Grandfather clocks, with their slow-moving pendulums, steady tick-tock sounds and human stature, can seem almost alive. Also called longcase or tallcase clocks, they need the tall cases to house their long pendulums. Technically, a true grandfather clock is 6 to 8 feet tall, while a clock 5 to 6 feet tall is a "grandmother" clock and one under 5 feet tall is a "granddaughter." In practice, any clock with a pendulum is sometimes mistakenly called a grandfather clock. The term should be reserved for the tall clocks which stand on their own bases. Shorter clocks which stand on another piece of furniture are more accurately termed "mantel clocks."
Pendulum clocks, which run solely on mechanical power supplied by winding the clock, were first invented in the 17th century and were the first truly accurate timepieces. They remained standard until the 1930s, when they were finally superseded by electric clocks. Because longer and slower pendulums need less power to run and so cause less wear to the rest of the clock movement, grandfather clocks are mechanically preferable to mantel clocks, even if less convenient. But the principles of operation are the same for all pendulum clocks.
The weights provide power to run the clock. Imagine a weight tied to one end of a long string, with the other end tied to a drum placed above the ground. Turning the drum winds the string and hoists the weight into the air; letting go of the drum causes the weight to plummet to the ground, unwinding the string and turning the drum. This is in effect how a grandfather clock runs. When you wind it, you are turning a drum and hoisting weights into the air. The hands on the clock face are attached to the drum and turn as the weight is released. The rest of the clock mechanism regulates the speed at which the weight falls and the drum turns.
Most grandfather clocks have either an 8-day or a 1-day (30-hour) movement. An 8-day clock needs to be wound once a week. It is powered by 2 weights: one for the pendulum and one for the chimes. There are 2 holes in the clockface, one for winding each weight.
A one-day movement has to be wound every day. It has only 1 weight, which drives both the timekeeping and striking mechanisms.
A pendulum is a small weight at the end of a rod. Galileo was the first to study pendulums; he discovered that a pendulum's period (the time it takes to go back and forth once) depends on the length of the rod, not the weight. (Test this yourself by tying a weight to a string, suspending it at different heights from a table top and counting how many swings it makes in a minute.) The steady pulse of the pendulum swinging is used to regulate the falling speed of the weights that power a clock. However, something is needed to keep the pendulum running and prevent friction from gradually bringing it to a standstill.
The escapement consists of several pieces that work together: the pendulum, a toothed gear and a piece called the anchor that engages the gear's teeth. At each swing of the pendulum, one gear tooth is allowed to escape. The two sides of the anchor engaging steadily with the gear, right, left, right, left, are what make the tick-tock, tick-tock sound when the clock is running. The reason the two sounds are different is that one sound is made when the anchor catches a gear tooth and the other is made when releasing a tooth.
The anchor nudges the pendulum just enough to overcome the force of friction, so that the pendulum keeps swinging.
The steady pulse of the pendulum regulates the speed at which the clock's hands go around. A grandfather clock usually has a two-second period, meaning it takes two seconds for the pendulum to go back and forth once. Smaller clocks have a one-second or half-second period.
A gear train is a series of gears of different sizes which engage each other in a long train. A grandfather clock has several gear trains: one for each hand (hour, second, and minute) and one to turn the drum. The different sizes of gears in each train cause the hands to turn at different, but precisely regulated, speeds. If the clock has chimes, these are regulated by a separate gear train called the striking train.
This mechanism disengages the gears from the hands so that the clock can be set without involving the rest of the clockworks. It is equivalent to the stem in a watch, which is pulled out, disengaging the gears, to set the time.