Pendulum clocks were first invented in the 1650s, and they represented an enormous increase in accuracy over any previous method of timekeeping. The physical principles of the pendulum were first investigated by Galileo, who discovered that the time period of a pendulum's swing, which is used to regulate pendulum clocks, depends not on the width of the swing or the mass of the weight used, but only on the length of the pendulum rod. The Dutch scientist Christiaan Huygens, using these observations, constructed the first pendulum clock. Because the accuracy of the new clocks was so important, scientists early discovered a problem that threatened their accuracy.
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If you own a pendulum clock and have discovered that it runs slower in summer than in winter, you are experiencing a problem that was first noted in the 1650s and 1660s on some of the earliest pendulum clocks ever built. The problem is that since the pendulum's period depends on the length of the rod, it changes between summer and winter, as the rod expands and contracts with changes in temperature.
Rods get longer in warmer weather, shorter in colder weather. With a shorter rod, a pendulum will swing faster and a clock will run faster. With a longer rod, the pendulum will swing slower and the clock will slow down.
Amount of Change
Quite a small change in length can affect the clock's time noticeably. An increase of 1/5 of a millimeter on a grandfather clock pendulum, which is usually about 1 meter long, will cause the clock to lose about 1 minute a week.
A clock with a steel rod can lose about a 1/4 of a second for each 1 degree F the temperature rises. So a 60-degree F temperature rise (from winter to summer) can result in a loss of 16 seconds a day. Wood varies less with the temperature than steel, so a clock with a wooden pendulum rod would lose only about 6 seconds a day after a 60-degree F temperature rise.
While seconds a day may not seem to be an enormous time difference, it adds up over the course of a season. And in an era when the pendulum clock was the premier method for keeping time, even small losses in accuracy were important. Clockmakers compensated for seasonal changes in rod length by adding a small screw to the bottom of the pendulum rod, below the weight (or "bob"). Adjusting the screw will change the length of the rod and so affect the clock's timekeeping. You may have to experiment to get the right length, but your clock can be made to keep quite accurate time, regardless of the season.