Grandfather clock

A longcase clock, also tall-case clock, floor clock, or grandfather clock, is a tall, freestanding, weight-driven pendulum clock with the pendulum held inside the tower or waist of the case. Clocks of this style are commonly 1.8–2.4 metres (6–8 feet) tall. The case often features elaborately carved ornamentation on the hood (or bonnet), which surrounds and frames the dial, or clock face. The English clockmaker William Clement is credited with the development of this form in 1670. Until the early 20th century, pendulum clocks were the world's most accurate timekeeping technology, and longcase clocks, due to their superior accuracy, served as time standards for households and businesses. Today they are kept mainly for their decorative and antique value.

The advent of the longcase clock is due to the invention of the anchor escapement mechanism by Robert Hooke around 1670. Prior to that, pendulum clock movements used an older verge escapement mechanism, which required very wide pendulum swings of about 80-100°. Long pendulums with such wide swings could not be fitted within a case, so most freestanding clocks had short pendulums.

The anchor mechanism reduced the pendulum's swing to around 4° to 6°, allowing clockmakers to use longer pendulums, which had slower "beats". These consumed less power allowing clocks to run longer between windings, caused less friction and wear in the movement, and were more accurate. Almost all longcase clocks use a seconds pendulum (also called a "Royal" pendulum) meaning that each swing (or half-period) takes one second. These are about a metre (39 inches) long (to the centre of the bob), requiring a long narrow case. The long narrow case actually predated the anchor clock by a few decades, appearing in clocks in 1660 to allow a long drop for the powering weights. However, once the seconds pendulum began to be used, this long weight case proved perfect to house it as well. British clockmaker William Clement, who disputed credit for the anchor escapement with Robert Hooke, produced the first longcase clocks around 1680. Within the year Thomas Tompion, the most prominent British clockmaker, was making them too.

Modern longcase clocks use a more accurate variation of the anchor escapement called the deadbeat escapement.

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Wikipedia