Verge escapement

[3][9][10] Starting in the 13th century, large tower clocks were built in European town squares, cathedrals, and monasteries.

The verge escapement probably evolved from an alarm mechanism to ring a bell which had appeared centuries earlier.

[11] In spite of the fact that these clocks were celebrated objects of civic pride which were written about at the time, it may never be known when the new escapement was first used.

[11] 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.

How accurate the first verge and foliot clocks were is debatable, with estimates of one to two hours error per day[31][13][2] being mentioned, although modern experiments with clocks of this construction show accuracies of minutes per day were achievable with enough care in design and maintenance.

A similar increase in accuracy in verge watches followed the introduction of the balance spring in 1658.

[13][36] 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.

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.

As the clock's gears turn the crown wheel (see animation), one of its teeth catches on a pallet, pushing on it.

The escape wheel tooth, pushing against the pallet each swing, provides an impulse which replaces the energy lost by the foliot to friction, keeping it oscillating back and forth.

In order to reduce the pendulum's swing to make it more isochronous, the French used larger pallet angles, upward of 115°.

Then the increase in accuracy due to the introduction of the pendulum and balance spring in the mid 17th century focused attention on error caused by the escapement.

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.

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 first successful marine chronometers, H4 and H5, made by John Harrison in 1759 and 1770, used verge escapements with diamond pallets.,[13][38][41] In trials they were accurate to within a fifth of a second per day.

One of the earliest existing drawings [ 8 ] 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 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).
Modern reproduction of an early verge and foliot clock. The pointed-tooth verge wheel is visible, with the wooden foliot rod and suspended weight above it.