From foliot to pendulum
How does a mechanical clock stop itself from running too fast? From the foliot used in the earliest medieval clocks to the compensated pendulum of Auguste Vérité’s astronomical clock, centuries of clockmaking have sought to answer the same question: how can time be made to beat regularly?
A clock must first learn how to slow down
In a mechanical clock, weights provide the energy needed to drive the movement. Without a regulating system, this energy would cause the wheels to turn far too quickly.
The mechanism therefore has to be periodically restrained and then released, tooth by tooth. This is the role of the escapement and its regulator.
For several centuries, that regulator was the foliot. From the seventeenth century onwards, the use of the pendulum transformed clock accuracy.
Foliot and pendulum: what is the difference?
Both systems oscillate, but they rely on different mechanical principles and do not offer the same degree of regularity.
The foliot
A foliot consists of a horizontal bar carrying adjustable weights. It is generally associated with a vertical shaft, known as the verge, and an escape wheel.
The mechanism alternately drives the verge in one direction and then the other. The foliot therefore performs an oscillating rotational movement.
Each oscillation allows the clock’s gear train to advance by a small amount.
- horizontal oscillating bar;
- adjustable weights;
- historically associated with the verge escapement;
- relatively limited accuracy.
The pendulum
With a pendulum, the regulator no longer performs an alternating rotation around a vertical axis. Instead, it swings around a fixed suspension point.
For small oscillations, the duration of its movement depends mainly on its length. This property provides a much more stable time reference.
- pendular motion;
- period mainly determined by length;
- better rate stability;
- far greater accuracy than a foliot.
Canon Musique’s clock: from foliot to pendulum
The clock known as the Canon Étienne Musique clock is one of the oldest surviving witnesses to the history of clockmaking preserved in Beauvais Cathedral.
A large part of its mechanism dates from the fourteenth century. The cathedral chapter archives record a repair to the clock as early as 1387.
The clock was originally regulated by a foliot, as were many mechanical clocks of the late Middle Ages.
Over the course of its history, this system was replaced by a pendulum. This change made it possible to achieve a much more regular rate without replacing the entire historic mechanism.
The way in which the time was displayed also evolved. Today, the clock has one dial and two hands, allowing hours and minutes to be read separately.
The dial visible today is more recent than the medieval mechanism. The clock therefore provides a remarkable record of several centuries of technical development in clockmaking.
From the medieval foliot to the pendulum
The foliot
A horizontal bar oscillates around the verge. The weights placed on the bar make it possible to adjust the behaviour of the regulator and therefore the overall rate of the clock.
The pendulum
The pendulum provides a much more regular time reference. The clock can therefore retain a large part of its old gear train while benefiting from a more efficient regulator.
A pendulum designed to resist temperature changes
When master clockmaker Auguste-Lucien Vérité built the great astronomical clock of Beauvais in the nineteenth century, clockmaking accuracy had already improved considerably.
Yet one physical problem remained: the thermal expansion of metals.
When a metal rod becomes warmer, it expands slightly. When it cools, it contracts. However, the duration of a pendulum’s oscillation depends directly on its effective length.
If the pendulum becomes longer, it swings slightly more slowly. If it becomes shorter, it swings slightly faster.
The pendulum of Vérité’s astronomical clock therefore combines steel and brass. These two metals do not expand by the same amount.
By arranging them carefully within the pendulum, the clockmaker uses their different expansion rates so that their effects compensate for one another.
Why combine steel and brass?
It is impossible to prevent a metal from expanding entirely. The clockmaker’s solution is therefore to use the different expansion rates of several materials so that their effects compensate for one another.
When the temperature rises
Metals expand. Without compensation, a pendulum would tend to become longer and the clock would run slightly slower.
When the temperature falls
Metals contract. The pendulum would therefore become slightly shorter and the clock would tend to gain time.
Steel + brass
Because the two metals behave differently as the temperature changes, combining them makes it possible to produce opposing movements and reduce changes in the pendulum’s effective length.
From the medieval foliot to the compensated pendulum
| Feature | Foliot | Pendulum |
|---|---|---|
| Movement | Alternating rotation around an axis | Oscillation around a suspension point |
| Adjustment | Position of the weights on the bar | Effective length of the pendulum |
| Regularity | Limited | Much greater |
| Historical use | Early mechanical clocks | Widely adopted from the seventeenth century |
| At Beauvais | Original regulator of Canon Musique’s clock | Later adopted on the Musique clock and used on Vérité’s clock |
| Further development | — | Steel/brass thermal compensation on Vérité’s clock |
Sources and further reading
This page complements the material devoted to the two clocks preserved in Beauvais Cathedral and helps explain the evolution of mechanical time regulation.