English: Drawing of a gridiron pendulum, a temperature compensated pendulum invented around 1725 by British clockmaker John Harrison. Ordinary pendulums' period of swing changes with temperature due to thermal expansion and contraction of the pendulum rod. The gridiron pendulum uses parallel rods of different metals to cancel out the length changes, so the length and period stay constant with temperature changes. This is a demonstration model mounted on a stand, for use in physics classes. Gridiron pendulums using brass and steel rods such as this one required 9 bars, while gridirons with zinc and steel bars only needed 5, because of the higher thermal expansion of zinc. The darker vertical rods such as (d) are steel, the lighter such as (e) are brass. Alterations to image: removed figure number.