Antique French clock with a mercury pendulum made by Charles Hour
A brief history on mercury pendulums
Early clocks dating as far back as the 17 and 18th centuries were accurate to around five minutes per week. Although impressive by the standards and materials used to make clocks, a higher degree of accuracy was required especially for scientific purposes and nautical exploration. Around 1720 in England, regulators started being made powered by a weighted and geared mechanism which now meant that they could be accurate to within 10 seconds per month.
Many of the early regulator clocks were fitted with mercury pendulums that were made to compensate for temperature variations that could alter the accuracy of the clock. Put simply, as metal gets warm it expand and can actually lengthen the length of the pendulum, causing the clock to run slower. However, using a mercury pendulum compensates this as the mercury – which is sealed safely in the vial – has nowhere to expand but up.