English: Drawing of a typical grandfather clock pendulum, from a 1901 physics book. It is a seconds pendulum, with each swing of the pendulum taking one second. Slightly less than 1 meter (39 in.) long. It consists of a metal weight suspended on a wooden rod, since wood expands and contracts with temperature less than metal, keeping the period of the pendulum more constant. Under the weight is a knurled rate adjustment nut, which adjusted the rate of the clock by moving the weight up and down on the pendulum rod. The weight has a streamlined disc shape to minimize air resistance. Alterations to image: removed caption.