Mercury compensated pendulum

To keep accurate time, a pendulum clock needs a pendulum whose centre of gravity remains constant. Unfortunately when the metal shaft of a pendulum warms up, it expands, shifting the centre of gravity down and causing the clock to run slow. To compensate for this, glass vials containing mercury can be placed at the bottom of the pendulum. Not only is mercury nice and heavy, making for a good solid pendulum, it also expands more than most other metals on heating. Thus as the pendulum warms up the bottom of the vials moves down (because the shaft gets longer) but the mercury gets taller, shifting the centre of gravity back up where it belongs. 


Now you know!