british bronze yard of 1855. yard defined distance between 2 lines scribed on bottom surface of holes @ each ends of bar, on neutral plane.
the property of remaining constant length under load has been made use of in length metrology. when metal bars developed physical standards length measures, calibrated marks made on length measured along neutral plane. avoided minuscule changes in length, owing bar sagging under own weight.
the first length standards use technique rectangular section solid bars. blind hole bored @ each end, depth of neutral plane, , calibration marks made @ depth. inconvenient, impossible measure directly between 2 marks, offset trammel down wells.
platinum-iridium standard metre bars of tresca section. neutral plane of bar designed coincide 1 surface of central web connecting 2 sides (in photo bottom surface) , meter defined 2 fine lines scribed near ends of bar on surface.
a more convenient approach used international prototype metre of 1870, bar of platinum-iridium alloy served definition of meter 1889 1960. made splayed h-shaped cross section, called tresca section. 1 surface of centre crossbar of h designed coincide neutral plane, , calibration marks defining meter scribed surface.
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