xlii INTRODUCTION 



ence standards are accurately compared copies, not necessarily duplicates, of 

 the primaries for use in the work of standardizing laboratories and the produc- 

 tion 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 o° 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, recommended 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 sec- 

 ond, 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 as 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 o L C of one meter of mercury, o° C, sea-level at latitude 45 . 

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

 condensing steam as ioo° under standard atmospheric pressure. This is known 

 as the Centigrade scale (abbreviated C). 



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

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

 In it the temperature is proportional to the average kinetic energy per molecule 

 of a perfect gas. The temperature of melting ice is taken as 273. 18 , that of 

 the boiling point, 373.1 8°. The scale of the hydrogen thermometer varies from 

 it only in the sense that the behavior of hydrogen departs from that of a perfect 

 gas. It is customary to refer to this scale as the Kelvin scale (abbreviated K.) 



