Anchor escapement
The anchor escapement is a type of escapement, the mechanism in a clock that regulates the swinging of a pendulum for accurate timekeeping. It was first invented by Robert Hooke around 1657, although some references credit William Clement, and led to the rapid introduction of a number of related designs over the next 50 years.
Almost all mechanical clocks use a main gear, known as the escapement, to control the motion of the pointers on the face. The escapement is driven in a fashion that allows it to travel only a certain angle before being stopped by some sort of lock. The lock is then attached to a periodic device, thereby allowing the escape to move only on set periods, for instance, once a second. In most cases the escape provides energy to the whole system, most often with a set of weights pulling on it.
Prior to the introduction of the anchor escapement, most mechanical clocks used the verge-and-foliot system. This was essentally a horizontal pendulum mounted on a vertical shaft (the verge) driving two small flat cams (the foliot). The cams were mounted to fit into the teeth of the escape. At every "tick", one of the cams would be forced out of the gear teeth due to the weights on the escape, forcing the pendulum to swing until the other cam entered the teeth again and stopped the motion. The foliots were positioned such that the escape could only move a short distance before hitting the other one and stopping, but the pendulum itself would continue swinging past that point before stopping and "turning around". For this portion of the motion, the pendulum was pushing the escapement back, lifting the weights. During the brief period of free motion of the escapement, the newly engaging foliot would hit the gearing and bounce off, an effect known as "recoil".
Although the verge-and-foliot had a fair amount of angular momentum, it also had a considerable amount of friction and drag, random forces that upset the ability for the mechanism to keep accurate time. Christian Huygens modified this basic system with the introduction of the vertical pendulum, using the verge-and-foliot horizontally as the mounting pivot for the pendulum. In this case the angular momentum could be greatly increased, reducing the relative influence of the random forces.
Hooke's breakthrough was the anchor, a small C-shaped metal piece that replaced the foliot's of the earier system. Whereas the foliots were driven off the face of the teeth in the escapement, the anchor's cams were rounded to fit into the gearing "from above" in such a way that they just sat inside the teeth. The teeth on the escapement were also modified into a rounded shape, such that the motion of the anchor past the point where they caught the tooth would ride smoothly on the face of the gear, and no longer forced the system to rotate backward. The overall force needed to operate the system was considerably lower, as well as the range of angles needed to operate the escapement. The result was a dramatic reduction in friction and drag, thereby reducing the influence of the random forces even more.
The anchor escapement system increased accuracy by about ten times over older systems, and from that point on almost every clock mechanism for the next 50 years was based on the anchor with additional modifications to further reduce drag or recoil. One direct modification was the Graham escapement, which eliminated recoil with a small modification to the motion. One of the very few mechanisms not based on the anchor was John Harrison's complex grasshopper escapement used in his famous longitude-measuring chronographs.