Gridiron pendulum
From Wikipedia, the free encyclopedia
The gridiron pendulum was an improvement of clock pendulums invented by John Harrison, consisting of alternating brass and iron rods assembled so that their different expansions (or contractions) cancel each other out. Its simplest form, attributed to John Arnold, consists of five rods. A central one runs up from the bob to just below the suspension. At that point a cross-piece (bridge) extends from the central rod and carries two rods, one on each side of the central rod, which reach down almost to the bob. These rods carry a broader bridge which clears the central rod and is wide enough to carry two further rods. These run back up to a final bridge at the top attached to the suspension. These two rods and the central one are made of steel and the other two are made of zinc. As the steel expands in heat, the bob drops relative to the first bridge and the second bridge drops relative to the final one. The expansion of the zinc, however, pushes the first bridge and the bob upwards by a greater distance. Simply - the upwards expansion of the zinc counteracts the combined downwards expansion of the steel (which has a greater total length).
The original construction using brass (zinc not being available) is more complex since brass doesn't expand as much as zinc does. A further set of rods and bridges is needed giving nine rods in all, five steel 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 steel. 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 or steel. 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.
A picture of such a pendulum can be seen at a German Wikipedia page
