Measures of Length

measures that are used to reproduce lengths of some given value. Measures of length can be classified as line standards, end blocks, and graduated end blocks. The value of a line standard is determined by the distance between lines marked on the standard; the value of an end block is given by the distance between the terminal faces of the end block. Graduated end blocks are end blocks that are also marked with lines corresponding to fractional units of the length.

Line standards can be single-valued or multivalued. They are usually made in the form of a rod (bar) or a tape and have rated values from 0.1 mm (measuring scales) to tens of meters (surveyor’s tapes, wires, or reels). Other examples of line standards are scales of optical-mechanical instruments (including measuring microscopes and micrometers) and devices for tool setting in machine tools.

Line standards are divided into six classes of precision: 0, 1, 2, 3, 4, and 5. Relative errors for these classes range from 0.5 × 10⁻⁶ (for class 0) to 5 × 10⁻⁵ (for class 5).

All end blocks are single-valued. They are divided into four classes of precision: 0, 1,2, and 3. Relative errors for these classes range from 2 × 10⁻⁶ (class 0) to 2 × 10⁻⁵ (class 3). Gauges are sometimes considered to be end blocks, although it is more correct to classify them as inspection tools rather than measuring tools.

Graduated end blocks are used mostly in trade, for selling cloth and similar merchandise (trade measures of length).

According to their metrological purpose, measures of length are classified as base standards and working standards.