A: exterior schematic B: normal temperature C: higher temperature
The gridiron pendulum was a temperature-compensated clockpendulum 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 a pendulum clock's 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 that 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 pendulum 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 accurate timekeeping that to this day many clocks have pendulums with decorative fake gridirons, which have no temperature compensating qualities.
How it works
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, upward to match the combined drop caused by the expanding iron.
In simple terms, the upward expansion of the zinc counteracts the combined downward 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 thermal 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 (pure zinc not being available then) is more complex because 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 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. Therefore, another type of temperature-compensated pendulum, the mercury pendulum, was used in the highest-precision clocks.
By 1900, the highest-precision astronomical regulator clocks used pendulum rods of low thermal expansion materials such as invar and fused quartz.
A pendulum clock is a clock that uses a pendulum, a swinging weight, as its timekeeping element. The advantage of a pendulum for timekeeping is that it is a harmonic oscillator: It swings back and forth in a precise time interval dependent on its length, and resists swinging at other rates. From its invention in 1656 by Christiaan Huygens, inspired by Galileo Galilei, until the 1930s, the pendulum clock was the world's most precise timekeeper, accounting for its widespread use. Throughout the 18th and 19th centuries, pendulum clocks in homes, factories, offices, and railroad stations served as primary time standards for scheduling daily life, work shifts, and public transportation. Their greater accuracy allowed for the faster pace of life which was necessary for the Industrial Revolution. The home pendulum clock was replaced by less-expensive, synchronous, electric clocks in the 1930s and '40s. Pendulum clocks are now kept mostly for their decorative and antique value.
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The year 1726 in science and technology involved some significant events.
Invar, also known generically as FeNi36, is a nickel–iron alloy notable for its uniquely low coefficient of thermal expansion. The name Invar comes from the word invariable, referring to its relative lack of expansion or contraction with temperature changes.
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A balance wheel, or balance, is the timekeeping device used in mechanical watches and small clocks, analogous to the pendulum in a pendulum clock. It is a weighted wheel that rotates back and forth, being returned toward its center position by a spiral torsion spring, known as the balance spring or hairspring. It is driven by the escapement, which transforms the rotating motion of the watch gear train into impulses delivered to the balance wheel. Each swing of the wheel allows the gear train to advance a set amount, moving the hands forward. The balance wheel and hairspring together form a harmonic oscillator, which due to resonance oscillates preferentially at a certain rate, its resonant frequency or "beat", and resists oscillating at other rates. The combination of the mass of the balance wheel and the elasticity of the spring keep the time between each oscillation or "tick" very constant, accounting for its nearly universal use as the timekeeper in mechanical watches to the present. From its invention in the 14th century until tuning fork and quartz movements became available in the 1960s, virtually every portable timekeeping device used some form of balance wheel.
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Elinvar is a nickel–iron–chromium alloy notable for having a modulus of elasticity which does not change much with temperature changes. The name is a contraction of the French élasticité invariable. It was invented by Charles Édouard Guillaume, a Swiss physicist who also invented Invar, another alloy of nickel and iron with very low thermal expansion. Guillaume won the 1920 Nobel Prize in Physics for these discoveries, which shows how important these alloys were for scientific instruments.
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A balance spring, or hairspring, is a spring attached to the balance wheel in mechanical timepieces. It causes the balance wheel to oscillate with a resonant frequency when the timepiece is running, which controls the speed at which the wheels of the timepiece turn, thus the rate of movement of the hands. A regulator lever is often fitted, which can be used to alter the free length of the spring and thereby adjust the rate of the timepiece.
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In internal combustion engines, the gudgeon pin connects the piston to the connecting rod, and provides a bearing for the connecting rod to pivot upon as the piston moves. In very early engine designs, including those driven by steam, and many very large stationary or marine engines, the gudgeon pin is located in a sliding crosshead that connects to the piston via a rod. A gudgeon is a pivot or journal. The origin of the word gudgeon is the Middle English word gojoun, which originated from the Middle French word goujon. Its first known use was in the 15th century.
A dilatometer is a scientific instrument that measures volume changes caused by a physical or chemical process. A familiar application of a dilatometer is the mercury-in-glass thermometer, in which the change in volume of the liquid column is read from a graduated scale. Because mercury has a fairly constant rate of expansion over ambient temperature ranges, the volume changes are directly related to temperature.
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