Physikalisch-Technische Bundesanstalt

Try to fasten a picture to the wall without using nails, screws or something similar fasteners. It is impossible? No, it is not; theoretically you have to polish the rear side of the picture and the wall so well that the atoms of picture and wall adhere to one another without any fastener being necessary. What practically cannot finally be realized for picture and wall can be achieved for the gauge block parts which adhere to one another without any mounting as if they were magnets. With gauge blocks of steel, ceramics or tungsten carbide, a handy tool is available for measuring a quantity which appears to be very easy to determine but has a most complicated definition: One metre is the length of the distance light covers in the vacuum in 1/299 792 458 seconds. Since 1983, the metre has been traceable to the fundamental constant "velocity of light".