Figure 1. Historical standard platinum iridium meter bar

In 1889, there was one prototype meter, a bar made of a platinum iridium alloy with lines inscribed at each end; the distance between them defined the meter (see Figure 1). The length standard was disseminated to the national laboratories through the use of artifact meters, which were accurate (but not identical) replicas of the prototype meter. Each artifact meter was calibrated against the prototype for use as a national standard. A serious problem with a prototype standard results from the fact that there is no method to detect a change in its value due to aging or misuse. As a consequence, it is not possible to state the accuracy or stability of the prototype meter, although calibration uncertainties of the artifact meters can be assigned.
 

From: "Length—Evolution from Measurement Standard to a Fundamental Constant," Howard P. Layer at:
http://www.mel.nist.gov/div821/museum/length.htm

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