Metrology

METROLOGY, a name for the science of pure measure ment, is in practice restricted to mean measurement of the three fundamental quantities, mass, length and time, from which all other quantities, such as volume, density, velocity, acceleration, force and power, are derived. If "science is measurement" then without metrology there is no science. Even in its most direct application it covers a very wide field (see MENSURATION ; SUR VEYING; GEODESY; FIGURE OF THE; MICROMETER; IN STRUMENTS, ELECTRICAL; TIME; CLOCKS; WEIGHTS AND MEAS URES ; WEIGHING MACHINES ; METRIC SYSTEM). The question of the time standard, moreover, is particularly an astronomical one. There is no "absolute" standard; all measurement is relative.

Problems Involved.

The problem of metrology is twofold. First to provide and maintain unaltered the standards of reference by which other quantities are compared and measured; and sec ondly to provide means by which the comparisons may be made with accuracy sufficient for the particular purpose in view. The demands of science and industry at the present day require, for certain purposes, an extremely high degree of accuracy in these fundamental comparisons, so that what, at first sight, would ap pear to be a fairly simple problem becomes in fact one of great difficulty, involving the most expensive and elaborate apparatus.

No measurement is ever absolutely correct. Some degree of ex perimental error is always necessarily present, and the approxi mate degrees of accuracy at present attainable in certain of the more fundamental operations in metrology, are as follows : Comparison of two platinum-iridium copies of the International Proto type Kilogramme: i part in 3[00,000,00o.

Comparison of ordinary chemical weights: i part in i,000,000. Comparison of smaller masses by micro-balance: i part in ioo,000,000. Comparison of two yard or metre (line or end) standards: i part in 10,000,000.

Comparison of end standard with line standard: i part in i,000,000. Determination of volume and density for very special work: i part in i,000,000.

Determination of volume and density, ordinary: i part in io,000. Calibration of set of end standards (not less than i in. in length): I part in 1,000,000.

Calibration of subdivisions of graduated yard or metal scale in terms of whole length: 0.000005 in. or o•000i millimetre.