekeepers are used in most modern timepieces. [2] [4] [5]

Verge and foliot clocks

One of the earliest existing drawings of a verge escapement, in Giovanni de Dondi's astronomical clock, the Astrarium, built 1364, Padua, Italy. This had a balance wheel (crown shape at top) instead of a foliot. The escapement is just below it. From his 1364 clock treatise, Il Tractatus Astrarii. Verge escapement Giovanni di Dondi.jpg
One of the earliest existing drawings of a verge escapement, in Giovanni de Dondi's astronomical clock, the Astrarium, built 1364, Padua, Italy. This had a balance wheel (crown shape at top) instead of a foliot. The escapement is just below it. From his 1364 clock treatise, Il Tractatus Astrarii.

The verge escapement dates from 13th-century Europe, where its invention led to the development of the first all-mechanical clocks. [3] [7] [8] Starting in the 13th century, large tower clocks were built in European town squares, cathedrals, and monasteries. They kept time by using the verge escapement to drive a foliot, a primitive type of balance wheel. [9] The foliot was a horizontal bar with weights near its ends affixed to a vertical bar called the verge which was suspended free to rotate. The verge escapement caused the foliot to oscillate back and forth about its vertical axis. [10] The rate of the clock could be adjusted by moving the weights in or out on the foliot.

The verge escapement probably evolved from an alarm mechanism to ring a bell which had appeared centuries earlier. [11] [12] There has been speculation that Villard de Honnecourt invented the verge escapement in 1237 with an illustration of a strange mechanism to turn an angel statue to follow the sun with its finger, [13] [14] but the consensus is that this was not an escapement. [15] [16] [17] [18] [19] [20]

It is believed that sometime in the late 13th century the verge escapement mechanism was applied to tower clocks, creating the first mechanical escapement clock. [9] In spite of the fact that these clocks were celebrated objects of civic pride which were written about at the time, it may never be known when the new escapement was first used. [11] This is because it has proven difficult to distinguish from the meager written documentation which of these early tower clocks were mechanical, and which were water clocks; the same Latin word, horologe, was used for both. [21] [9] None of the original mechanisms have survived unaltered. Sources differ on which was the first clock 'known' to be mechanical, depending on which manuscript evidence they regard as conclusive. One candidate is the Dunstable Priory clock in Bedfordshire, England built in 1283, because accounts say it was installed above the rood screen, where it would be difficult to replenish the water needed for a water clock. [22] [9] Another is the clock built at the Palace of the Visconti, Milan, Italy, in 1335. [23] Astronomer Robertus Anglicus wrote in 1271 that clockmakers were trying to invent an escapement, but hadn't been successful yet. [24] [9] However, there is agreement that mechanical clocks existed by the late 13th century. [3] [21] [25]

Salisbury Cathedral, medieval clock.JPG
Salisbury Cathedral 2012 12.jpg
Salisbury Cathedral clock, 1386?, Salisbury, England, shows what the first verge clocks looked like. It did not have a clock face but was built to ring the hours. The few original verge clock mechanisms like this surviving from the Middle Ages have all been extensively modified. This example, like others, was found with the original verge and foliot replaced by a pendulum; a reproduction verge and foliot, shown in the righthand picture, was added in 1956.

The earliest description of an escapement, in Richard of Wallingford's 1327 manuscript Tractatus Horologii Astronomici on the clock he built at the Abbey of St. Albans, was not a verge, but a variation called a 'strob' escapement. [26] [27] It consisted of a pair of escape wheels on the same axle, with alternating radial teeth. [9] The verge rod was suspended between them, with a short crosspiece that rotated first in one direction and then the other as the staggered teeth pushed past. Although no other example is known, it is possible that this design preceded the more usual verge in clocks. [26]

For the first two hundred years or so of the mechanical clock's existence, the verge, with foliot or balance wheel, was the only escapement used in mechanical clocks. In the sixteenth century alternative escapements started to appear, but the verge remained the most used escapement for 350 years until mid-17th century advances in mechanics, resulted in the adoption of the pendulum, and later the anchor escapement. [28] Since clocks were valuable, after the invention of the pendulum many verge clocks were rebuilt to use this more accurate timekeeping technology, so very few of the early verge and foliot clocks have survived unaltered to the present day.

How accurate the first verge and foliot clocks were is debatable, with estimates of one to two hours error per day [29] [11] [2] being mentioned, although modern experiments with clocks of this construction show accuracies of minutes per day were achievable with enough care in design and maintenance. [30] [31] Early verge clocks were probably no more accurate than the previous water clocks, [14] but they did not require water to be manually hauled to fill the reservoir, did not freeze in winter, and were a more promising technology for innovation. By the mid-17th century, when the pendulum replaced the foliot, the best verge and foliot clocks had achieved an accuracy of 15 minutes per day.

Verge pendulum clocks

Most of the gross inaccuracy of the early verge and foliot clocks was due not to the escapement itself, but to the foliot oscillator. The first use of pendulums in clocks around 1656 suddenly increased the accuracy of the verge clock from hours a day to minutes a day. Most clocks were rebuilt with their foliots replaced by pendulums, [32] [33] to the extent that it is difficult to find original verge and foliot clocks intact today. A similar increase in accuracy in verge watches followed the introduction of the balance spring in 1658.

How it works

Verge Escapement Labelled.png
Verge escapement showing (c) crown wheel, (v) verge, (p,q) pallets
VergeEscapementCycle.gif
Verge escapement in motion
The second verge pendulum clock built by Christiaan Huygens, inventor of the pendulum clock, 1673. Huygens claimed an accuracy of 10 seconds per day. In a pendulum clock, the verge escapement is turned 90 degrees so that the crown wheel faces up (top). Huygens clock.png
The second verge pendulum clock built by Christiaan Huygens, inventor of the pendulum clock, 1673. Huygens claimed an accuracy of 10 seconds per day. In a pendulum clock, the verge escapement is turned 90 degrees so that the crown wheel faces up (top).

The verge escapement consists of a wheel shaped like a crown, called the escape wheel, with sawtooth-shaped teeth protruding axially toward the front, and with its axis oriented horizontally. [11] [34] In front of it is a vertical rod, the verge, with two metal plates, the pallets, that engage the teeth of the escape wheel at opposite sides. The pallets are not parallel, but are oriented with an angle in between them so only one catches the teeth at a time. Attached to the verge at its top is an inertial oscillator, a balance wheel or in the earliest clocks a foliot, a horizontal beam with weights on either end. This is the timekeeper of the clock.

As the clock's gears turn the crown wheel (see animation), one of its teeth catches on a pallet, pushing on it. [11] This rotates the verge and foliot in one direction, and rotates the second pallet into the path of the teeth on the opposite side of the wheel, until the tooth slides off the end of the pallet, releasing it. Then the crown wheel rotates freely a short distance until a tooth on the wheel's opposite side contacts the second pallet, pushing on it. This reverses the direction of the verge rod and foliot, rotating the verge back the other direction, until this tooth pushes past the second pallet. Then the cycle repeats. The result is to change the rotary motion of the wheel to an oscillating motion of the verge and foliot. Each swing of the balance wheel thus allows one tooth of the escape wheel to pass, advancing the wheel train of the clock by a fixed amount, moving the hands forward at a constant rate. The moment of inertia of the foliot or balance wheel controls the oscillation rate, determining the rate of the clock. The escape wheel tooth, pushing against the pallet each swing, provides an impulse which replaces the energy lost by the foliot to friction, keeping it oscillating back and forth.

In a verge pendulum clock (see picture) which appeared after the pendulum was invented in 1656, the escapement was turned 90° so the verge rod was horizontal, while the escape wheel's axis was vertical, located under the verge rod. In the first pendulum clocks the pendulum was attached to the end of the verge rod instead of the balance wheel or foliot. In later pendulum clocks the pendulum was suspended by a short straight spring of metal ribbon from the clock frame, and a vertical arm attached to the end of the verge rod ended in a fork which embraced the pendulum rod; this avoided the friction of suspending the pendulum directly from the pivoted verge rod. Each swing of the pendulum released an escape wheel tooth.

The escape wheel must have an odd number of teeth for the escapement to function. [34] With an even number, two opposing teeth will contact the pallets at the same time, jamming the escapement. The usual angle between the pallets was 90° to 105°, [11] [34] resulting in a foliot or pendulum swing of around 80° to 100°. In order to reduce the pendulum's swing to make it more isochronous, the French used larger pallet angles, upward of 115°. [34] This reduced the pendulum swing to around 50° and reduced recoil (below), but required the verge to be located so near the crown wheel that the teeth fell on the pallets very near the axis, reducing initial leverage and increasing friction, thus requiring lighter pendulums. [34] [35]

Disadvantages

As might be expected from its early invention, the verge is the most inaccurate of the widely used escapements. It suffers from these problems:

Modern reproduction of an early verge and foliot clock. The pointed-tooth verge wheel is visible, with the wooden foliot rod and suspended weight above it. Waagbalkenuhr Metall Nachbau 02.jpg
Modern reproduction of an early verge and foliot clock. The pointed-tooth verge wheel is visible, with the wooden foliot rod and suspended weight above it.

Decline

Verge escapements were used in virtually all clocks and watches for 400 years. Then the increase in accuracy due to the introduction of the pendulum and balance spring in the mid 17th century focused attention on error caused by the escapement. By the 1820s, the verge was superseded by better escapements, though inexpensive verge watches continued to be made through the 19th century.

In pocketwatches, besides its inaccuracy, the vertical orientation of the crown wheel and the need for a bulky fusee made the verge movement unfashionably thick. French watchmakers adopted the thinner cylinder escapement, invented in 1695. In England, high end watches went to the duplex escapement, developed in 1782, but relatively inexpensive verge fusee watches continued to be produced until the mid 19th century, when the lever escapement took over. [36] [37] These later verge watches were colloquially called 'turnips' because of their bulky build.

The verge was only used briefly in pendulum clocks before it was replaced by the anchor escapement, invented around 1660 probably by Robert Hooke, and widely used beginning in 1680. [38] The problem with the verge was that it required the pendulum to swing in a wide arc of 80° to 100°. Christiaan Huygens in 1674 showed that a pendulum swinging in a wide arc is an inaccurate timekeeper, because its period of swing is sensitive to small changes in the drive force provided by the clock mechanism. [38]

Although the verge is not known for accuracy, it is capable of it. The first successful marine chronometers, H4 and H5, made by John Harrison in 1759 and 1770, used verge escapements with diamond pallets., [11] [36] [39] In trials they were accurate to within a fifth of a second per day. [40]

Today the verge is seen only in antique or antique-replica timepieces. Many original bracket clocks have their Victorian-era anchor escapement conversions undone and the original style of verge escapement restored. Clockmakers call this a verge reconversion.

See also

Related Research Articles

<span class="mw-page-title-main">John Harrison</span> English clockmaker (1693–1776)

John Harrison was an English carpenter and clockmaker who invented the marine chronometer, a long-sought-after device for solving the problem of calculating longitude while at sea.

<span class="mw-page-title-main">Pendulum clock</span> Clock regulated by a pendulum

A pendulum clock is a clock that uses a pendulum, a swinging weight, as its timekeeping element. The advantage of a pendulum for timekeeping is that it is an approximate 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, inspired by Galileo Galilei, 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 for the faster pace of life which was necessary for the Industrial Revolution. The home pendulum clock was replaced by less-expensive synchronous electric clocks in the 1930s and '40s. Pendulum clocks are now kept mostly for their decorative and antique value.

<span class="mw-page-title-main">Grasshopper escapement</span> Low friction clock escapement

The grasshopper escapement is a low-friction escapement for pendulum clocks invented by British clockmaker John Harrison around 1722. An escapement, part of every mechanical clock, is the mechanism that gives the clock's pendulum periodic pushes to keep it swinging, and each swing releases the clock's gears to move forward by a fixed amount, thus moving the hands forward at a steady rate. The grasshopper escapement was used in a few regulator clocks built during Harrison's time, and a few others over the years, but has never seen wide use. The term "grasshopper" in this connection, apparently from the kicking action of the pallets, first appears in the Horological Journal in the late 19th century. The term “escapement” is not really applicable here, since the wheel never escapes the pallets: there is always a tooth in contact, so there is never a short period where the wheel “escapes” and free-wheels forward to impact the other pallet.

<span class="mw-page-title-main">Escapement</span> Mechanism for regulating the speed of clocks

An escapement is a mechanical linkage in mechanical watches and clocks that gives impulses to the timekeeping element and periodically releases the gear train to move forward, advancing the clock's hands. The impulse action transfers energy to the clock's timekeeping element to replace the energy lost to friction during its cycle and keep the timekeeper oscillating. The escapement is driven by force from a coiled spring or a suspended weight, transmitted through the timepiece's gear train. Each swing of the pendulum or balance wheel releases a tooth of the escapement's escape wheel, allowing the clock's gear train to advance or "escape" by a fixed amount. This regular periodic advancement moves the clock's hands forward at a steady rate. At the same time, the tooth gives the timekeeping element a push, before another tooth catches on the escapement's pallet, returning the escapement to its "locked" state. The sudden stopping of the escapement's tooth is what generates the characteristic "ticking" sound heard in operating mechanical clocks and watches.

<span class="mw-page-title-main">Movement (clockwork)</span> Mechanism of a watch or clock

In horology, a movement, also known as a caliber or calibre, is the mechanism of a watch or timepiece, as opposed to the case, which encloses and protects the movement, and the face, which displays the time. The term originated with mechanical timepieces, whose clockwork movements are made of many moving parts. The movement of a digital watch is more commonly known as a module.

<span class="mw-page-title-main">Anchor escapement</span> Type of mechanism used in pendulum clocks

In horology, the anchor escapement is a type of escapement used in pendulum clocks. The escapement is a mechanism in a mechanical clock that maintains the swing of the pendulum by giving it a small push each swing, and allows the clock's wheels to advance a fixed amount with each swing, moving the clock's hands forward. The anchor escapement was so named because one of its principal parts is shaped vaguely like a ship's anchor.

<span class="mw-page-title-main">Balance wheel</span> Time measuring device

A balance wheel, or balance, is the timekeeping device used in mechanical watches and small clocks, analogous to the pendulum in a pendulum clock. It is a weighted wheel that rotates back and forth, being returned toward its center position by a spiral torsion spring, known as the balance spring or hairspring. It is driven by the escapement, which transforms the rotating motion of the watch gear train into impulses delivered to the balance wheel. Each swing of the wheel allows the gear train to advance a set amount, moving the hands forward. The balance wheel and hairspring together form a harmonic oscillator, which due to resonance oscillates preferentially at a certain rate, its resonant frequency or "beat", and resists oscillating at other rates. The combination of the mass of the balance wheel and the elasticity of the spring keep the time between each oscillation or "tick" very constant, accounting for its nearly universal use as the timekeeper in mechanical watches to the present. From its invention in the 14th century until tuning fork and quartz movements became available in the 1960s, virtually every portable timekeeping device used some form of balance wheel.

<span class="mw-page-title-main">Pallet fork</span>

The pallet fork is a component of the lever escapement of a mechanical watch. The pallet fork and the lever form one component that sits between the escape wheel and the balance wheel. Its purpose is to lock the escape wheel, and release it one tooth at a time at each swing of the balance wheel, and also give the balance wheel small pushes to keep it going.

<span class="mw-page-title-main">Lever escapement</span>

The lever escapement, invented by the English clockmaker Thomas Mudge in 1754, is a type of escapement that is used in almost all mechanical watches, as well as small mechanical non-pendulum clocks, alarm clocks, and kitchen timers.

<span class="mw-page-title-main">Pin-pallet escapement</span> Mechanical watch component

A Roskopf, pin-lever, or pin-pallet escapement is an inexpensive, less accurate version of the lever escapement, used in mechanical alarm clocks, kitchen timers, mantel clocks and, until the 1970s, cheap watches now known as pin lever watches. It was popularized by German watchmaker Georges Frederic Roskopf in its "proletarian watch" from 1867. It was invented by Louis Perron, of Besançon, suggested to Roskopf by Jules Grossmann.

<span class="mw-page-title-main">Fusee (horology)</span> Mainspring force equalizing pulley in timepieces

A fusee is a cone-shaped pulley with a helical groove around it, wound with a cord or chain attached to the mainspring barrel of antique mechanical watches and clocks. It was used from the 15th century to the early 20th century to improve timekeeping by equalizing the uneven pull of the mainspring as it ran down. Gawaine Baillie stated of the fusee, "Perhaps no problem in mechanics has ever been solved so simply and so perfectly."

<span class="mw-page-title-main">History of watches</span> Aspect of history

The history of watches began in 16th-century Europe, where watches evolved from portable spring-driven clocks, which first appeared in the 15th century.

<span class="mw-page-title-main">Japanese clock</span> Mechanical clock that has been made to tell traditional Japanese time

A Japanese clock is a mechanical clock that has been made to tell traditional Japanese time, a system in which daytime and nighttime are always divided into six periods whose lengths consequently change with the season. Mechanical clocks were introduced into Japan by Jesuit missionaries or Dutch merchants. These clocks were of the lantern clock design, typically made of brass or iron, and used the relatively primitive verge and foliot escapement. Tokugawa Ieyasu owned a lantern clock of European manufacture.

<span class="mw-page-title-main">Salisbury Cathedral clock</span> Ancient clock of Salisbury Cathedral

The Salisbury Cathedral clock is a large iron-framed tower clock without a dial, in Salisbury Cathedral, England. Thought to date from about 1386, it is a well-preserved example of the earliest type of mechanical clock, called verge and foliot clocks, and is said to be the oldest working clock in the world, although similar claims are made for other clocks. Previously in a bell-tower which was demolished in 1790, the clock was restored to working condition in 1956 and is on display in the North nave aisle of the cathedral, close to the West front.

<span class="mw-page-title-main">Riefler escapement</span> Mechanical escapement for pendulum clocks

The Riefler escapement is a mechanical escapement for precision pendulum clocks invented and patented by German instrument maker Sigmund Riefler in 1889. It was used in the astronomical regulator clocks made by his German firm Clemens Riefler from 1890 to 1965, which were perhaps the most accurate all-mechanical pendulum clocks made.

<span class="mw-page-title-main">Turret clock</span> Large prominently located clock used as a public amenity

A turret clock or tower clock is a clock designed to be mounted high in the wall of a building, usually in a clock tower, in public buildings such as churches, university buildings, and town halls. As a public amenity to enable the community to tell the time, it has a large face visible from far away, and often a striking mechanism which rings bells upon the hours.

<span class="mw-page-title-main">Mechanical watch</span> Type of watch which uses a clockwork mechanism to measure the passage of time

A mechanical watch is a watch that uses a clockwork mechanism to measure the passage of time, as opposed to quartz watches which function using the vibration modes of a piezoelectric quartz tuning fork, or radio watches, which are quartz watches synchronized to an atomic clock via radio waves. A mechanical watch is driven by a mainspring which must be wound either periodically by hand or via a self-winding mechanism. Its force is transmitted through a series of gears to power the balance wheel, a weighted wheel which oscillates back and forth at a constant rate. A device called an escapement releases the watch's wheels to move forward a small amount with each swing of the balance wheel, moving the watch's hands forward at a constant rate. The escapement is what makes the 'ticking' sound which is heard in an operating mechanical watch. Mechanical watches evolved in Europe in the 17th century from spring powered clocks, which appeared in the 15th century.

<span class="mw-page-title-main">History of timekeeping devices</span>

The history of timekeeping devices dates back to when ancient civilizations first observed astronomical bodies as they moved across the sky. Devices and methods for keeping time have gradually improved through a series of new inventions, starting with measuring time by continuous processes, such as the flow of liquid in water clocks, to mechanical clocks, and eventually repetitive, oscillatory processes, such as the swing of pendulums. Oscillating timekeepers are used in all modern timepieces.

In horology, a wheel train is the gear train of a mechanical watch or clock. Although the term is used for other types of gear trains, the long history of mechanical timepieces has created a traditional terminology for their gear trains which is not used in other applications of gears.

<span class="mw-page-title-main">Cotehele clock</span> Turret clock in Cornwall, England

The Cotehele clock is situated at Cotehele House, Calstock, Cornwall. It is the earliest turret clock in the United Kingdom still working in an unaltered state and in its original position. It was probably installed between 1493 and 1521.

References

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