Astonishing Science. Spectacular museum.
Until the 17th century, most mechanical clocks, such as the one at Wells Cathedral used a foliot balance and a verge escapement to make them tick at a steady rate.
The toothed wheel (called a crown wheel) is turned anticlockwise by a weight hanging from a rope wrapped round the horizontal shaft (the rope and weight are not shown). This movement is controlled by the flat plates (called pallets) attached to the vertical shaft (or verge) on top of which there is a bar (or foliot) with weights hanging from each end. The pallets engage with the teeth of the crown wheel. In this diagram, the top tooth is pushing the pallet to the left, turning the verge anticlockwise. The verge turns until the lower pallet meets the bottom tooth, which pushes the verge in the other direction.
The back-and-forth movement of the verge and foliot stop the crown wheel turning freely. The time taken for each swing of the foliot, and the rate of rotation of the crown wheel, can be controlled by moving the weights on the foliot. The hands of the clock are connected to the shaft of the crown wheel by a system of gears.
The mechanism for controlling the movement of a clock is called an escapement and this type is known as a verge escapement. The time taken for each swing of the foliot is affected by many things and varies unpredictably. Typically, clocks which used foliot balances could gain or lose up to fifteen minutes a day.
The time taken for a pendulum to swing (its period) is much more reliable. It depends almost entirely on the pendulum's length. It is only slightly altered by the type of escapement used or the size of the swing, as long as this is not very large.
Huygens' first pendulum clock used a verge escapement similar to those used in foliot balance clocks. Later pendulum clocks used anchor escapements, as shown in this model. This kind of escapement also allows one tooth of the wheel to pass each time the pendulum completes a swing but it requires a smaller swing angle than a verge escapement does. This is an improvement because, if a pendulum swings through a very large angle, its accuracy is reduced.
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