COM'PARA'TOR (Lat.. comparer. from corn parare, to compare, from coin-, together pat-, equal). An instrument, used in comparing the lengths of two graduated scales and in deter mining accurately the amount of their differenee. It consists essentially of two microscopes, each fitted with cross-hairs and capable of being moved by a micrometer screw, or else supplied with a micrometer eyepiece, or often both. (See MICROMETER.) These micrometer microscopes are so mollified that they may he moved both laterally and forward and backward. and through them the observer looks down on the scale which is being examined. The microscopes are then moved so that their cross-hairs are either at the end or over some division of the scale, which often is so magnified that in making a setting it is convenient to bisect the enlarged image of the mark. By means of rollers or other mechanism the platform containing the first scale is temporarily removed and a second intro dueed in its place. In this case the intersections of the cross-hairs are probably some small dis tanec away from the corresponding division of the other scale, and this amount, as determined with the micrometers, is their difference at that temperature. While the operation of comparing two scales does not seem particularly •mnplicat ed, nevertheless, in order to secure the desired high degree of precision, elaborate precautions are taken both in the construction and manipula tion of the apparatus. The temperature at
which the scales are compared is of course no un important consideration. and in order that this should be uniform and easily determined, they are immersed in,a. bath of liquid whose temper ature can be maintained at some constant point. In the comparison of standards there must of course be an ultimate standard to which all measures of length are referred, and this bar is considered correct at some one stated tempe• ature. Copies of this are made and serve as secondary standards whose coefficient of expan sion, errors, and other constants are known. Such comparisons are performed by the International Bureau of Weights and Measures at Paris, where comparators susceptible of the most accurate results and possessing all possible refinements are employed. (See Wiamyrs AND MEASURES.) Similar lint usually less precise paratus is found also in the various national standard bureaus and physical laboratories. The da International des Folds et Jlesures (Paris) contain full and technical descriptions of the most. accurate and approved apparatus and methods.