Temperature Compensation
The largest source of error in early pendulums was slight changes in length due to thermal expansion and contraction of the pendulum rod with changes in ambient temperature. This was discovered when people noticed that pendulum clocks ran slower in summer, by as much as a minute per week (one of the first was Godefroy Wendelin, as reported by Huygens in 1658). Thermal expansion of pendulum rods was first studied by Jean Picard in 1669. A pendulum with a steel rod will expand by about 11.3 parts per million (ppm) with each degree Celsius increase (6.3 ppm/°F), causing it to lose about 0.27 seconds per day, or 16 seconds per day for a 33 °C (60 °F) change. Wood rods expand less, losing only about 6 seconds per day for a 33 °C (60 °F) change, which is why quality clocks often had wooden pendulum rods. However, care had to be taken to reduce the possibility of errors due to changes in humidity.
Read more about this topic: Pendulum, Use For Time Measurement
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