Graduation

Graduation means the process of marking scales on measuring instruments. If the instrument is to measure some physical quantity directly, the scale reading may be marked in terms of that physical quantity, or it may be marked in inches of length, degrees of angular measurement, or in some other convenient numerical units, whose physical interpretation is obtained by calibrating the instrument. [Calibration.] The simplest example of graduation is that of scales of linear measurement. This is done by a dividing engine, which consists essentially of a well-turned screw that can be made to move a sharp cutter through exactly equal distances, and thus mark the scale uniformly. Rough graduation may be effected by hand with the aid of compasses. For the accurate marking of a circular scale it is necessary, in the first place, to subdivide a circular arc into any number of equal parts. Though such an operation can be effected in the case of a straight line, no geometrical process is known by which a circular arc can be so divided; but any arc may be bisected, and the process of bisection of each half repeated indefinitely. Other subdivision may- be done approximately by systematic trial with compasses. Circular dividing-engines consist essentially of a circular horizontal plate, carefully graduated and capable of rotation by means of a tangent screw. The cutter moves radially in one direction, and the scale to be marked is rotated with the graduated plate. The markings on metal or glass scales are made by coating the scale with wax, cutting lines on the wax with the compass or dividing-engine, ance thus exposing the metal or glass underneath. This is then subjected to the action of acid, usually nitric for metals and hydrofluoric for glass. When the wax is cleared away, the scale will be found permanently engraved.