Pendulum clock

# Pendulum clock

Overview
A pendulum clock is a clock
Clock
A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...

that uses a pendulum
Pendulum
A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...

, a swinging weight, as its time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

keeping element. The advantage of a pendulum for timekeeping is that it is a resonant device; it swings back and forth in a precise time interval dependent on its length, and resists swinging at other rates. From its invention in 1656 by Christiaan Huygens until the 1930s, the pendulum clock was the world's most precise timekeeper, accounting for its widespread use.
Discussion

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Encyclopedia
A pendulum clock is a clock
Clock
A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...

that uses a pendulum
Pendulum
A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...

, a swinging weight, as its time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

keeping element. The advantage of a pendulum for timekeeping is that it is a resonant device; it swings back and forth in a precise time interval dependent on its length, and resists swinging at other rates. From its invention in 1656 by Christiaan Huygens until the 1930s, the pendulum clock was the world's most precise timekeeper, accounting for its widespread use. Pendulum clocks must be stationary to operate; any motion or acceleration
Acceleration
In physics, acceleration is the rate of change of velocity with time. In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity. ...

s will affect the motion of the pendulum, causing inaccuracies, so other mechanisms must be used in portable timepieces. They are now kept mostly for their decorative and antique value.

## History

The pendulum clock was invented in 1656 by Dutch scientist Christiaan Huygens, and patented the following year. Huygens contracted the construction of his clock designs to clockmaker Salomon Coster
Salomon Coster
Salomon Coster is a Dutch clockmaker of the Hague, who in 1657 was the first clockmaker to make a pendulum clock, which had been invented by Christiaan Huygens . Costers' earliest pendulum clocks were signed "Samuel Coster Haghe met privilege" indicating that he had been authorized by the inventor...

, who actually built the clock. Huygens was inspired by investigations of pendulum
Pendulum
A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...

s by Galileo Galilei
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

beginning around 1602. Galileo discovered the key property that makes pendulums useful timekeepers: isochronism
Isochronous
Isochronous : From Greek iso, equal + chronos, time. It literally means regularly, or at equal time intervals. In general English language, it refers to something that occurs at a regular interval, of the same duration; as opposed to synchronous which refers to more than one thing happening at the...

, which means that the period
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

of swing of a pendulum is approximately the same for different sized swings. Galileo had the idea for a pendulum clock in 1637, which was partly constructed by his son in 1649, but neither lived to finish it. The introduction of the pendulum, the first harmonic oscillator
Harmonic oscillator
In classical mechanics, a harmonic oscillator is a system that, when displaced from its equilibrium position, experiences a restoring force, F, proportional to the displacement, x: \vec F = -k \vec x \, where k is a positive constant....

used in timekeeping, increased the accuracy of clocks enormously, from about 15 minutes per day to 15 seconds per day leading to their rapid spread as existing 'verge and foliot' clocks were retrofitted with pendulums.
These early clocks, due to their verge escapement
Verge escapement
The verge escapement is the earliest known type of mechanical escapement, the mechanism in a mechanical clock that controls its rate by advancing the gear train at regular intervals or 'ticks'. Its origin is unknown. Verge escapements were used from the 14th century until about 1800 in clocks...

s, had wide pendulum swings of up to 100°. In his 1673 analysis of pendulums, Horologium Oscillatorium, Huygens showed that wide swings made the pendulum inaccurate, causing its period
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

, and thus the rate of the clock, to vary with unavoidable variations in the driving force provided by the movement
Movement (clockwork)
In horology, a movement is the internal mechanism of a clock or watch, 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 movements are made of many moving parts...

. Clockmakers' realization that only pendulums with small swings of a few degrees are isochronous
Isochronous
Isochronous : From Greek iso, equal + chronos, time. It literally means regularly, or at equal time intervals. In general English language, it refers to something that occurs at a regular interval, of the same duration; as opposed to synchronous which refers to more than one thing happening at the...

motivated the invention of the anchor escapement
Anchor escapement
In horology, the recoil or anchor escapement is a type of escapement used in pendulum clocks. An escapement is the mechanism in a mechanical clock that maintains the swing of the pendulum and allows the clock's wheels to advance a fixed amount with each swing, moving the hands forward...

around 1670, which reduced the pendulum's swing to 4°-6°. The anchor became the standard escapement used in pendulum clocks. In addition to increased accuracy, the anchor's narrow pendulum swing allowed the clock's case to accommodate longer, slower pendulums, which needed less power and caused less wear on the movement. The seconds pendulum
Seconds pendulum
A seconds pendulum is a pendulum whose period is precisely two seconds; one second for a swing in one direction and one second for the return swing, a frequency of 1/2 Hz....

(also called the Royal pendulum) in which each swing takes one second, which is about one metre (39.37 in) long, became widely used. The long narrow clocks built around these pendulums, first made by William Clement around 1680, became known as grandfather clock
Longcase clock
A longcase clock, also tall-case clock, floor clock, or grandfather clock, is a tall, freestanding, weight-driven pendulum clock with the pendulum held inside the tower, or waist of the case. Clocks of this style are commonly 1.8–2.4 metres tall...

s. The increased accuracy resulting from these developments caused the minute hand, previously rare, to be added to clock faces beginning around 1690.

The 18th and 19th century wave of horological innovation that followed the invention of the pendulum brought many improvements to pendulum clocks. The deadbeat escapement invented in 1675 by Richard Towneley and popularized by George Graham
George Graham (clockmaker)
George Graham was an English clockmaker, inventor, and geophysicist, and a Fellow of the Royal Society.He was born to George Graham in Kirklinton, Cumberland. A Friend like his mentor Thomas Tompion, Graham left Cumberland in 1688 for London to work with Tompion...

around 1715 in his precision regulator clocks gradually replaced the anchor escapement and is now used in most modern pendulum clocks. Observation that pendulum clocks slowed down in summer brought the realization that thermal expansion
Thermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...

and contraction of the pendulum rod with changes in temperature was a source of error. This was solved by the invention of temperature-compensated pendulums; the mercury pendulum by George Graham
George Graham (clockmaker)
George Graham was an English clockmaker, inventor, and geophysicist, and a Fellow of the Royal Society.He was born to George Graham in Kirklinton, Cumberland. A Friend like his mentor Thomas Tompion, Graham left Cumberland in 1688 for London to work with Tompion...

in 1721 and the gridiron pendulum
Gridiron pendulum
The gridiron pendulum was an improved clock pendulum invented by British clockmaker John Harrison around 1726. It didn't change its effective length with temperature, so its period of swing stayed constant with changes in ambient temperature...

by John Harrison
John Harrison
John Harrison was a self-educated English clockmaker. He invented the marine chronometer, a long-sought device in solving the problem of establishing the East-West position or longitude of a ship at sea, thus revolutionising and extending the possibility of safe long distance sea travel in the Age...

in 1726. With these improvements, by the mid-18th century precision pendulum clocks achieved accuracies of a few seconds per week.

Until the 19th century, clocks were handmade by individual craftsmen and were very expensive. The rich ornamentation of pendulum clocks of this period indicates their value as status symbols of the wealthy. The clockmaker
Clockmaker
A clockmaker is an artisan who makes and repairs clocks. Since almost all clocks are now factory-made, most modern clockmakers only repair clocks. Modern clockmakers may be employed by jewellers, antique shops, and places devoted strictly to repairing clocks and watches...

s of each country and region in Europe developed their own distinctive styles. By the 19th century, factory production of clock parts gradually made pendulum clocks affordable by middle class families.

During the Industrial Revolution
Industrial Revolution
The Industrial Revolution was a period from the 18th to the 19th century where major changes in agriculture, manufacturing, mining, transportation, and technology had a profound effect on the social, economic and cultural conditions of the times...

, daily life was organized around the home pendulum clock. More accurate pendulum clocks, called regulators, were installed in places of business and used to schedule work and set other clocks. The most accurate, known as astronomical regulators, were used in observatories for astronomy, surveying, and celestial navigation
Celestial navigation, also known as astronavigation, is a position fixing technique that has evolved over several thousand years to help sailors cross oceans without having to rely on estimated calculations, or dead reckoning, to know their position...

. Beginning in the 19th century, astronomical regulators in naval observatories served as primary standards for national time distribution services
Time signal
A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.-Audible and visible time signals:...

. From 1909, US National Bureau of Standards (now NIST) based the US time standard on Riefler
Riefler escapement
The Riefler escapement is a mechanical escapement for precision pendulum clocks invented and patented by German instrument maker Sigmund Riefler in 1889...

pendulum clocks, accurate to about 10 milliseconds per day. In 1929 it switched to the Shortt-Synchronome free pendulum clock
Shortt-synchronome clock
The Shortt-Synchronome free pendulum clock was a complex precision electromechanical pendulum clock invented in 1921 by British railway engineer William Hamilton Shortt in collaboration with horologist Frank Hope-Jones, and manufactured by the Synchronome Co., Ltd. of London, UK...

before phasing in quartz
Quartz clock
A quartz clock is a clock that uses an electronic oscillator that is regulated by a quartz crystal to keep time. This crystal oscillator creates a signal with very precise frequency, so that quartz clocks are at least an order of magnitude more accurate than good mechanical clocks...

standards in the 1930s.
With an error of around one second per year, the Shortt was the most accurate commercially-produced pendulum clock.

Pendulum clocks remained the world standard for accurate timekeeping for 270 years, until the invention of the quartz clock
Quartz clock
A quartz clock is a clock that uses an electronic oscillator that is regulated by a quartz crystal to keep time. This crystal oscillator creates a signal with very precise frequency, so that quartz clocks are at least an order of magnitude more accurate than good mechanical clocks...

in 1927, and were used as standards through World War 2. The French Time Service used pendulum clocks as part of their ensemble of standard clocks until 1954 The most accurate experimental pendulum clock to date (2007) may be the Littlemore clock, built by Edward T. Hall
Edward Thomas Hall
Edward Thomas Hall CBE, Hon. FBA, FSA, D.Phil was a British scientist.-Life:Born in London, Hall was also a hot-air-balloon pilot and owner of Cameron O-84 Flaming Pearl G-AYAJ 1970-1990....

in the 1990s.

## Mechanism

All mechanical pendulum clocks have these five parts:
• A power source; either a weight on a cord that turns a pulley, or a mainspring
Mainspring
A mainspring is a spiral torsion spring of metal ribbon that is the power source in mechanical watches and some clocks. Winding the timepiece, by turning a knob or key, stores energy in the mainspring by twisting the spiral tighter. The force of the mainspring then turns the clock's wheels as it...

• A gear train
Gear train
A gear train is formed by mounting gears on a frame so that the teeth of the gears engage. Gear teeth are designed to ensure the pitch circles of engaging gears roll on each other without slipping, this provides a smooth transmission of rotation from one gear to the next.The transmission of...

(wheel train
Wheel train (horology)
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.Watch...

) that steps up the speed of the power so that the pendulum can use it
• An escapement
Escapement
In mechanical watches and clocks, an escapement is a device that transfers energy to the timekeeping element and enables counting the number of oscillations of the timekeeping element...

that gives the pendulum precisely timed impulses to keep it swinging, and which releases the gear train wheels to move forward a fixed amount at each swing. This is the source of the "ticking" sound of an operating pendulum clock.
• The pendulum
Pendulum
A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...

, a weight on a rod
• An indicator or dial that records how often the escapement has rotated and therefore how much time has passed, usually a traditional clock face
Clock face
A clock face is the part of an analog clock that displays the time through the use of a fixed numbered dial or dials and moving hands. In its most basic form, recognized universally throughout the world, the dial is numbered 1–12 indicating the hours in a 12-hour cycle, and a short hour hand...

with rotating hands.

Additional functions in clocks besides basic timekeeping are called complication
Complication (horology)
In horology , complication refers to any feature in a timepiece beyond the simple display of hours, minutes, and seconds.A timepiece indicating only hours, minutes, and seconds is otherwise known as a simple movement...

s. More elaborate pendulum clocks may include these complications:
• Striking train
Striking clock
A striking clock is a clock that sounds the hours audibly on a bell or gong. In 12 hour striking, used most commonly in striking clocks today, the clock strikes once at 1 AM, twice at 2 AM, continuing in this way up to twelve times at 12 noon, then starts over, striking once at 1 PM, twice at 2...

– strikes a chime on every hour, with the number of strikes equal to the number of the hour. More elaborate types, technically called chiming clocks, strike on the quarter hours, and may play tunes, usually Westminster quarters.
• Calendar dials – show the day, date, and sometimes month
• Moon phase dial – Shows the phase of the moon with a painted picture of the moon on a rotating disk.
• Equation of time
Equation of time
The equation of time is the difference between apparent solar time and mean solar time. At any given instant, this difference will be the same for every observer...

dial – this rare complication was used in early days to set the clock by the passage of the sun overhead at noon. It displays the difference between the time indicated by the clock and the time indicated by the position of the sun, which varies by as much as ±16 minutes during the year.
• Repeater attachment – repeats the hour chimes when a knob is pressed. This rare complication was used before artificial lighting to check what time it was at night.

In electromechanical pendulum clocks such as used in mechanical Master clock
Master clock
A master clock is a precision clock that provides timing signals to synchronize slave clocks as part of a clock network. The master clock in such installations is controlled by an accurate quartz crystal oscillator, usually referenced to an external frequency standard such as MSF, which is part of...

s the power source is replaced by an electrically powered solenoid
Solenoid
A solenoid is a coil wound into a tightly packed helix. In physics, the term solenoid refers to a long, thin loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electric current is passed through it. Solenoids are important because they can create...

that provides the impulses to the pendulum by magnetic force, and the escapement is replaced by a switch
Switch
In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....

or photodetector
Photodetector
Photosensors or photodetectors are sensors of light or other electromagnetic energy. There are several varieties:*Active pixel sensors are image sensors consisting of an integrated circuit that contains an array of pixel sensors, each pixel containing a both a light sensor and an active amplifier...

that senses when the pendulum is in the right position to receive the impulse. These should not be confused with more recent quartz pendulum clocks in which an electronic quartz clock
Quartz clock
A quartz clock is a clock that uses an electronic oscillator that is regulated by a quartz crystal to keep time. This crystal oscillator creates a signal with very precise frequency, so that quartz clocks are at least an order of magnitude more accurate than good mechanical clocks...

module swings a pendulum. These are not true pendulum clocks because the timekeeping is controlled by a quartz crystal in the module, and the swinging pendulum is merely a decorative simulation.

## Gravity-swing pendulum

The pendulum swings with a period
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

that varies with the square root of its effective length. The rate of pendulum clocks is adjusted by moving the pendulum bob
Bob (physics)
A bob is the weight on the end of a pendulum most commonly, but not exclusively, found in pendulum clocks.- Reason for use :Although a pendulum can theoretically be any shape, any rigid object swinging on a pivot, clock pendulums are usually made of a weight or bob attached to the bottom end of a...

up or down on its rod, often by means of an adjusting nut under the bob. In some pendulum clocks, fine adjustment is done with an auxiliary adjustment, which may be a small weight that is moved up or down the pendulum rod. In some turret clock
Turret clock
A Turret clock is a clock mounted in a tower or turret, usually to show the current time on a dial with hand or to announce the time by strike, or both. It can also have more than one dial to show days, moon phases, and other astronomical data.-Sundials:...

s, adjustment is accomplished by a small tray mounted on the rod where small weights are placed or removed to change the effective length, so the rate can be adjusted without stopping the clock.

### Thermal compensation

To keep time accurately, pendulums are usually made to not vary in length as the temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

changes. Owing to the expansion of metal, the length of a simple pendulum will vary with temperature, slowing the clock as the temperature rises. Early high-precision clocks used the liquid metal mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

to lift a portion of the pendulum mass in compensation for the increased length of the suspension. John Harrison
John Harrison
John Harrison was a self-educated English clockmaker. He invented the marine chronometer, a long-sought device in solving the problem of establishing the East-West position or longitude of a ship at sea, thus revolutionising and extending the possibility of safe long distance sea travel in the Age...

invented the gridiron pendulum
Gridiron pendulum
The gridiron pendulum was an improved clock pendulum invented by British clockmaker John Harrison around 1726. It didn't change its effective length with temperature, so its period of swing stayed constant with changes in ambient temperature...

, which uses a sliding "banjo" of solid metals with differing thermal expansion rates such as brass
Brass
Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...

or zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...

and steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

to achieve a zero-expansion pendulum while avoiding the use of toxic mercury.

By the end of the 19th century, materials were available that had a very low inherent change of length with temperature and these were used to make a simple pendulum rod. These included Invar
Invar
Invar, also known generically as FeNi36 , is a nickel steel alloy notable for its uniquely low coefficient of thermal expansion . The name, Invar, comes from the word invariable, referring to its lack of expansion or contraction with temperature changes.It was invented in 1896 by Swiss scientist...

, a nickel/iron alloy; and fused silica, a glass. The latter is still used for pendulums in gravimeter
Gravimeter
A gravimeter or gravitometer is an instrument used in gravimetry for measuring the local gravitational field of the Earth. A gravimeter is a type of accelerometer, specialized for measuring the constant downward acceleration of gravity, which varies by about 0.5% over the surface of the Earth...

s.

### Atmospheric drag

The viscosity of the air through which the pendulum swings will vary with atmospheric pressure, humidity, and temperature. This drag also requires power that could otherwise be applied to extending the time between windings. Pendulums are sometimes polished and streamlined to reduce the effects of air drag (which is where most of the driving power goes) on the clock's accuracy. In the late 19th century and early 20th century, pendulums for clocks in astronomical observatories were often operated in a chamber that had been pumped to a low pressure to reduce drag and make the pendulum's operation even more accurate.

### Leveling and 'beat'

To keep time accurately, pendulum clocks must be absolutely level. If they are not, the pendulum swings more to one side than the other, upsetting the symmetrical operation of the escapement
Escapement
In mechanical watches and clocks, an escapement is a device that transfers energy to the timekeeping element and enables counting the number of oscillations of the timekeeping element...

. This condition can often be heard audibly in the ticking sound of the clock. The ticks or 'beats' should be at precisely equally spaced intervals to give a sound of, "tick...tock...tick...tock"; if they are not, and have the sound "tick-tock...tick-tock..." the clock is out of beat and needs to be leveled. This problem can easily cause the clock to stop working, and is one of the most common reasons for service calls. A spirit level
Spirit level
A spirit level or bubble level is an instrument designed to indicate whether a surface ishorizontal or vertical . Different types of spirit levels may be used by carpenters, stonemasons, bricklayers, other building trades workers, surveyors, millwrights and other metalworkers, and in some...

or watch timing machine
Watch timing machine
A watch timing machine is a machine that acoustically measures the ticks of a watch so that its accuracy can be measured and effectively calibrated.These companies manufacture watch timing machines:* * * * TYMC* Vibrograf* * - External links :*...

can achieve a higher accuracy than relying on the sound of the beat; precision regulators often have a built in spirit level for the task. Older freestanding clocks often have feet with adjustable screws to level them, more recent ones have a levelling adjustment in the movement. Some modern pendulum clocks have 'auto-beat' or 'self-regulating beat adjustment' devices, and don't need this adjustment.

### Local gravity

Since the pendulum rate will increase with an increase in gravity, and local gravity varies with latitude and elevation on Earth, pendulum clocks must be readjusted to keep time after a move. For example, a pendulum clock moved from sea level to 4000 feet will lose 16 seconds per day. Even moving a clock to the top of a tall building will cause it to lose measurable time due to lower gravity.

## Torsion pendulum

Also called torsion-spring pendulum, this is a wheel-like mass (most often four spheres on cross spokes) suspended from a vertical strip (ribbon) of spring steel, used as the regulating mechanism in torsion pendulum clock
Torsion pendulum clock
A torsion pendulum clock, or torsion clock, is a mechanical clock which keeps time with a mechanism called a torsion pendulum. This is a weighted disk or wheel, often a decorative wheel with 3 or 4 chrome balls on ornate spokes, suspended by a thin wire or ribbon called a torsion spring...

s. Rotation of the mass winds and unwinds the suspension spring, with the energy impulse applied to the top of the spring. As the period of a cycle is quite slow compared to the gravity swing pendulum, it is possible to make clocks that need to be wound only every 30 days, or even only once a year. clock requiring only annual winding is sometimes called a "400-Day clock", "perpetual clock" or "anniversary clock", the latter sometimes given as a wedding memorialisation gift. Schatz and Kundo, both German firms, were once the main manufacturers of this type of clock. This type is independent of the local force of gravity but is more affected by temperature changes than an uncompensated gravity-swing pendulum.

## Escapement

The escapement drives the pendulum, usually from a gear
Gear
A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine....

train, and is the part that ticks. Most escapements have a locking state and a drive state. In the locking state, nothing moves. The motion of the pendulum switches the escapement to drive, and the escapement then pushes on the pendulum for some part of the pendulum's cycle. A notable but rare exception is Harrison's grasshopper escapement
Grasshopper escapement
The grasshopper escapement is an unusual, 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 causes the clock's gears to move forward by a fixed distance at regular intervals and...

. In precision clocks, the escapement is often driven directly by a small weight or spring that is re-set at frequent intervals by an independent mechanism called a remontoire
Remontoire
In mechanical horology, a remontoire, is a small secondary source of power, a weight or spring, which runs the timekeeping mechanism and is itself periodically rewound by the timepiece's main power source, such as a mainspring...

. This frees the escapement from the effects of variations in the gear train. In the late 19th century, electromechanical escapements were developed. In these, a mechanical switch or a phototube
Phototube
A phototube is a type of gas-filled or vacuum tube that is extremely sensitive to light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum.-Operating principles:...

turned an electromagnet
Electromagnet
An electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current is turned off...

on for a brief section of the pendulum's swing. These were used on some of the most precise clocks known. They were usually employed with vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

pendulums on astronomical clocks. The pulse of electricity that drove the pendulum would also drive a plunger to move the gear train.

In the 20th century, W.H. Shortt invented a free pendulum clock / Shortt-Synchronome clock
Shortt-synchronome clock
The Shortt-Synchronome free pendulum clock was a complex precision electromechanical pendulum clock invented in 1921 by British railway engineer William Hamilton Shortt in collaboration with horologist Frank Hope-Jones, and manufactured by the Synchronome Co., Ltd. of London, UK...

with an accuracy of one-hundredth of a second per day. In this system, the timekeeping pendulum does no work and is kept swinging by a push from a weighted arm (gravity arm) that is lowered onto the pendulum by another (slave) clock just before it is needed. The gravity arm then pushes on the free pendulum, which releases it to drop out of engagement at a time that is set entirely by the free pendulum. Once the gravity arm is released, it trips a mechanism to reset itself ready for release by the slave clock. The whole cycle is kept synchronised by a small blade spring on the pendulum of the slave clock. The slave clock is set to run slightly slow, and the reset circuit for the gravity arm activates a pivoted arm that just engages with the tip of the blade spring. If the slave clock has lost too much time, its blade spring pushes against the arm and this accelerates the pendulum. The amount of this gain is such that the blade spring doesn't engage on the next cycle but does on the next again. This form of clock became the standard for use in observatories from the mid-1920s until superseded by quartz technology.

## Time Indication

The indicating system is almost always the traditional dial
Clock face
A clock face is the part of an analog clock that displays the time through the use of a fixed numbered dial or dials and moving hands. In its most basic form, recognized universally throughout the world, the dial is numbered 1–12 indicating the hours in a 12-hour cycle, and a short hour hand...

with moving hour and minute hands. Many clocks have a small third hand indicating seconds on a subsidiary dial. Pendulum clocks are usually designed to be set by opening the glass face cover and manually pushing the minute hand around the dial to the correct time. The minute hand is mounted on a slipping friction sleeve which allows it to be turned on its arbor. The hour hand is driven not from the wheel train
Wheel train (horology)
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.Watch...

but from the minute hand's shaft through a small set of gears, so rotating the minute hand manually also sets the hour hand.

## Styles

Pendulum clocks were more than simply utilitarian timekeepers; they were status symbol
Status symbol
A status symbol is a perceived visible, external denotation of one's social position and perceived indicator of economic or social status. Many luxury goods are often considered status symbols...

s that expressed the wealth and culture of their owners. They evolved in a number of traditional styles, specific to different countries and times as well as their intended use. Case styles somewhat reflect the furniture styles popular during the period. Experts can often pinpoint when an antique clock was made within a few decades by subtle differences in their cases and faces. These are some of the types of pendulum clocks:
• Act of Parliament clock
Act of Parliament clock
An "Act of Parliament clock" is a type of large clock originally hung in inns and taverns in the United Kingdom, beginning in the mid-18th century. Such clocks were plain in design, the faces were around two to five feet in diameter, and they were hung on the wall. They are also commonly called...

• Banjo clock
Banjo clock
The banjo clock, or banjo timepiece, is an American wall clock with a banjo-shaped case. It was invented by Simon Willard, originally of Grafton, Massachusetts, later of Roxbury, Massachusetts, and patented in 1802...

• Bracket clock
Bracket clock
A bracket clock is a style of antique portable table clock made in the 17th and 18th centuries. The term originated with small weight driven clocks that had to be mounted on a bracket on the wall to allow room for their hanging weights. When spring driven clocks were invented they continued to...

• Cartel clock
Cartel clock
A Cartel Clock is a clock designed to hang directly on the wall. Most often used in reference to highly ornate mid-18th-century Rococo examples. These flowing, curvilinear designs are executed in gilt bronze ....

• Comtoise or Morbier clock
• Crystal regulator
• Cuckoo clock
Cuckoo clock
A cuckoo clock is a clock, typically pendulum-regulated, that strikes the hours with a sound like a common cuckoo's call and typically has a mechanical cuckoo that emerges with each note...

• Longcase clock
Longcase clock
A longcase clock, also tall-case clock, floor clock, or grandfather clock, is a tall, freestanding, weight-driven pendulum clock with the pendulum held inside the tower, or waist of the case. Clocks of this style are commonly 1.8–2.4 metres tall...

(commonly known as a grandfather clock)
• Lantern clock
Lantern clock
A lantern clock is a type of antique weight-driven wall clock, shaped like a lantern. They were the first type of clock widely used in private homes. They probably originated before 1500 but only became common after 1600; in Britain around 1620. They became obsolete in the 19th century.- Origin...

• Mantel clock
Mantel clock
Mantel clocks — or shelf clocks — are relatively small house clocks traditionally placed on the shelf, or mantel, above the fireplace. The form, first developed in France in the 1750s, can be distinguished from earlier chamber clocks of similar size due to a lack of carrying handles.These clocks...

• Master clock
Master clock
A master clock is a precision clock that provides timing signals to synchronize slave clocks as part of a clock network. The master clock in such installations is controlled by an accurate quartz crystal oscillator, usually referenced to an external frequency standard such as MSF, which is part of...

• Ogee clock
Ogee
An ogee is a curve , shaped somewhat like an S, consisting of two arcs that curve in opposite senses, so that the ends are parallel....

• Pillar clock
• Schoolhouse regulator
• Shortt-Synchronome clock
Shortt-synchronome clock
The Shortt-Synchronome free pendulum clock was a complex precision electromechanical pendulum clock invented in 1921 by British railway engineer William Hamilton Shortt in collaboration with horologist Frank Hope-Jones, and manufactured by the Synchronome Co., Ltd. of London, UK...

• Torsion pendulum clock
Torsion pendulum clock
A torsion pendulum clock, or torsion clock, is a mechanical clock which keeps time with a mechanism called a torsion pendulum. This is a weighted disk or wheel, often a decorative wheel with 3 or 4 chrome balls on ornate spokes, suspended by a thin wire or ribbon called a torsion spring...

(uses a torsion pendulum)
• Turret clock
Turret clock
A Turret clock is a clock mounted in a tower or turret, usually to show the current time on a dial with hand or to announce the time by strike, or both. It can also have more than one dial to show days, moon phases, and other astronomical data.-Sundials:...

• Vienna regulator
• Zaandam clock

• Pendulum
Pendulum
A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...

• Escapement
Escapement
In mechanical watches and clocks, an escapement is a device that transfers energy to the timekeeping element and enables counting the number of oscillations of the timekeeping element...

• Steam clock
Steam clock
A steam clock is a clock which is fully or partially powered by a steam engine. Only a few functioning steam clocks exist, most designed and built by Canadian horologist Raymond Saunders for display in urban public spaces...

• Odd sympathy
Odd sympathy
The phrase odd sympathy appears in the record of a letter by Dutch mathematician and physicist Christiaan Huygens to Sir Robert Moray as presented to the Royal Society of London, relating to the tendency of two pendulum clocks to synchronize with opposite phases when suspended side by side...

• Torsion pendulum clock
Torsion pendulum clock
A torsion pendulum clock, or torsion clock, is a mechanical clock which keeps time with a mechanism called a torsion pendulum. This is a weighted disk or wheel, often a decorative wheel with 3 or 4 chrome balls on ornate spokes, suspended by a thin wire or ribbon called a torsion spring...