Brief History a THE E SYSTE UNITED STATES DEPARTMENT OF COMMERCE C. R. SMITH, Secretary NATIONAL BUREAU OF STANDARDS/ A. v. astin, Director *i _ Special Publication JO'lA Issued It AND METRIC F MEASUREMENT with a CHART OF THE MODERNIZED METRIC SYSTEM "Weights and measures may be ranked among the necessaries of life to every individual of human society. They enter into the eco- nomical arrangements and daily concerns o) every family. They are necessary to every occupation of human industry; to the distribution and security of every species of property; to every transaction of trade and commerce; to the labors of the husbandman; to the in- genuity of the artificer; to the studies of the philosopher; to the researches of the antiquarian, to the navigation of the mariner, and the marches of the soldier; to all the exchanges of peace, and all the operations of tear." — JOHN QUINCY ADAMS When the American Colonies sepa- rated from the mother country to assume among the nations of the earth a separate and individual sta- tion, they retained, among other things, the weights and measures that had been used when they were colonies, namely, the weights and measures of England. It is probable that these were at that time the most firmly established and widely used weights and measures in the world. England a highly coherent nation, separated by sea from many of the turmoils of the European continent, had long before established stand- ards for weights and measures that have remained essentially unchanged up to the present time. The yard, established by Henry II, differs only by about 1 part in a thousand from the yard of today. The pound of Queen Elizabeth I shows similar agreement with the present avoir- dupois pound. No such uniformity of weights and measures existed on the European continent. Weights and measures dif- fered not only from country to coun- try, hut even from town to town and from one trade to another. This lack of uniformity led the National As- For sole by the Superintendent of Do. sembly of France on May 8, 1790, to enact a decree, sanctioned by Louis XVI. which called upon the French Academy of Sciences in concert with the Royal Society of London to "de- duce an invariable standard for all of the measures and all weights." Hav- ing already an adequate system of weights and measures, the English were not interested in the French undertaking, so the French proceeded with their endeavor alone. The result is what is known as the metric system. The metric system was conceived as a measurement system to the base ten: that is. the units of Ihe system. their multiples, and submultiples should be related to each other by simple factors of ten. This is a great convenience because it conforms to our common system for numerical no- tation, which is also a base ten system. Thus to convert between units, their multiples, and submultiples, it is not necessary to perform a difficult mul- tiplication or division process, but simply to shift the decimal point. The system seems to have been first proposed by Gabriel Mouton. a vicar of Lyons, France, in the late 17th cen- tury. He proposed to define the unit of length for the system as a fraction of the length of a great circle of the earth. This idea found favor with the French philosophers at the time of the French Revolution, men who were generally opposed to any vestige of monarchical authority and preferred a standard based on a constant of nature. The French Academy assigned the name metre (meter), from the Greek mctron, a measure, to the unit of length which was supposed to be one ten millionth of the distance from the north pole to the equator, along the meridian running near Dunkirk, Paris, and Barcelona. An attempt was made to ineasure this meridian from northern France to southern France. from which the true distance from the pole to the equator could be calculated. The best techniques then available were used. Although the op- erations were carried out during a politically disturbed time, the results were in error only by about 2<>(l(l meter-, .i remarkable achievement in those days, Meanwhile the National Assembly had preempted the geodetic survey, upon which the meter was to be based, and established a provisional meter. Tin- unit of mass called the gram was ■nts, U.5. Govi I Printing Off,«*, Woshmglon, D C 20402— Pri : 20 < The Modernized Metric Sy« l |l |l| l|l| . | lj l | l|l ^ | l|l [-|l |l|l ,^^ j i INCH ] 1 FOOT Y A --.'■•--.■■•-■'-•-■^■.•■■■J..-H-l^ THE International System of Units— viated SI— is a modernized versioi system. It was established by inte ment to provide a logical and interconne for all measurements in science, industry, SI is built upon a foundation of base unit: nitions, which appear on this chart. All ol The Six Base Units of Measurer The meter is defined as 1 650 763.73 wavelengths : vacuum ol Ihe orange-red line of the spectrum i krypton-86. The SI ). Land is otten measured by Ihe hectare (10 000 square meters, or approximately 2.5 acres). The SI unit ol volume is Ihe cubic meter (m 1 )- Fluid volume is often measured by the liter (0.001 cubic lthig Office. Washington, Handbook 102. flSTM Ir SECOND -S The second is defined as Ihe duration of 9 192 631 770 cycles of the radiation associated with a specified transilion of the cesium atom. It is realized by tun- ing an oscillator to the resonance frequency of the cesium atoms as they pass through a system of mag- nets and a resonant cavity into a detector. -REGION - (CAVITY) OSCILLATING FIELD ♦ ' / I MAGNET DEFLECTING MAGNET FROM OSCILLATOR The number of periods or cycles per second is called frequency. The 51 unil for frequency is the hertz (Hz). One hertz equals one cycle per second. Standard frequencies and correct time are broad* cast from NBS stations WWV, WWVB, WWVH, and WWVL. and stations of the U.S. Navy- Many shortwave receivers pick up WWV on fre- quencies of 2.5, 5, 10, 15, 20, and 25 megahertz. The standard radio broadcast band extends from 535 to 1605 kilohertz. Oividing distance by time gives speed. The SI unit for speed is Ihe meter per second (m/s), approxi- mately 3 feet per second. Rate of change in speed is called acceleration. The SI unit for acceleration is the meter per second per second (m/s 1 ). KILOGRAM-hg The standard for the unit of mass, the kilogram, is a cylinder of platinum-lridium alloy kept by the Inter- national Bureau of Weights and Measures at Paris. A duplicate in the custody of the National Bureau of Standards serves as the mass standard for the United States. This is the only base unit still defined by an artifact. U.S. PROTOTYPE KILOGRAM NO. 20 Closely allied to Ihe concept of mass is that of force. The SI unit of force is the newton (N). A force of 1 newton, when applied for 1 second, will give to a 1 kilogram mass a speed of 1 meter per second (an acceleration of 1 meter per second per second). IN .- ACCELERATION OF 1 . 1kg- One newton equals approximately two tenlhs of a pound of force. The weight ol an objecl is the force exerted on it by gravity. Gravity gives a mass a downward accelera- tion of about 9. Bm/s. The SI unil for work and energy ol any kind is the joule (J). U = lN«lm The SI unit for power of any kind Is the watt (W). s one-third the actual size of the full-scale wall chart, NBS Special Publication 304, which is i ! The Modernized Metric System The International System of Units (SI, and its relationship to U.S. customary units U.S. DEPARTMENT OF COMMERCE National Bureau of Standards ,.ULi.L,J..uLd.Ll,.L.U- 1-i-l.l !■.' ■'■ J.J.,L.U.,. 1 ^d,J.,J..,l. L .L,L,l,.L..L,J..U^l, METER '- ULJ L,].,J..aj..,J..,i.d,J.,J.,l..,],.L, 5 f,l.d,J.. l J.,,L, >., !,i.J. j. lib.! Li.l.i.l-1, ,!.,i,.i,,i"..1,.!.U,I. I...i axI.,.L,L,.l,.l,.i;.l,.|,|,l,.q,I..U I. i 1 INCH .,.I...i...I-i.,.!...i.=.! ■...■■..t.-.i...!.,.;. one-third the actual size of the full-scale wall chart, NBS Special Publication 304, which is av; from the Superintendent ot Documents, U.S. Goverr t Printing Office. Washington, DC 20402, for 50 C PENN STATE UNIVERSITY LIBRARIES AQQDD711SDSDM decided on as the mass of one cubic centimeter of water at its temperature of maximum density. Since this was too small a quantity to be measured with the desired precision the deter- mination was made on one cubic decimeter of water, but even at that the results were found to be in error by about 28 parts in a million. Thus, the meter that was established as the foundation of the system did not ap- proximate the idealized definition on which it was based with the desired accuracy. Also the unit of mass dif- fered from the idealized definition even as given in terms of the errone- ously defined meter. So the new sys- tem was actually based on two metal- lic standards not differing greatly in nature from the yard of Henry II or the pound of Elizabeth I. As a unit for fluid capacity, the founders selected the cubic decimeter and as a unit for land area they se- lected the are, equal to a square ten meters on the side. In this manner, while decimal relationships were pre- served between the units of length, fluid capacity, and area, the relation- ships were not kept to the simplest possible form. Although there was some discussion at the time of deci- malizing the calendar and the time of day. the system did not include any unit for time. The British system of weights and measures, and the metric system as well, had been developed primarily for use in trade and commerce rather than for purposes of science and en- gineering. Because technological achievement depends to a considerable extent upon the ability to make physical measure- ments, the Americans and the Brit- ish proceeded to adapt their system of measurements to the requirements of the new technology of the 10th cen- tury, despite the fact that the newly developed metric system seemed to have certain points of superiority. Both the United States and Great Britain soon had vast investments in a highly industrialized society based on their own system. The new metric system found much favor with scientists of the 10th cen- tury, partly because it was intended to be an international system of measurement, partly because the units of measurement were theoretically supposed to be independently re- producible, and partly because of the simplicity of its decimal nature. These scientists proceeded to derive new units for the various physical quantities with which they had to deal, basing the new units on ele- mentary laws of physics and relating them to the units of mass and length of the metric system. The system found increasing acceptance in vari- ous European countries winch had been plagued by a plethora of unre- lated units for different quantities. Because of increasing technological development there was a need for international standardization and im- provements in the accuracy of stand- ards for units of length and mass. This led to an international meeting in France in 1872. attended by 26 countries including the United States. The meeting resulted in an interna- tional treaty, the Metric Convention, which was signed by 17 countries, including the United States in 1875. This treaty set up \v ell defined metric standards for length and mass, and established the International Bureau of Weights and Measures. Also es- tablished was the General Conference of Weights and Measures, which would meet every six years to con- sider any needed improvements in the standards and to serve as the authority governing the International Bureau. An International Committee of Weights and Measures was also set up to implement the recommenda- tions of the General Conference and to direct the activities of the Interna- tional Bureau; this Committee meets every two years. Since its inception nearly 175 years ago. the number of countries using the metric system has been growing rapidly. The original metric system of course had imperfections; and H has since undergone many revisions. the more recent ones being accom- plished through the General Confer- ence of Weights and Measures. An extensive revision and simplification in 1060 by the then 40 members of the General Conference resulted in a modernized metric system — the Inter- national System of Units — which is described in detail in the accompany- ing chart. NOTE: For further information see the references listed on the chart. In addition, a more complete treat- ment of the English and metric sys- tems of measurement will soon be available. ELECTRICITY PHOTOMETRY TEMPERATURE LENGTH TIME IONIZING RADIATIONS UNITS GCWM. GENERAL CONFERENCE OF WEIGHTS AND MEASURES ICWM: INTERNATIONAL COMMITTEE OF WEIGHTS AND MEASURES 81PM: INTERNATIONAL BUREAU OF WEIGHTS AND MEASURES INTERNATIONAL COORDINATION OF MEASUREMENT STANDARDS . S. GOVhHNMENT PRINTING OF VIL V. •