14 



purposes, its value having been fixed by certain measuring processes. Second- 

 ary or reference standards are accurately compared copies, not necessarily 

 duplicates, of the primaries for use in the work of standardizing laboratories 

 and the production of working standards for everyday use. 



Standard of length. — The primary standard of length which now almost 

 universally serves as the basis for physical measurements is the meter. It is 

 defined as the distance between two lines at 0° C on a platinum-iridium bar 

 deposited at the International Bureau of Weights and Measures. This bar is 

 known as the International Prototype Meter, and its length was derived from 

 the "metre des Archives," which was made by Borda. Borda, Delambre, 

 Laplace, and others, acting as a committee of the French Academy, recom- 

 mended that the standard unit of length should be the ten-millionth part of the 

 length, from the equator to the pole, of the meridian passing through Paris. In 

 1795 the French Republic passed a decree making this the legal standard of 

 length, and an arc of the meridian extending from Dunkirk to Barcelona was 

 measured by Delambre and Mechain for the purpose of realizing the standard. 

 From the results of that measurement the meter bar was made by Borda. The 

 meter is now defined as above and not in terms of the meridian length ; hence, 

 subsequent measures of the length of the meridian have not affected the length 

 of the meter. 



Standard of mass. — The primary standard of mass now almost universally 

 used as the basis for physical measurements is the kilogram. It is defined as 

 the mass of a certain piece of platinum-iridium deposited at the International 

 Bureau of Weights and Measures. This standard is known as the International 

 Prototype Kilogram. Its mass is equal to that of the older standard, the "kilo- 

 gram des Archives," made by Borda and intended to have the same mass as a 

 cubic decimeter of distilled water at the temperature of 4° C. 



Copies of the International Prototype Meter and Kilogram are possessed by 

 the various governments and are called National Prototypes. 



Standard of time. — The unit of time universally used is the mean solar 

 second, or the 86400th part of the mean solar day. It is based on the average 

 time of one rotation of the earth on its axis relatively to the sun as a point of 

 reference = 1.002 737 91 sidereal second. 



Standard of temperature. — The standard scale of temperature, adopted by 

 the International Committee of Weights and Measures (1887), depends on 

 the constant-volume hydrogen thermometer. The hydrogen is taken at an 

 initial pressure at 0° C of 1 meter of mercury, 0° C, sea-level at latitude 45°. 

 The scale is defined by designating the temperature of melting ice as 0° and of 

 condensing steam as 100° under standard atmospheric pressure. 



Thermodynamic (Kelvin) Scale (Centigrade degrees). — Such a scale 

 independent of the properties of any particular substance, and called the 

 thermodynamic, or absolute scale, was proposed in 1848 by Lord Kelvin. The 

 temperature is proportional to the average kinetic energy per molecule of a 

 perfect gas. 



International temperature scale. — See Table 37. 



Numerically different systems of units. — The fundamental physical 

 quantities which form the basis of a system for measurements have been chosen 

 and the fundamental standards selected and made. Custom has not however 



SMITHSONIAN PHYSICAL TABLES 



