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 , meaning "stick" or "rod".

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. Oscillating timekeepers keep time for all modern clocks.

Verge and foliot clocks

left|thumb|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. 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. 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. 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, but the consensus is that this was not an escapement.

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. Astronomer Robertus Anglicus wrote in 1271 that clockmakers were trying to invent an escapement, but hadn't been successful yet.

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. It consisted of a pair of escape wheels on the same axle, with alternating radial teeth. 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.

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 Early verge clocks were probably no more accurate than the previous water clocks, 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

thumb|upright=0.9|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. 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.

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:

  • Verge watches and clocks are sensitive to changes in the drive force; they slow down as the mainspring unwinds. The cause of this problem is that the crown wheel teeth are always pushing on the pallets, driving the pendulum or balance wheel throughout its cycle; the timekeeping element is never allowed to swing freely. 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. 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. In trials they were accurate to within a fifth of a second per day.

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

  • Galileo's escapement – the first proposed pendulum clock escapement by Galileo Galilei.
  • Dover Castle Clock – the oldest-known operating clock with its original verge and foliot escapement, now in the Science Museum, London.

References

Further reading

  • Weight Driven Clock Encyclopædia Britannica article has animation showing operation of a verge and foliot.
  • Drawing of verge and foliot on commercial website
  • Elytra Design, Diagram of verge and foliot escapement on commercial website
  • Mark Frank (2005), The Evolution of Tower Clock Movements Paper with much technical information on early verge clocks by tower clock restorer, with many unique pictures of movements, references.