How Pendulum Clocks Work

The mechanics and history of pendulum clocks: how they work and how they keep accurate time.

Pendulum clocks were developed in the mid-1600's in Europe. Galileo discovered that pendulums swung at a constant rate, no matter how much energy was in each swing. He noted that pendulums might be used to regulate clocks or other devices. Christiaan Huygens (a Dutch scientist) is credited with the patent for the first pendulum clock in 1657. Adding a pendulum to the time-keeping device improved its accuracy and allowed more even and consistent measurement of time. These clocks rely on several properties of pendulums and a series of other mechanical devices to work properly.

A simple pendulum is a weight attached to the end of a string or wire. The weight hangs from a fixed point and will swing from side to side if pulled to one side and released. Gravity causes the pendulum to swing at an even rate. Because the wire holding the weight is of fixed length, the weight's path describes part of a circle. The path is called the arc of the pendulum. The time it takes for the weight to pass back and forth over the arc one time is the period of oscillation. If the pivotal point is well-lubricated and there are no other sources of friction to slow it down, a pendulum will swing freely and the arc will get smaller as the pendulum slows very gradually due to drag from the air around it.

Pendulum clocks rely on this regularity to help them keep time accurately. Most pendulum clocks are weight-driven and must be wound regularly. When the clock is wound, the heavy weight is pulled up by a chain or wire. This cord wraps around a drum as the clock is wound. Gravity pulls the weight down, causing energy to be released into the pendulum as the cord slowly unwinds from the drum. The drum turns a series of gears that operate the clock. These are called the train. The gears of the train turn the clock's hands. The gears are different sizes, so they turn at different speeds, allowing the minute hand to move more rapidly than the hour hand. It is the pendulum that regulates the speed of the unwinding and hence the speed of the clock's hands.

In order for the clock to work properly, designers had to devise a mechanism to keep the weights from unwinding too rapidly or unevenly. The pendulum, together with a device called an escapement, accomplished the task. The escapement consists of a gear that is connected to the train (the escape wheel) and a piece called a verge that looks like a rainbow-shaped piece of metal. The escape wheel turns at the same pace as the train, which turns because of the unwinding cord pulled by the weight. The verge has hooks on each end that stop the escape wheel from turning freely. When the pendulum swings to the side of its arc, the verge rocks and releases the escape wheel for an instant. The escape wheel turns a small amount and the hands move a small amount. When the pendulum reaches the other side of its arc, the verge is pulled the other way and releases the escape wheel once again. The hands move another small amount. The on and off movements of the verge and the escape wheel are responsible for the familiar tick-tock sound that pendulum clocks are known for. The swing of the pendulum slows the fall of the weight and the speed of the clock hands in this way. The energy of the falling weight keeps the pendulum swinging until the weight reaches the bottom of its cord and cannot fall any farther. This causes the pendulum to slowly swing with a shorter and shorter arc until it finally stops all together. When the clock is rewound, the entire process begins again.

The idea was wonderful in theory, but did not allow for several problems that could occur. Pendulums behave differently under various conditions of gravity and weather. Pendulums have a broader period when they are swinging in lower gravity, such as on mountains. They are also affected by changes in weather because the metal that holds the pendulum weight expands and contracts with the temperature. A pendulum with a longer cord will take longer to complete its arc and the clock will run more slowly.

Designers needed to create a way to adjust the pendulum mechanism to allow for these changes so that the clock could still keep accurate time.

Today, most pendulum clocks have two mechanisms that allow adjustments to the pendulum. There is a screw at the top of the pendulum that allows miniscule adjustments in the length of the cord or wire, and so will adjust the period of oscillation and the speed of the clock. This can be used if the clock seems to be running too slowly or too fast. Making the pendulum longer will slow the clock's speed and shortening it will pick up its pace.

Many pendulums also are equipped with regulators that help avoid the clock running more slowly in warm weather and faster during cold weather. One common type of regulator looks like a flat grid of metal rods around the pendulum's cord or wire. These are made of brass and steel because each of these materials expands and contracts differently at different temperatures. The bars are arranged so that they expand and contract when needed to keep the pendulum's length constant. They work together to maintain a consistent length. When the pendulum's length remains constant, the clock will keep accurate time instead of running more slowly or more quickly in different kinds of weather.

Clocks have come a long way from the first devices of the middle ages that simply chimed on the hour. These early clocks had no hands at all and were not terribly accurate. The invention of the pendulum clock ushered in an age of keeping time to the minute and even to the second and changed our sense of time permanently.

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