Balance spring

hairspringisochronousbalance spring or "hair springBreguet overcoilisochronous balance springspiral balancespiral balance springspiral springspringspringy
A balance spring, or hairspring, is a spring attached to the balance wheel in mechanical timepieces.wikipedia
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Balance wheel

foliotcompensation balanceauxiliary temperature compensation
A balance spring, or hairspring, is a spring attached to the balance wheel in mechanical timepieces. The balance spring is a fine spiral or helical torsion spring used in mechanical watches, alarm clocks, kitchen timers, marine chronometers, and other timekeeping mechanisms to control the rate of oscillation of the balance wheel. Before that time, balance wheels or foliots without springs were used in clocks and watches, but they were very sensitive to fluctuations in the driving force, causing the timepiece to slow down as the mainspring unwound.
It is a weighted wheel that rotates back and forth, being returned toward its center position by a spiral torsion spring, the balance spring or hairspring.

Clock

clockstimepiecemechanical clock
A balance spring, or hairspring, is a spring attached to the balance wheel in mechanical timepieces.
In 1675, Huygens and Robert Hooke invented the spiral balance spring, or the hairspring, designed to control the oscillating speed of the balance wheel.

Mechanical watch

mechanicalmechanical watchesmanual winding
The balance spring is a fine spiral or helical torsion spring used in mechanical watches, alarm clocks, kitchen timers, marine chronometers, and other timekeeping mechanisms to control the rate of oscillation of the balance wheel.
It consists of a weighted wheel which rotates back and forth, which is returned toward its center position by a fine spiral spring, the balance spring or "hair spring".

Marine chronometer

chronometerchronometersmarine chronometers
The balance spring is a fine spiral or helical torsion spring used in mechanical watches, alarm clocks, kitchen timers, marine chronometers, and other timekeeping mechanisms to control the rate of oscillation of the balance wheel. John Harrison, in the course of his development of the marine chronometer, solved the problem by a "compensation curb" – essentially a bimetallic thermometer which adjusted the effective length of the balance spring as a function of temperature.
In 1675, Huygens, who was receiving a pension from Louis XIV, invented a chronometer that employed a balance wheel and a spiral spring for regulation, instead of a pendulum, opening the way to marine chronometers and modern pocket watches and wristwatches.

Torsion spring

torsion balancetorsion pendulumtorsion
The balance spring is a fine spiral or helical torsion spring used in mechanical watches, alarm clocks, kitchen timers, marine chronometers, and other timekeeping mechanisms to control the rate of oscillation of the balance wheel.
The balance spring or hairspring in mechanical watches is a fine, spiral-shaped torsion spring that pushes the balance wheel back toward its center position as it rotates back and forth. The balance wheel and spring function similarly to the torsion pendulum above in keeping time for the watch.

John Arnold (watchmaker)

John ArnoldJohn Arnold (1736–1799)
Some watchmakers, for example John Arnold, used gold, which avoids the problem of corrosion but retains the problem of gradual weakening.
Certainly one of his most important inventions, the overcoil balance spring is still to be found in most mechanical wristwatches to this day.

Robert Hooke

HookeDr Robert HookeHooke, Robert
The addition of the balance spring to the balance wheel around 1657 by Robert Hooke and Christiaan Huygens greatly increased the accuracy of portable timepieces, transforming early pocketwatches from expensive novelties to useful timekeepers.
Hooke's work on elasticity culminated, for practical purposes, in his development of the balance spring or hairspring, which for the first time enabled a portable timepiece – a watch – to keep time with reasonable accuracy.

Thomas Tompion

Tompion
A few early watches had a Barrow regulator, which used a worm drive, but the first widely used regulator was invented by Thomas Tompion around 1680.
Due to his relationship with the scientist Robert Hooke he made some of the first watches with balance springs, these had the potential to be much more accurate than earlier watches.

Christiaan Huygens

HuygensHuygens, ChristiaanChristiaan
The addition of the balance spring to the balance wheel around 1657 by Robert Hooke and Christiaan Huygens greatly increased the accuracy of portable timepieces, transforming early pocketwatches from expensive novelties to useful timekeepers.
Huygens developed a balance spring watch in the same period as, though independently of, Robert Hooke.

John Harrison

H-4H4H4 and H5
Hardened and tempered steel was first used by John Harrison and subsequently remained the material of choice until the 20th century. John Harrison, in the course of his development of the marine chronometer, solved the problem by a "compensation curb" – essentially a bimetallic thermometer which adjusted the effective length of the balance spring as a function of temperature.
Huygens ran trials using both a pendulum and a spiral balance spring clock as methods of determining longitude, with both types producing inconsistent results.

Pierre Le Roy

PierreLe Roy
Around 1765, Pierre Le Roy (son of Julien Le Roy) invented the compensation balance, which became the standard approach for temperature compensation in watches and chronometers.
He was the inventor of the detent escapement, the temperature-compensated balance and the isochronous balance spring.

Verge escapement

verge and foliotfoliotcrown wheel and verge escapement
Before that time, balance wheels or foliots without springs were used in clocks and watches, but they were very sensitive to fluctuations in the driving force, causing the timepiece to slow down as the mainspring unwound.
A similar increase in accuracy in verge watches followed the introduction of the balance spring in 1658.

Bimetallic strip

bi-metallic stripbimetallicbi-metallic
John Harrison, in the course of his development of the marine chronometer, solved the problem by a "compensation curb" – essentially a bimetallic thermometer which adjusted the effective length of the balance spring as a function of temperature.
He made it for his third marine chronometer (H3) of 1759 to compensate for temperature-induced changes in the balance spring.

Mainspring

springspring motorspring-wound
Before that time, balance wheels or foliots without springs were used in clocks and watches, but they were very sensitive to fluctuations in the driving force, causing the timepiece to slow down as the mainspring unwound. Modern balance springs are made of special low temperature coefficient alloys like nivarox to reduce the effects of temperature changes on the rate, and carefully shaped to minimize the effect of changes in drive force as the mainspring runs down.
This was especially true of the primitive verge and foliot type used before the advent of the balance spring in 1657.

Pocket watch

pocketwatchfob watchwatch fob
The addition of the balance spring to the balance wheel around 1657 by Robert Hooke and Christiaan Huygens greatly increased the accuracy of portable timepieces, transforming early pocketwatches from expensive novelties to useful timekeepers.
Positional adjustments are attained by careful poising (ensuring even weight distribution) of the balance-hairspring system as well as careful control of the shape and polish on the balance pivots.

Nivarox

Modern balance springs are made of special low temperature coefficient alloys like nivarox to reduce the effects of temperature changes on the rate, and carefully shaped to minimize the effect of changes in drive force as the mainspring runs down.
Hairsprings made of this alloy have a spring constant which does not vary with temperature, allowing the watch's balance wheel, its timekeeping element, to keep better time.

Watch

wristwatchwatchesdigital watch
Watch
A great leap forward in accuracy occurred in 1657 with the addition of the balance spring to the balance wheel, an invention disputed both at the time and ever since between Robert Hooke and Christiaan Huygens.

Elinvar

new materials
Around 1900, a fundamentally different solution was created by Charles Édouard Guillaume, inventor of elinvar.
The largest use of elinvar was in balance springs for mechanical watches and chronometers.

Spiral

spiralswhorlspiral patterns
The balance spring is a fine spiral or helical torsion spring used in mechanical watches, alarm clocks, kitchen timers, marine chronometers, and other timekeeping mechanisms to control the rate of oscillation of the balance wheel.

Helix

helicalheliceshelically
The balance spring is a fine spiral or helical torsion spring used in mechanical watches, alarm clocks, kitchen timers, marine chronometers, and other timekeeping mechanisms to control the rate of oscillation of the balance wheel.

Alarm clock

clock radioalarmalarm clocks
The balance spring is a fine spiral or helical torsion spring used in mechanical watches, alarm clocks, kitchen timers, marine chronometers, and other timekeeping mechanisms to control the rate of oscillation of the balance wheel.

Timer

timerstiming devicesoftware timer
The balance spring is a fine spiral or helical torsion spring used in mechanical watches, alarm clocks, kitchen timers, marine chronometers, and other timekeeping mechanisms to control the rate of oscillation of the balance wheel.

Harmonic oscillator

harmonic oscillatorsharmonic oscillationharmonic
The balance spring and balance wheel together form a harmonic oscillator, which oscillates with a precise period or "beat" resisting external disturbances, and is responsible for timekeeping accuracy.

Frequency

frequenciesperiodperiodic
The balance spring and balance wheel together form a harmonic oscillator, which oscillates with a precise period or "beat" resisting external disturbances, and is responsible for timekeeping accuracy.

Temperature coefficient

negative temperature coefficientpositive temperature coefficientPTC
Modern balance springs are made of special low temperature coefficient alloys like nivarox to reduce the effects of temperature changes on the rate, and carefully shaped to minimize the effect of changes in drive force as the mainspring runs down.