The gridiron pendulum was an improved clock

Clock

A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...

 pendulum

Pendulum

A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...

 invented by British clockmaker John Harrison

John Harrison

John Harrison was a self-educated English clockmaker. He invented the marine chronometer, a long-sought device in solving the problem of establishing the East-West position or longitude of a ship at sea, thus revolutionising and extending the possibility of safe long distance sea travel in the Age...

 around 1726. It didn't change its effective length with temperature, so its period

Frequency

Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 of swing stayed constant with changes in ambient temperature. It consisted of alternating brass

Brass

Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...

 and iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 rods arranged so that their different thermal expansion

Thermal expansion

Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...

s (or contractions) counteracted each other.

How it works

Its simplest and later form consists of five rods. A central iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 rod runs up from the bob

Bob (physics)

A bob is the weight on the end of a pendulum most commonly, but not exclusively, found in pendulum clocks.- Reason for use :Although a pendulum can theoretically be any shape, any rigid object swinging on a pivot, clock pendulums are usually made of a weight or bob attached to the bottom end of a...

 to just below the suspension. At that point a cross-piece (middle bridge) extends from the central rod and connects to two zinc

Zinc

Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...

 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.

In simple terms, the upwards expansion of the zinc counteracts the combined downwards expansion of the iron (which has a greater total length). The rod lengths are calculated so that the effective length of the zinc rods multiplied by zinc's expansion coefficient equals the effective length of the iron rods multiplied by iron's expansion coefficient thereby keeping the pendulum the same length.

Harrison's original construction using brass

Brass

Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...

 (pure zinc not being available then) is more complex since brass does not expand as much as zinc does. A further set of rods and bridges is needed giving nine rods in all, five iron and four brass. The exact degree of compensation can be adjusted by having a section of the central rod which is partly brass and partly iron. These overlap (like a sandwich) and are joined by a pin which passes through both metals. A number of holes for the pin are made in both parts and moving the pin up or down the rod changes how much of the combined rod is brass and how much is iron. In the late 19th century the Dent

Edward John Dent

Edward John Dent was a famous English watchmaker noted for his highly accurate clocks and marine chronometers.He founded the Dent company.- Early years :...

 company marketed a further development of the zinc gridiron in which the four outer rods were replaced by two concentric tubes which were linked by a tubular nut which could be screwed up and down to alter the degree of compensation.

Disadvantages

Scientists in the 1800s found that the gridiron pendulum had disadvantages that made it unsuitable for the highest precision clocks. The friction of the rods sliding in the holes in the frame caused the rods to adjust to temperature changes in a series of tiny jumps, rather than with a smooth motion. This caused the rate of the pendulum, and therefore the clock, to change suddenly with each jump. Later it was found that zinc is not very stable dimensionally; it is subject to creep

Creep (deformation)

In materials science, creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material....

. Therefore another type of temperature-compensated pendulum, the mercury pendulum, was used in the highest precision clocks. Both types were superseded by pendulums using low-expansion materials such as Invar

Invar

Invar, also known generically as FeNi36 , is a nickel steel alloy notable for its uniquely low coefficient of thermal expansion . The name, Invar, comes from the word invariable, referring to its lack of expansion or contraction with temperature changes.It was invented in 1896 by Swiss scientist...

 and fused silica.