Gridiron pendulum

The gridiron pendulum was a temperature-compensated clock pendulum invented by British clockmaker John Harrison around 1726 and later modified by John Ellicott. It was used in precision clocks. In ordinary clock pendulums, the pendulum rod expands and contracts with changes in temperature. The period of the pendulum's swing depends on its length, so pendulum clocks rate varied with changes in ambient temperature, causing inaccurate timekeeping. The gridiron pendulum consists of alternating parallel rods of two metals with different thermal expansion coefficients, such as steel and brass. The rods are connected by a frame in such a way that their different thermal expansions (or contractions) compensate for each other, so the overall length of the pendulum, and thus its period, stays constant with temperature.

The gridiron pendulum was used during the Industrial Revolution period in regulator clocks, precision clocks employed as time standards in factories, laboratories, office buildings, and post offices to schedule work and set other clocks. The gridiron became so associated with quality timekeeping that to this day many clocks have pendulums with decorative fake gridirons, which have no temperature compensating qualities.

Its simplest and later form consists of five rods. A central iron rod runs up from the bob to a point immediately below the suspension. At that point a cross-piece (middle bridge) extends from the central rod and connects to two zinc rods, one on each side of the central rod, which reach down to, and are fixed to, the bottom bridge just above the bob. The bottom bridge clears the central rod and connects to two further iron rods which run back up to the top bridge attached to the suspension. As the iron rods expand in heat, the bottom bridge drops relative to the suspension, and the bob drops relative to the middle bridge. However, the middle bridge rises relative to the bottom one because the greater expansion of the zinc rods pushes the middle bridge, and therefore the bob, upwards to match the combined drop caused by the expanding iron.

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