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Verge escapement

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 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 and pocketwatches. The name verge comes from the Latin virga, 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 are at the heart of every clock today.

Verge and foliot clocks

In China, escapement-like devices were employed in a water-powered celestial globe built by Yi Xing in 725 CE and Su Song's astronomical clock-tower in 1088. However, these were not true mechanical escapements as employed later in mechanical clocks, since the time was still measured by flowing water:

The first hard evidence of the verge escapement dates from 14th century Europe, where its invention led to the development of the first all-mechanical clocks. Starting in the 1200s, large tower clocks began appearing in town squares and cathedrals. They kept time by using the verge escapement to drive a horizontal bar with weights on the ends called the foliot, a primitive type of balance wheel, to oscillate back and forth. The rate of the clock could be adjusted by sliding the weights in or out on the foliot bar.

There is speculation that Villard de Honnecourt invented the verge escapement in 1237 with an illustration of a questionable device. It probably evolved from the alarum, invented centuries earlier, which used the same mechanism to ring a bell. However, it may never be known when the escapement was first used, because it has proven impossible to distinguish from existing documentation which of these early tower clocks were mechanical, and which were water clocks . The same Latin word, horologe, was used for both. 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 clock built at the Palace of the Visconti, Milan, Italy, in 1335. However, there is agreement that mechanical clocks existed by the late 1200s.

Actually, 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 verge in clocks.

How accurate these early verge and foliot clocks were is debatable, with estimates of two hours to 15 minutes per day being mentioned. Early verge clocks were probably no more accurate than the previous water clocks, but they did not freeze in winter and were a more promising technology for innovation.

Verge pendulum clocks

Most of the gross inaccuracy of verge and foliot clocks was not due to the escapement itself, but to the foliot oscillator. The invention of the pendulum 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, 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

The verge escapement consists of a wheel shaped like a crown, with sawtooth-shaped teeth protruding axially to the front. In front of it is a vertical rod, the verge, with two metal plates, the pallets, that engage the teeth at opposite sides of the crown wheel. The balance wheel (or the pendulum) is attached to the verge. The pallets are positioned so only one catches the teeth at a time. As the clock’s gears turn the crown wheel, it pushes the first pallet, rotating the verge in one direction, and rotating the second pallet into the path of the teeth, until the tooth pushes past the first pallet. Then a tooth on the wheel’s opposite side catches the second pallet, rotating the verge back the other direction, and the cycle repeats. The result is to change the rotary motion of the wheel to an oscillating motion of the verge. Each stroke of the foliot or pendulum thus advances the wheel train of the clock, moving the hands forward at a constant rate.

The crown wheel must have an odd number of teeth for the escapement to function. The usual angle between the pallets was 90° to 105°, 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, upwards of 115°. 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.

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 clocks are sensitive to changes in the drive force; they slow down as the mainspring unwinds. This is called lack of isochronism. It was much worse in verge and foliot clocks due to the lack of a balance spring, but is a problem in all verge movements. In fact, the common method of adjusting the rate of early verge watches was to alter the force of the mainspring. The cause of this problem is that the verge is a frictional type escapement. The crown wheel teeth are always pushing on the pallets, driving the pendulum (or balance wheel) throughout its cycle; it is never allowed to swing freely. All later verge watches and some clocks required fusees to equalize the force of the mainspring.
  • It is a recoil type escapement, meaning that the force of the pendulum causes the wheel train to move backward during part of its cycle. This increases friction and wear, resulting in inaccuracy. One way to tell whether a watch has a verge escapement is to observe the second hand closely; if it moves backward a little during each cycle, the watch is a verge.
  • In pendulum clocks, the wide pendulum swing angles of 80°-100° required by the verge cause an additional lack of isochronism due to circular error.
  • The wide pendulum swings also cause a lot of air friction, reducing the accuracy of the pendulum, and requiring a lot of power to keep it going, increasing wear. So verge pendulum clocks had lighter bobs, which reduced accuracy.

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 1600s focused attention on error caused by the escapement. In the 17th and 18th century wave of horological innovation that followed, the verge was superseded by better escapements.

In pendulum clocks, it was replaced by the anchor escapement, invented around 1670. By locating the pallets farther from the axis of rotation, the anchor escapement reduced the pendulum swing from around 100° in verge clocks to 4°-6°. In addition to eliminating circular error, this allowed room in the clock case for longer and slower pendulums, reducing wear. In England the anchor escapement took over, but the French continued to use verge escapements until about 1800. Many verge clocks were rebuilt with anchor escapements.

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 inexpensive verge fusee watches continued to be produced until the early 1800s, when the lever escapement took over.

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 ruby pallets. 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.

References

  • Bolter, David J. (1984). Turing's Man: Western Culture in the Computer Age. University of North Carolina Press. . Summary of the role of the clock in setting the direction of philosophic movement for the Western world.
  • Glasgow, David (1885). Watch and Clock Making. London: Cassel & Co.. , p.124-126. Construction info on verge escapements.
  • Headrick, Michael (2002). "Origin and Evolution of the Anchor Clock Escapement". Control Systems magazine, 22 (2): Retrieved on 2007-06-06.. Engineering overview of development of clock and watch escapements, starting with verge, focusing on sources of error.
  • Macey, Samuel L. (1980). Clocks and the Cosmos: Time in Western Life and Thought. Archon Books. .
  • Milham, Willis I. (1923). Time and Timekeepers. MacMillan. . Comprehensive reference by astronomy professor, good account of origin of first clocks, but historical info dated. Long bibliography.
  • A Walk Through Time, part 3: A Revolution in Timekeeping. National Inst. of Standards and Technology. (2002). Retrieved on 2007-06-06... Brief nontechnical history of clocks.
  • Usher, Abbot Payson (1988). A History of Mechanical Inventions. Courier Dover. Chapter on invention of mechanical clock debunks many mistaken historical claims.
  • White, Lynn Jr. (1966). Medieval Technology and Social Change. Oxford Press. , p.119-127. Analysis of medieval clock development, copious references to original sources.

Notes

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