MEASURES AND WEIGHTS. This subject may be most conveniently discussed under two headings—I. Scientific and II. Commercial. The scientific basis of the systems of units employed is also dealt with in the article PHYSICAL UNITS and the unit of TIME is discussed in the article bearing that title.
I. In Great Britain, the Irish Free State, Northern Ireland and the United States of America two systems of weights and measures are now recognized—the imperial and the metric. The funda mental units of these systems are—of length, the yard and the metre ; and of mass, the pound and the kilogram. The term "weight" denotes a magnitude of the same nature as a f orce; the weight of a body is the product of the mass of the body by the acceleration of gravity, a quantity which varies with the locality in which it is measured. (See MECHANICS.) The normal weight of a body is the product of the mass of the body by the normal acceleration of gravity, the value adopted for this latter quantity by the Comite International des Poids et Mesures (C.I.P.M.) at Paris being 980.665 2. The Metric System.—The standards of the International Metric System are fixed by an International Conference on Weights and Measures established by a treaty—the Convention du Metre—signed in Paris on May 20th, 1875. This treaty created an International Bureau, which was built at Sevres on a piece of land declared by the French Government to be inter national property, and governed by an International Committee (C.I.P.M.). (See La Creation du Bureau International des Poids et Mesures by its Director, Dr. Ch.-Ed. Guillaume.) The inter national prototype standards are kept at Sevres, and copies have been made to serve as national standards for the 31 Governments which subscribe to the treaty.
The Conference meets once every six years and, at its seventh meeting in 1927, the metre (Metre-a-traits) was defined:— The unit of length is the metre, defined by the distance at the temperature of melting ice between the centres of two lines traced on the platinum-iridium bar deposited at the International Bureau of Weights and Measures, and declared prototype of the metre by the first general conference on weights and measures, this bar being subjected to normal atmospheric pressure and supported by two rollers, at least 1 centimetre diameter, situated symmetrically in the same horizontal plane and at a distance of 572 millimetres from each other.
This metre (m.) (fig. I) is the only unit of metric extension by which all other metric measures of extension, whether linear, or superficial, are ascertained.
The legal definition does not now refer to any natural standards or to physical constants, though originally the metre (metre-des archives) was intended to be one ten millionth part of the quad rant of the earth's meridian. It has, however, been shown by A. A. Michelson that a standard of length might be restored, if neces sary, by reference to the measurement of wave-lengths of light (see INTERFEROMETER), and the Conference decided (1927) to adopt as an alternative and provisional definition of the metre 1,553,164.13 of the wave lengths of the red light emitted by a cadmium vapour lamp excited under certain specified conditions. The relative accuracy of the value of the metre in terms of light waves is one part in ten millions.
The international prototype kilogram (kg.) is a cylinder of platinum, alloyed with 10 per cent. of irid ium, of approximately equal height and diameter. Originally the kilogram was in tended to be the mass of a cubic decimeter of water at its temperature of maximum density, C. All other metric weights, and all measures having reference to metric weight are referred to secondary standards in Great Britain and the United States.
The international unit of volume in the metric system is the volume occupied by the mass of 1 kilogram of pure water at its maximum density and under normalatmos pheric pressure; this volume is known as a litre (1). (Normal atmospheric pressure is the pressure exerted by a column of mer cury 76cm. high at the temperature of melting ice and at a place where gravity has its standard value. The density of mercury at o° C. is 13.5951 g. per cu.cm.