The motive-power of a clock is a either a weight, or fusee and spring which drives a train of wheels.
The fusee is conical-shaped brass drum, with the gut line from the spring wound round it like a spindle filled with thread, which controls, by different ratios, the power output of the spring to enable the clock to keep a regular time. Fusees were used on clock and watches until the late 19th century, not given up until slimmer Swiss watches became fashionable in early 20th century.
The escapement is the part of the clock, watch or timepiece which allows the power driving the mechanism to escape, and controls the speed at which a clock runs down. The various forms of escapement release the escape wheel at regular short intervals (giving a tick-tock sound), allowing the driving force to operate, lock and release again, in a regulated sequence.
The earliest types of clock mechanism had verge escapements, with a balance wheel on a short pendulum. These were used from the second half of the 17th century in bracket or table clocks, lantern clocks and, more rarely, longcase clocks. The verge and pendulum continued to be used until 1800, but it was an inaccurate method of timekeeping.
The anchor escapement was developed around 1670. Skaped like an anchor, it allowed either a short or long pendulum to be used and gave greater accuracy than was possible with the verge escapement. It was used in longcase clocks and in bracket and wall clocks. The arc of the swing was much smaller than that required by the verge and so it was seen as an enormous advance.
|Pendulum and anchor escapement.|
(a) pendulum rod
(b) pendulum bob
(c) rate adjustment nut
(d) suspension spring
(g) escape wheel
Clocks with a verge escapement generally use a small, pear-shaped bob on the end of the pendulum as a large weight is not needed. Anchor escapements require a larger disc-shaped bob which is lead-filled to give it weight.
Cylinder, lever and balance wheel escapements were mounted onto platforms on the top of carriage clocks or mantel clocks from c. 1840 and continue to be used today. The platform of which these escapements are carried is detachable as a unit, as opposed to being an integral part of the clock.
The earliest striking mechanisms determined the hours with a locking plate incorporated into the mecchanism, with notches cut out of it to determine the number of hours are controlled by a snail-shaped disc fixed to the hour hand, with steps cut away equalling the number of hour to be struck. This is a more reliable type of mechanism and ensures that the wrong number of hours cannot be struck, as can happen with the locking plate strike. The ‘rack and snail’ is still used on clocks today.
The majority of clocks strike on the hour only. Chiming clocks were developed in the early 18th century and could sound on a number of bells at the hours and quarter hours, playing popular tunes.
Wire gongs became popular in the 1840s and developed in the latter half of the 19th century into large gong rods, like pipes, found in high quality longcase clocks made in England and Germany in c. 1900. In the 20th century, German clocks often had two wire gongs to provie a ‘ting tang’ strike at the quarters.