This report- will deal only with length measurements in the field of large accelerator surveying, and in particular, with the device for length measurement;perfected at CERN, which offers a new and original solution to the problem.nThe CERN Survey Group had to construct a comparator to calibrate the invar wire-c used for length measurements, in order to make a distinction between the apparent precision (obtained by comparing the measurements with one, another), and the real precision (which above all depends on the real length of the mesuring bar used), and to avoid the slight, uncertainty that exists as to the real length of the measuring bar, at the, moment of measuring.
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