File:Bronze Yard No. 11.jpg

English: Bronze Yard No. 11 length standard, the official standard of length for the United states from 1855, when it replaced the Troughton Scale, until 1893, when the US adopted Metric standards. It was an exact copy of the British Imperial Standard Yard, one of a number of Imperial prototype yard length standards constructed by Great Britain in 1855 to replace the British Imperial Troy Yard prototype, which was destroyed in the 1834 Houses of Parliament fire. The bronze bar with square cross section was a "line standard"; when the bar is at 62° F the yard is defined by the distance between two fine lines ruled on gold "plugs" which were set into recesses in the ends of the bar to protect them. Protective caps (shown) fit in the holes. The reason for recessing the defining lines in holes in the bar is to locate them on the bar's "neutral plane", its axis of symmetry, where length error due to sag of the bar under its own weight is minimum. The bar is now in the NIST Museum, Bethesda, Maryland. Alterations to image: Composite of two separate images; a closeup image of the bar end was rotated, cropped, and pasted next to an image of the entire bar.