Editing Pendulum (section)

==== Gridiron pendulum ====
[[Image:BanjoPendulum.svg|thumb|Diagram of a gridiron pendulum{{ordered list
 | list_style=margin-left:0;
 | item_style=list-style-position:inside;
 | list-style-type=upper-alpha
 | exterior schematic
 | normal temperature
 | higher temperature
}}]]
{{Main|Gridiron pendulum}}
The most widely used compensated pendulum was the [[gridiron pendulum]], invented in 1726 by [[John Harrison]].<ref name="Milham1945" /><ref name="BritannicaCompensation" /><ref name="MatthysCompensation" />  This consists of alternating rods of two different metals, one with lower thermal expansion ([[Coefficient of thermal expansion|CTE]]), [[steel]], and one with higher thermal expansion, [[zinc]] or [[brass]]. The rods are connected by a frame, as shown in the drawing at the right, so that an increase in length of the zinc rods pushes the bob up, shortening the pendulum.  With a temperature increase, the low expansion steel rods make the pendulum longer, while the high expansion zinc rods make it shorter. By making the rods of the correct lengths, the greater expansion of the zinc cancels out the expansion of the steel rods which have a greater combined length, and the pendulum stays the same length with temperature.

Zinc-steel gridiron pendulums are made with 5 rods, but the thermal expansion of brass is closer to steel, so brass-steel gridirons usually require 9 rods. Gridiron pendulums adjust to temperature changes faster than mercury pendulums, but scientists found that friction of the rods sliding in their holes in the frame caused gridiron pendulums to adjust in a series of tiny jumps.<ref name="MatthysCompensation" /> In high precision clocks this caused the clock's rate to change suddenly with each jump. Later it was found that zinc is subject to [[Creep (deformation)|creep]]. For these reasons mercury pendulums were used in the highest precision clocks, but gridirons were used in quality regulator clocks.

Gridiron pendulums became so associated with good quality that, to this day, many ordinary clock pendulums have decorative 'fake' gridirons that don't actually have any temperature compensation function.