Editing Pendulum (section)

==== Mercury pendulum ====
The first device to compensate for this error was the mercury pendulum, invented by [[George Graham (clockmaker)|George Graham]]<ref name="Graham1726" /> in 1721.<ref name="Milham1945" /><ref name="BritannicaCompensation" />  The liquid metal [[mercury (element)|mercury]] expands in volume with temperature. In a mercury pendulum, the pendulum's weight (bob) is a container of mercury. With a temperature rise, the pendulum rod gets longer, but the mercury also expands and its surface level rises slightly in the container, moving its [[centre of mass]] closer to the pendulum pivot. By using the correct height of mercury in the container these two effects will cancel, leaving the pendulum's centre of mass, and its period, unchanged with temperature. Its main disadvantage was that when the temperature changed, the rod would come to the new temperature quickly but the mass of mercury might take a day or two to reach the new temperature, causing the rate to deviate during that time.<ref name="MatthysCompensation">[https://books.google.com/books?id=Lx0v2dhnZo8C&pg=PA7 Matthys 2004], p.7-12</ref>  To improve thermal accommodation several thin containers were often used, made of metal. Mercury pendulums were the standard used in precision regulator clocks into the 20th century.<ref>Milham 1945, p.335</ref>