Vernier

(engineering) A short, auxiliary scale which slides along the main instrument scale to permit accurate fractional reading of the least main division of the main scale.

A short, auxiliary scale placed along the main instrument scale to permit accurate fractional reading of the least main division of the main scale. The auxiliary, or vernier, scale is graduated in one or both directions from the fiducial (index) mark in numbered divisions which are fractionally shorter (in a direct vernier) or longer (in a retrograde vernier) than those on the main scale (see illustration). The position of the fiducial mark (the zero mark of the vernier scale) between divisions on the main scale is indicated by the number of the graduation on the vernier scale which lines up exactly with a graduation on the main scale.

Two types of vernier scales. (a) Direct (reading 3.6). (b) Retrograde (reading 12.7)