What is "gridiron pendulum"

Gridiron \Grid"i`ron\, n. [OE. gredire, gredirne, from the same source as E. griddle, but the ending was confused with E. iron. See Griddle.]

1. A grated iron utensil for broiling flesh and fish over coals.

2. (Naut.) An openwork frame on which vessels are placed for examination, cleaning, and repairs.

3. (Sport) A football field; -- so called because of the resemblance of the parallel marked yard lines to a gridiron[1].

   Gridiron pendulum. See under Pendulum.

   Gridiron valve (Steam Engine), a slide valve with several parallel perforations corresponding to openings in the seat on which the valve moves.

The gridiron pendulum was a temperature-compensated clock pendulum invented by British clockmaker John Harrison around 1726 and later improved 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.