Metrology.— Many sciences and industries have given the science of metrology their best thought and labors. Chemistry opened up new chapters in the science of the refractory metals, developing iridium and platinum of purity requisite for the world's fundamental standards of mass and length, producing in conjunction with metallurgy the remarkable nickel-steel alloy %nine° (qv.), having practically a zero temperature coefficient of expansion; the alloy numganin free from temperature coefficient for electrical resistance; Jena glass, a material for measuring flasks and thermometer bulbs which practically eliminates temperature disturbances. To optics and the glass industry we are in debted for the microscopes, telescopes, polari scopes and other optical instruments which form essential parts of modern measuring ap paratus. Optical methods based upon the in terference of light waves have made possible the detection of changes of dimension far beyond the power of the microscope to detect. By such methods plane glass surfaces are produced hav ing errors of less than a millionth of an inch, and in the interferometer is provided the must delicate and senstive length-measuring instru ment known to man. Metallurgy and engineer ing have developed methods of fusing, casting and tempering materials to be used in measur ing apparatus, and have developed the accurate machining of such instruments. Astronomy has given the precision measurements of time, the •second' being the unit of time in the interna tional C. G. S. system of units. Mathematics has devoted a large section of its science to the theory of observations and added rigur in their adjustment, and in the elimination of accidental errors, and by developing the theoretical side of precision measurements has rendered the great est service to metrolosia..
Unit Standards.— The standards of length and mass are fundamental. From these and the unit of time practically all other standards are derived, either directly or indirectly. The derived standards include those used in the measurement of volume, density, capacity, ve locity, pressure, energy, electricity, tempera ture. illumination and the like. The production of copies, multiples and subdivisions of the fun damental standards, the construction of the de rived standards, and the comparison of the standards used in scientific work manu facturing and commerce with the fundamental or derived standards of the government are functions of the Bureau of Standards in the United States and of similar bureaus in other countries. In the United States the standard of the weights and measures of trade is main tained through the State, county and city scalers of weights and measures with an efficiency de pendent upon local administration. For more precise purposes, however, uniformity and pre cision are attained and maintained only by reg ular reference of the local standards to the fun damental standards of the government, at the Bureau of Standards in Washington.
The units of weight and measure in the United States are practically those used in the colonies prior to the formation of our govern ment. While Congress has never definitely authorized the weights and measures in com mon use, it has sanctioned their use by its act of 14 June 1836, providing that accurate copies of the yard, pound, etc. be furnished as stand ards to each State of the Union. The consti tutional power "to fix the standard of weights and measures," vested in Congress (I. 3, 5, United States Constitution) has rarely been ex ercised, so that legislation on weights and measures has been confined to the individual States, and in this manner numerous differ ences in usage have grown up in the several States, although the same general system of weights and measures prevails throughout the country. The customary units of length are de fined by reference to the yard. The yard itself was formerly defined as the length of a certain brass bar, the standard yard, kept at the Office of Standard Weights and Measures in Wash ington. Since 1893 the yard is defined in terms of the international metre. Theard appears to have been a unit of length in England since very early times. The name signified in Old English and in Anglo-Saxon (gyrd) a rod or stick. A standard bronze yard of 1496 still ex ists in England, having been used for the veri fication of other yards until 1588, when a new standard was made which is also in existence to-day. This yard, known as the Winchester standard, was the legal standard in England until 1824, when new standards, the imperial standards, were authorized.
When the Coast Survey was organized it was found necessary to have a definite standard of length, and an 82-inch bar was secured from the instrument maker, Troughton, of London. This bar was nearly a copy of Troughton's scale, and had not been compared with the Brit ish standard yard, but the distance between the 27th and 63d inch marks on the bar was taken a3 representing the standard yard, and was made the basis of the standards sent out in ac cordance with the act of Congress of 1836. In 1834 the British imperial yard was destroyed by fire and when a new standard had been pre pared, copies of it were made, two of which %%ere presented to the United States in 1856. These copies were compared with the Trot t( ,n scale, and the latter was found to be 0. inch longer than the new imperial yard at the standard temperature of 62' F. One of the copies was then accepted as standard by the United States Office of Standard Weights and Measures in place of the Troughton scale. This copy of the imperial yard was the stand ard on which all measurements of length in the United States were thereafter based until 1so3, when the yard was defined in terms of the international prototype metre. The matter had been left entire) to the Treasury Department, which had adopted different yard: from time to time, assumed to be equal to the British im perial yard.