Pendulum - (Clock)

Pendulum - (Clock) The incorporation of a pendulum in a clock is attributed to a Dutchman, in 1657, though there is some evidence to suggest that Italian clock-makers had done so earlier. Early bob-pendulums are short and, as used with the verge escapement, swung through an arc of 35 to 40 degrees. With the invention of the anchor escapement about 1670 a much longer pendulum became practicable; known as the Royal pendulum, it was 39-1393 inches long and moved through an arc of 4 or S degrees to a one-second beat, thus permitting a second hand and leading to the long-case clock. Early pendulum rods, often of simple iron wire, were liable to expansion and contraction with changes of temperature. Two main improvements, invented about the same time (c. 1726), were the mercury pendulum and the grid-iron pendulum. The former, invented by George Graham, had as a bob a glass jar of mercury suspended from a brass pendulum rod; heat that lengthened the rod also expanded the mercury and thus the centre of oscillation remained constant (and vice versa in cold weather). The grid-iron pendulum of John Harrison is based on the fact that the expansion of brass to steel is in the ratio of 3 to 2; in his pendulum the upper extremities of alternate brass and steel rods, in the aforementioned ratio as regards length, will therefore if joined at their lower ends remain the same distance apart whatever the changes of temperature. Both the mercury and grid-iron pendulum are still in use today. Very rare is the `second-and-a-quarter' pendulum in which the duration of each arc of the swing is 1 1/4 seconds and the pendulum is 5 feet in length: introduced c. 1675, probably by William Clement, but abandoned by the end of the next decade.

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