Pendulum clocks, with their tick-tock sound, can feel like the living heart of a home. But just how they work remains a mystery to most people. Pendulum clocks were invented in the 17th century, before batteries existed or electricity had been harnessed. They run purely on mechanical power supplied by winding the clock, and are so accurate they represented an enormous improvement over earlier timepieces, such as sundials and water clocks. Pendulum clocks were so accurate that they remained in use until the 1930s, when they were finally superseded by electric clocks. They use the swing of the pendulum to regulate the clock, the energy of weights gradually dropping to keep the pendulum moving, and one or more series of gears to make sure the hands move at a precise rate.

The Weight

The weight is the engine that drives the clock. Imagine that you had a weight tied to a string, and you wound the string around a drum so that the weight hung above the floor. If you let it go, the weight would drop, turning the drum, until it reached bottom. This is what you are doing when you wind a clock: turning a drum and pulling weights up by winding their string around it. If hands (like those on a clock) were attached to the drum, you would see those hands turn as the weight dropped. This, in simple terms, is how a clock runs. But something is needed to regulate how fast the weight drops.

The Pendulum

The pendulum is a round weight, or bob, set on the end of a metal rod. As the rod swings, the pendulum goes back and forth in a regular beat. The period of a pendulum (the time it takes to go back and forth once) is based on the length of the pendulum rod and affected by the pull of gravity at a particular location on earth. It is possible to design pendulums of the correct length and weight so that their period is exactly one second long, or some other regular amount of time. If the pendulum could somehow be kept going, so that it wasn't stopped by friction, it could be used to exactly measure the passage of time.

The Escapement

The escapement is a series of parts, including the pendulum, a toothed gear and a device called the anchor which engages the gear's teeth. At each swing of the pendulum, one tooth of the gear is allowed to escape. The anchor touching the gear in two places, right, left, right, left, is what makes the tick, tock, tick, tock sound we hear when a pendulum clock is running. The anchor nudges the pendulum just enough to overcome friction, so that the pendulum swings without stopping.

The Gear Trains

A gear train is a series of gears, each connecting to the next. In a clock, one gear train connects the drum (turned by the falling weight) to each hand of the clock. The different sizes of gears cause each hand to revolve at a different speed. Another gear train holds the weight up so it doesn't fall so quickly. In most clocks with chimes, the striking of the chimes is regulated by a separate gear train and weight. The gear trains regulate the speed at which the drums turn, so the weights fall at precisely regulated speeds.

Setting Mechanism

The setting mechanism disengages the gear trains so that a clock's hands can be set to the correct time. It corresponds to the stem of a watch, which has to be pulled out (disengaging the gears) so that the hands can be set.