By Aanchal Mahajan
Take a guess as to where did the physicists get the first insight into the wonderful mechanism behind the working of pendulum clocks… Of course, in the 16th century, it was bound to be a church. Believe it or not, Galileo was the first person who came up with the idea of a pendulum clock – when he noticed that a lamp kept on swinging back and forth in the same period of time while he was in a Pisa cathedral. The lamp could have been said to be a pendulum, and Galileo came up with a theory that all pendulums have the same time period. The first proper pendulum clock developed by Huygens – 15 years after Galileo’s death – worked on the same mechanism.
The time period of a pendulum (the total time take to swing back and forth to the same position, i.e., to finish one oscillation) is given by:
Where T is the time period, L the length of a pendulum, and g is the acceleration due to gravity (≈9.8). The acceleration of a body is the rate at which the speed of a body changes.
In the most basic pendulum clock, a pendulum with the time period of one second strikes rocks against a lever, called an escapement, every second. This escapement is connected to the gear moving the seconds’ hand of the clock, while this gear is connected to a weight that pulls it down, making it rotate. Every time the escapement and the gear aren’t in contact, the seconds’ hand moves. Since the escapement strikes the gear every second, making the tick-tock sound associated with the pendulum clocks, the seconds’ hand can only move for one second before the gear is locked by the escapement. The gear of the seconds’ hand is connected to another gear which controls the minutes’ hand. For the distance covered by the seconds’ gear in one second, the minutes’ gear and thus, the minutes’ hand covers 1/60th of the distance covered by the seconds’ hand. Similarly, an hours’ gear moves the hours’ hand 1/60th of the distance covered by the minutes’ hand in one second. Thus, because the gear was locked every second due to the constant swinging of the pendulum, time keeping was very precise. In fact, pendulum clocks were the most reliable time keeping devices seen by humans of that time, the erring by only 10 seconds everyday. The idea behind them was astonishingly simple, and seemingly perfect.
However, while theoretically, the given formula seems to indicate that a pendulum will never stop swinging as the time period is constant, you and I know that practically, this is impossible. The air surrounding the pendulum will naturally oppose the motion of the pendulum, making it lose speed and slowly come to a halt. Another problem that arose was that length of the metal strings of the pendulum could change according to the temperature, affecting the time period. Even gravity is not constant all over the earth’s surface, and thus, pendulum clocks have different time periods at different regions of the earth. The problem of the drag, in fact, didn’t even need to be addressed, however, the other two were major drawbacks of the clocks. As for the drag, even though the pendulum slows down due to it, the total time period does not change, after all, if it did, pendulum clocks won’t have been one of the more reliable time keeping devices known to man. The pendulum on slowing down only covers lesser distance in the same time, thus the time period stays constant. The escapement mechanism also provided the pendulum with an extra push from the gears, ensuring that the drag was overcome. The property to keep the same time period, known as isochronism (iso = same, chronism = time) is what made pendulum clocks so popular as reliable time keepers, paving the path for even more precise time keeping. And all of this was a result of Galileo paying attention to a swinging lamp in a church.