Who Is N.I.S.T.?
Every Country has a government body that controls the measurement standards, and has in its possession the ultimate standards of physical parameters, such as dimensional parameters, electrical parameters, optical parameters etc.

In the UK N.P.L. (National Physics Laboratory) for instance  carries out the task.

In the USA, N.I.S.T. (National Institute of Standards and Technology) carries out this task ensuring that via correct calibration all similar instruments measure the same parameter producing the same reading within their capabilities. The process is very similar to ensuring that every choirboy sings from the same hymn sheet!