UNITED STATES DEPARTMENT OF COMMERCE

C. R. SMITH, Secretary

NATIONAL BUREAU OF STANDARDS / a. v. astin, Director

Special Publication 304A. Issued 1968.

AUG 13 1971 161670 -^4

lUj h

Brief History and Use of

THE ENGLISH AND METRIC

SYSTEMS OF 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 of 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 war." —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 country, but even from town to town and

from one trade to another. This lack

of uniformity led the National Assembly 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." Having 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 the 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 notation, which is also a base ten system.

Thus to convert between units, their multiples, and submultiples. it is not necessary to perform a difficult multiplication 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 century. 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

metron, 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 measure 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 2000

meters, a 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. The unit of mass called the gram was

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 —Price 20 cents
The Modernized Metric Syt

M E 1

j 1 2 | 8 4 i 7

iiiiiiLiiiii

9 1 0 Centimeter-! LLuJ,j,,ii,l!d,.,i,,I ! ii,,Li,..,L,,LL,L.iJ 11,

W ' ill ' m Inchu 1>3 1 lU ' 1*6 ' VS ' VI 1'8 119 '

1 INCH 1 FOOT Y A

1,1,1,1,1,1,1, 1, J, I 1, 1, 1, nl,l,l,l,ln, I, ill, 1, 1,1,1,1,1, 1,1, 1,1, lJ.1.1, 1, 1,1,1,1, 1,1,1, 1, 1,1,1, 1,1,1,1,1^^ 1.1, 1,1, 1, 1,1,1, 1,1,1, 1,1,1,1,1,1,1,1, ill, 1,1,1, hi,!,!, I 1, 1,1.1,1,1,1,1, 1, 1,1, 1,1, 1,1,1,1,1

1

1

:ub c c ECI METER

also called

I liter

] 00c cm

1 LIQUID QUART

(approximately) 0.946 liter

THE International System of Units—

0

viated SI—is a modernized version

system. It was established by inter ment to provide a logical and interconnec

for all measurements in science, industry,

;

SI is built upon a foundation of base units

nitions, which appear on this chart. All oil The Six Base Units of Measured!

METER-m

The meter is defined as 1 650 763.73 wavelengths in vacuum of the orange-red line of the spectrum of krypton-86.

SECOND s

An interferometer is used to measure length by means of light waves.

The SI unit of area is the square meter (m ). Land

is often measured by the hectare (10 000 square

meters, or approximately 2.5 acres). The SI unit of volume is the cubic meter (m ). Fluid volume is often measured by the liter (0.001 cubic

meter). 1968

National Bureau of Standards Special Publication 304 A

(Supersedes Miscellaneous Publication 232)

For sale by the Superintendent of Documents, U.S. Government

Printing Office, Washington, D.C. 20402 - Price 20 cents

References: NBS Handbook 102, ASTM Metric Practice Guide, 40 cents NBS Misc. Publ. 247, Weights and Measures Standards of the United States, A Brief History, 35 cents NBS Misc. Publ. 286. Units of Weight and Measure, Definitions and Tables of Equivalents, $1.50

The second is defined as the duration of 9 192 631 770 cycles of the radiation associated with a specified

transition 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 magnets and a resonant cavity into a detector.

DIRECTION OF

MAGNETIC FIELD

KILOGRAM-kg

The standard for the unit of mass, the kilogram, is a

cylinder of platinum-iridium 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

FROM OSCILLATOR

A schematic of an atomic beam spectrometer. The trajectories are drawn for those atoms whose magnetic moments are "flipped" in the transition region.

The number of periods or cycles per second is called frequency. The SI unit for frequency is the

hertz (Hz). One hertz equals one cycle per second. Standard frequencies and correct time are broadcast from NBS stations WWV, WWVB, WWVH, and WVWL, 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. Dividing distance by time gives speed. The SI unit

for speed is the 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=).

Closely allied to the 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).

—- ACCELERATION OF lm/j'

Is2 One newton equals approximately two tenths of a pound of force. The weight of an object is the force exerted on it by gravity. Gravity gives i mass l downward acceleration of about 9.8m s The SI unit for work and energy of any kind is the joule (J). 1J — 1 N • lm

The SI unit for power of any kird is the watt (W). iw= 4^

Is

This chart is one-third the actual size of the full-scale wall chart, NBS Special Publication 304, which is availabl m
'<t

^mrmr^ The International System of Units (SI)

61 1 and its relationship to U.S. customary units

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

6,0

„i„j ,, m l„i,,,l,i,Ii,!!i,,Jm„i ,,,,i j LL,LJi.a.i.iJ

SL±, lj lj l.l i ,.i SLlu i ,LuLa,,

-

|

l| l |l

| l|

| ll|l|l

| l| l |l

|

l «W^^

1

1 zl« 1 2I3 1 214 m« 2l5 1 zk 1 2l7 1 2>S

1 219 1 3l0 1 3ll 1 3l2 1 3l3 1 3l4 1 3l5 D 2 FEET

'''''''''''' llhlllllllllLlliJllllllllllllllllllllllllllllllll^

ially abbrethe metric

tional agreei framework

S commerce,

d their defiSl units are ent

derived from these base units. Multiples and submultiples

are expressed in a decimal system. Use of metric weights

and measures was legalized in the United States in 1866,

and our customary units of weights and measures are de- fined in terms of the meter and the kilogram. The only

legal units for electricity and illumination in the United

States are SI units.

definitions, abbreviations, and some SI units derived from them

The kilogram shown here approximates one third the size of the platinum-iridium standard of mass. One

pound of the same material would be three times the size shown below.

1 pound = 0.453 592 37 kg

KELV1N-K

;the thermodynamic or Kelvin scale of temperature

jused in SI has its origin or zero point at absolute zero

land has a fixed point at the triple point of water deifined as 273.16 kelvins. The Celsius scale is derived

jrrom the Kelvin scale. The triple point is defined as

0.01 °C on the Celsius scale, which is approximately 32.02 °F on the Fahrenheit scale. The relationship of the Kelvin, Celsius, and Fahrenheit temperature

scales is shown below. THERMOMETER

{(ELECTRICAL RESISTANCE TYPE)

WATER - BOILS

1 WATER VAPORWATER

FREEZES

BODY _310.15 37 TEMPERATURE

...273.15 JJ.0 --32

°?

ABSOLUTE Mo

ZER07\

REENTRANT

WELL KELVIN

REFRIGERATING BATH

.+ 233.15 4J--40

1L -273.15 J o o

CELSIUS FAHRENHEIT

Temp F+40=1.8(Temp C+40) Temp F=1.8(Temp C)+32 Temp C=(Temp F-32)/1.8 Temp K=Temp C+273.15

TRIPLE POINT CELL The triple point cell, an evacuated glass cylinder

filled with pure water, is used to define a known fixed temperature. When the cell is cooled until a mantle

of ice forms around the reentrant well, the temperature at the interface of solid, liquid, and vapor is 0.01 °C. Thermometers to be calibrated are placed

in the reentrant well.

AMPERE—

A

The ampere is defined as the magnitude of the current that, when flowing through each of two long

parallel wires separated by one meter in free space,

results in a force between the two wires (due to their magnetic fields) of 2x10 ' newton for each meter of length. 1A

t I t I lm FORCE = 2 x 10-' N

i 111

CANDELA-cd

[«—lm-»|

- 1A

The SI unit of voltage is the volt (V). 1W

1A

The SI unit of electrical resistance is the ohm (;>). IV1A

IV =

l a -

The candela is defined as the luminous intensity of

1 600 000 of a square meter of a radiating cavity

at the temperature of freezing platinum (2042 K). LIGHT EMITTED HERE

INSULATING MATERIAL

The SI unit of light flux is the lumen (lm). A source

having an intensity of 1 candela in all directions

radiates a light flux of 4 it lumens.

A 100-watt light bulb emits about 1700 lumens

COMMON EQUIVALENTS AND CONVERSIONS

Approximate Common Equivalents Conversions Accurate to Parts Per Million

1 inch -.25 millimeters inches x 25,4" —millimeters

1 foot -0.3 meter feet x 0.3048' -meters

1 yard -0.9 meter yards x 0.9144* -meters

1 mile =1,6 kilometers miles x 1.609 34 ^kilometers

1 square inch =6.5 sq centimeters square inches x 6.4516* —sq centimeters

1 square foot 0.D9 square meter square feet x 0.092 903 0 .-square meters

1 square yard O.B square meter square yards x 0.836 127 .^square meters

1 acre . 0.4 hectare f

acres x 0.404 6B6 —hectares

1 cubic inch 16 cu centimeters cubic inches x 16.3871 = cu centimeters

1 cubic foot 0.03 cubic meter cubic feet x 0.028 31G 8 — cubic meters

1 cubic yard 0.8 cubic meter cubic yards x 0.764 555 —cubic meters

1 quart (tq) ---1 liter f quarts (|q)x 0.946 353 =liters

1 gallon 0.004 cubic meter gallons x 0 003 7B5 41 — cubic meters

t ounce (avdp} - 28 grams ounces (avdp) x 23.349 5 ^grants

1 pound (avdp) 0.45 kilogram pounds (avdp) i 0.453 592 -kilograms

1 horsepower =0.75 kilowatt horsepower i 0.745 700 — kilowatts

1 millimeter - 0.04 inch millimeters x 0.039 370 1 = inches

1 meter -. 3.3 teet meters x 3.280 84 —feet

1 meter . 1.1 yards meters x 1.093 61 =: yards

1 kilometer 0.6 mile kilometers x 0.621 371 -miles

1 sq centimeter 0.16 square inch sq centimeters x 0.155 DM --square inches

1 square meter = 11 square teet square meters i 10.7639 — square feet

1 square meter 12 square yards square meters x 1.195 99 ^square yards

1 hectare + -2.5 acres hectares x 2.471 05

1 cu centimeter 0 06 cubic inch cu centimeters x 0.061 023 7 -cubic inches

1 cubic meter -35 cubic feet cubic meters x 35.3147 =cubie teet

1 cubic meter .1.3 cubic yards cubic meters t 1.307 95 — cubic yards

1 liter t =1 quart (lq) liters x 1.056 69 —quarts (lq)

t cubic meter . 250 gallons cubic meters x 264.172 ^gallons

1 gram 0.035 ounces (avdp] grams » 0.035 274 0 = ounces (avdp)

1 kilogram 2.2 pounds (avdp) kilograms x 2.204 62 ^pounds (avdp)

1 kilowatt =1.3 horsepower kilowatts x 1.341 02 -horsepower

THESE PREFIXES MAY BE APPLIED TO ALL SI UNITS

Multiples and Submultiples

1 OOO 000 OOO OOO 10" tera (ter'a) T

1 OOO OOO OOO 10

'

giga (jl'ga) G

1 OOO OOO 10« mega (meg' a) M « lOOO 10' kilo (kil o) k

• 100 10= hecto (nek 'to) h 10 10 deka (dek'a) da

O.l 10 ' deci (des'i) d 0.01 io-= centi (sen ' ti) c

• O.OOl 10-3 milli (mil'i) m • 0.000 001 10-6 micro (mi'kro) • 0.000 OOO OOl 10-» nano (nan'o) n 0.000 OOO OOO OOl 10' = pico (pe ' ko) P 0.000 OOO OOO OOO OOl 10" femto (fern 'to) f 0.000 OOO OOO OOO OOO OOl 10-" atto (at to) a

n term not used in SI •Most commonly used

om the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402, for 50 cents.
1

J
The Modernized Metric System *»«-«-.%•«-»* m U S DEPARTMENT ( and its relationship to U.S. customary units METER

-

:— as THE International System of Units—officially abbreviated SI—is a modernized version of the metric

system. It was established by international agreement to provide a logical and interconnected framework

for all measurements in science, industry, and commerce.

SI is built upon a foundation of base units and their definitions, which appear on this chart. All other SI units are

derived from these base units. Multiples and submultiples

are expressed in a decimal system. Use of metric weights and measures was legalized in the United States in 1866. and our customary units of weights and measures are defined in terms of the meter and the kilogram. The only

legal units for electricity and illumination in the United

States are SI units. The Six Base Units of Measurement

t 1 t 1 —

:::2=_ l|iS

COMMON EQUIVALENTS AND CONVERSIONS

0.000 OM 001 10-' wTwiTb n

0.000 000 000 000 000 001 10-" Iili'JiVrt)

'

i.

ing Office, Washington, I
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 erroneously defined meter. So the new sys- tem was actually based on two metallic 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 phvsical measurements, the Americans and the British proceeded to adapt their system of measurements to the requirements of

the new technology of the 19th century, despite the fad 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 19th century, 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 which had

been plagued by a plethora of unrelated 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 international treaty, the Metric Convention,

which was signed by 17 countries,

including the United States in 1875. This treaty set up well 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 consider 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 recommendations of the General Conference and

to direct the activities of the International 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 it has since undergone many revisions, the more recent ones being accomplished through the General Confer- ence of Weights and Measures. An

extensive revision and simplification

in 1960 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 accompanying 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.

^CONVENTION OF THE METER|

DIPLOMATIC

TECHNICAL

GCWM: GENERAL CONFERENCE OF WEIGHTS AND MEASURES

ICWM: INTERNATIONAL COMMITTEE OF WEIGHTS AND MEASURES

BIPM: INTERNATIONAL BUREAU OF WEIGHTS AND MEASURES

ELECTRICITY PHOTOMETRY TEMPERATURE

LENGTH

TIME

IONIZING RADIATIONS

UNITS

INTERNATIONAL COORDINATION OF MEASUREMENT STANDARDS

U. S. GOVERNMENT PRINTING OFFICE : 1069 O - 325-509
UNITED STATES DEPARTMENT OF COMMERCE

FREDERICK B. DENT, Secretary NATIONAL BUREAU OF STANDARDS / RICHARD W. ROBERTS, Director

Special Publication 304 A. Reissued September, 1974

Brief History of

MEASUREMENT SYSTEMS

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 of 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 war. The knowledge of them, as in established use, is among the first elements of education, and is often learned by those

who learn nothing else, not even to read and write. This knowledge

is riveted in the memory by the habitual application of it to the employments of men throughout life." JOHN QUINCY ADAMS

Report to the Congress, 1821

Weights and measures were among

the earliest tools invented by man.

Primitive societies needed rudimentary

measures for many tasks: constructing

dwellings of an appropriate size and

shape, fashioning clothing, or bartering

food or raw materials. Man understandably turned first to

parts of his body and his natural sur- roundings for measuring instruments. Early Babylonian and Egyptian records and the Bible indicate that length was

first measured with the forearm, hand,

or finger and that time was measured by

the periods of the sun, moon, and other heavenly bodies. When it was necessary

to compare the capacities of containers such as gourds or clay or metal vessels, they were filled with plant seeds which

were then counted to measure the vol- umes. When means for weighing were

invented, seeds and stones served as standards. For instance, the "carat," still used as a unit for gems, was derived

from the carob seed. As societies evolved, weights and

measures became more complex. The

invention of numbering systems and the

science of mathematics made it possible

to create whole systems of weights and

measures suited to trade and commerce,

land division, taxation, or scientific re- search. For these more sophisticated uses it was necessary not only to weigh

and measure more complex things—it was also necessary to do it accurately

time after time and in different places.

However, with limited international ex- change of goods and communication of

ideas, it is not surprising that different systems for the same purpose developed

and became established in different parts of the world—even in different parts of

a single continent.

The English System

The measurement system commonly

used in the United States today is nearly

the same as that brought by the colonists from England. These measures had

their origins in a variety of cultures

Babylonian, Egyptian, Roman, AngloSaxon, and Norman French. The ancient "digit," "palm," "span," and

"cubit" units evolved into the "inch,"

"foot," and "yard" through a complicated transformation not yet fully understood. Roman contributions include the use

of the number 12 as a base (our foot is divided into 12 inches) and words from

which we derive many of our present

weights and measures names. For example, the 12 divisions of the Roman

"pes," or foot, were called unciae. Our

words "inch" and "ounce" are both de- rived from that Latin word.

The "yard" as a measure of length can be traced back to the early Saxon

kings. They wore a sash or girdle

around the waist—that could be re- moved and used as a convenient measuring device. Thus the word "yard" comes

from the Saxon word "gird" meaning

the circumference of a person's waist. Standardization of the various units and their combinations into a loosely

related system of weights and measures sometimes occurred in fascinating ways.

Tradition holds that King Henry I de- creed that the yard should be the dis- tance from the tip of his nose to the end of his thumb. The length of a fur- long (or furrow-long) was established by early Tudor rulers as 220 yards. This

led Queen Elizabeth I to declare, in the 16th century, that henceforth the tradi- tional Roman mile of 5,000 feet would

be replaced by one of 5,280 feet, making the mile exactly 8 furlongs and providing a convenient relationship between

two previously ill-related measures. Thus, through royal edicts, England

by the 18th century had achieved a greater degree of standardization than

the continental countries. The English

units were well suited to commerce and

trade because they had been developed

and refined to meet commercial needs. Through colonization and dominance of world commerce during the 17th, 18th,

For sale by the Superintendent of Documents. U.S. Government Printing Office, Washington. D.C. 20402 Stock No. 0303-01073
THE MODERNIZED

metric

system The International System of Units-SI

is a modernized version of the metric system established by international agreement. It provides a logical and interconnected framework for all measurements in science, industry, and commerce. Officially abbreviated

SI, the system is built upon a foundation of seven base units, plus two

supplementary units, which appear on this chart along with their definitions. All other SI units are derived from these units. Multiples and submultiples are expressed in a decimal system. Use of metric weights and measures was legalized in the United States in 1866, and since 1893 the

yard and pound have been defined in terms of the meter and the kilogram.

The base units for time, electric current, amount of substance, and lumi- nous intensity are the same in both the customary and metric systems.

COMMON CONVERSIONS

Accurate to Six Significant Figun MULTIPLES AND PREFIXES

These Prefixes May Be Applied To AM SI Unils

Symbol When You Know

in inches

ft feet yd yards

mi miles

yd-' square yards

acres yd3 cubic yards

qt quarts (Iq) oz ounces (avdp)

lb pounds (avdp) °F Fahrenheit temperature

kilometers square meters

chectares cubic meters

"liters grams

kilograms

Celsius temperature

Multiply by

*25.4 *0.3048

' 0.91 44

1.609 34

0.836 127

0.404 686

0.764 555

0.946 353

28.349 5

0 453 592 *5 9(aflersub trading 32)

0 039 370 1 3.280 84

1.093 61 0 621 371

1 195 99

2.471 05

1 307 95

1.056 69 0 035 274 0

2.204 62 *9/5 (then add 32)

To Find Symb

Bmillimeters mm

melers m

meters m

kilometers km

square meters m? checlares ha

cubic meters m3

°liters I grams g kilograms kg

Celsius °C

temperature

inches in feel ft yards yd

miles mi square yards yd? acres

cubic yards yd3 quarts (Iq) qt ounces (avdp) oz pounds (avdp) lb Fahrenheit °F

temperature

Multiples and Submultiples Prefixes Symbols

"lor example, l in 25 4 mm. so 3 inches would be

(3 in) |25 4 mTM ) 762 mm

cnectar* is a common name for 10 000 square meters

Dliter is a common name for fluid volume of 0 001 cubic meter

Note; Most symbols are written with lower case letters, exceptions are

unils named after persons for which the symbols are capitalized. Periods are nol used with any symbols.

I 000 000 000 000 10' ' lera(tera) T 1000 000 000 IV giqn{|iga) G

1 000 000 lO*- mcqn (meg'a) M

1 000 103 kilo (kilo") k

10 10' deka (dek a) da Base Unit 1 10"

0.1 10' dccrfdes'O d

0.01 10 * centi (scn'li) c

VJAJ 0 001 10 3 milh (mil i) m

0 000 001 10 ° micro (rrii'kro) \i 0 000 000 001 10 9 nano (nan o) n 0000 000 000 001 10" pico(pe'ko) p

0 000 000 000 000 001 10. •* lemlo.(lem'to) . f

0 000 000 000 000 000 001 10 1B allo(al'to) a

National Bureau of Standards

Special Publication 304A i Revised October 1972)

For sale by trie Suponntcndcnl of Documents. U.S. Government Printing Office, Washinqton, D C. 20402 SDCal.ilogNo.CI3 10: 304A - Price 75 cents REFERENCES NQS Spcct.il Publication 330. 1972 Edition. International System of Unils (SI), available by purchase from the Superintendent of Documenls. Government Printing

Office. Washington. D.C. 20402. order as 013.10 330/2; 30 cents a copy. ASTM Metric Practice Quid" E380-72. available by pur- chase from the Amuriran Society for Testing and

Materials. 1916 Race Street. Philadelphia. Pa. 19103.

SI .50 a copy, minimum order S3.00.

SI Units and Recommendations for the Use ol Their

Multiples and of Certain Oilier Units, order as ISO Standard 1000. $1.50 a copy, from the American National Standards Institute. 1430 Broadway. N.Y.. N.Y. 10018.

Illlrlr^

LENGTH

MASS

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r agnets and a i

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1 IK S3! 770 Mdlla

ELECTRIC CURRENT

TEMPERATURE

The kelvin isdet

tion 1/273.16 of namic temperati

point of water. T OK is called "ab

OUNTOF SUBSTANt

[rftiiT' ih teM

LUMINOUS INTENSITY

[

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PLANE ANGLE

The radian Is the vertex at the cer subtended by an

the radius.

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A full-scale wall chart, NBS Special Publication 304 (Revised October 1972,) is available from the f u
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards SEVEN BASE UNITS

national spelling, metre) wavelengths in vacuum ol spectrum of krypton-86.

1 650 76173 WAVELENGTHS - / ML WAVELENGTH A A\AAAAAA/

J \# W An Interferon* interferometer le used to measure length by means of light waves

The SI unit of area is the square meter (m ').

The SI unit of volume is the cubic meter (m>). The

liter (0.001 cubic meter), although not an SI unit, is commonly used to measure fluid volume.

i for the unit of mass, the kilogram, is a

ilatinum-iridium alloy kept by the Internaj of Weights and Measures at Paris. A dus custody of the National Bureau of Standis the mass standard for the United States,

nly base unit still defined by an artifact. rYThe SI unit of force is the newton (N). One newton is the force which, when

u s. prototype applied to a 1 kilogram mass, will give the kilogram mass an acceleration of

1 (meter per second) per second. 1N = 1kg-m/s2

The SI unit for pressure is the pascal (Pa).

in 1Pa = 1N/m*

The SI unit for work and energy of any

:celeration kind i s the joule (J). -oMm/s" 1J = 1N-m

The SI unit for power of any kind is the watt (W). 1W = 1J/s

the duration of 9 192631 770

issociated with a specified

-133 atom. It is realized by

ie resonance frequency of

y pass through a system of cavity into a detector.

team spectrometer or "dock." Onry those atoms whose

' In the transition region reach ths detector When

occurred, the clock Indicates one second has passed

TRANSITION REGION (CAVITY) OSCILLATING FIELD CESIUM SOURCE ,

DEFLECTION MAGNET

The number of periods or cycles per second

is called frequency. The SI unit for frequency

is the hertz (Hz). One hertz equals one cycle per second. The SI unit for speed is the meter per second

(m/s). The SI unit for acceleration is the (meter per second) per second (m/s!

).

Standard frequencies and correct time are broadcast from WWV, WWVB. and WWVH,

and stations of the U.S. Navy. Many short- wave receivers pick up WWV and WWVH, on

frequencies of 2 5, 5, 10, 15, and 20 megahertz.

pere is defined as that current which, if maintained in each of two

rallel wires separated by one meter in free space, would produce between the two wires (due to their magnetic fields) of 2 x 10"7 for each meter of length.

FORCE =2x10"'N

The SI unit of voltage is the volt (V). 1V = 1W/A

The SI unit of electric resistance is the ohm (O).

1*fi = 1V/A

•*>![' TEMPERATURE 1

Water MEASUREMENT

" i SYSTEMS

Body Temperature

FAHRENHEIT °F

0 Water FreezesOn the commonly used Celsius temperature scale, water freezes at about 0 °C and boils at about 100 °C. The °C is defined as an interval of 1 K, and the Celsius temperature 0 °C is defined as 273.15 K.

1.8 Fahrenheit degrees are equal to 1.0 °C or 1.0 K; the Fahrenheit scale uses 32 °F as a temperature cor- responding to 0 °C.

THERMOMETER

(ELECTRICAL RESISTANCE

TYPE)

WATER VAPOR The standard temperature at the triple point of water is provided by a special

cell, an evacuated glass cylinder containing pure water. When the cell is cooled

until a mantle of ice forms around the re- entrant well, the temperature at the inter- face of solid, liquid, and vapor is 273.16 K. Thermometers to be calibrated are placed

in the reentrant well.

The mole is the amount of substance of a system that contains as many elementary entities as there are atoms in 0.01

kilogram of carbon 12.

When the mole is- used, the elementary

entities must be specified and may be

atoms, molecules, ions, electrons, other

particles, or specified groups of such

particles.

The SI unit of concentration (of amount of substance) is the mole per cubic meter (mol/m3

).

The candela is defined as the luminous

intensity of 1/600 000 of a square meter

of a blackbody at the temperature of freezing platinum (2045 K).

The SI unit of light flux is the lumen

(Im). A source having an intensity of 1 candela in all directions radiates a light flux of 4 7T lumens.

TWO SUPPLEMENTARY UNITS

angle with Its M raoianV\\

circle that is

f 1 r

al in length to I ^^^-<

W U Bl] iy J L* 1 The steradian is the solid angle with ^rr^-v. JA ~~atL?

,ea Bb/i ri, —1 Td 1 1 r| '—1 -a i its vertex at the center of a sphere /mlrJ^^W ^1 ai l J K L L, that is subtended by an area of the /

spherical surface equal to that of a wL [ . \ SOLID ANGLE ess?"******** j^^aa^

ident of Documents, U.S. Government Printing Office, Washington, D.C. 20402 V

>

KlMftlilnllEi

SOLID ANGLE

YARD
and 19th centuries, the English system

of weights and measures was spread to and established in many parts of the world, including the American colonies. However, standards still differed to an

extent undesirable for commerce among

the 13 colonies. The need for greater

uniformity led to clauses in the Articles of Confederation (ratified by the origi- nal colonies in 1781) and the Constitution of the United States (ratified in 1790) giving power to the Congress to

fix uniform standards for weights and

measures. Today, standards supplied to

all the States by the National Bureau of Standards assure uniformity throughout

the country.

The Metric System

The need for a single worldwide coor- dinated measurement system was recog- nized over 300 years ago. Gabriel Mouton, Vicar of St. Paul in Lyons, proposed

in 1670 a comprehensive decimal meas- urement system based on the length of one minute of arc of a great circle of

the earth. In 1671 Jean Picard, a French

astronomer, proposed the length of a pendulum beating seconds as the unit of length. (Such a pendulum would

have been fairly easily reproducible,

thus facilitating the widespread distribu- tion of uniform standards.) Other proposals were made, but over a century

elapsed before any action was taken. In 1790, in the midst of the French

Revolution, the National Assembly of France requested the French Academy

of Sciences to "deduce an invariable standard for all the measures and all the weights." The Commission appointed by

the Academy created a system that was,

at once, simple and scientific. The unit of length was to be a portion of the

earth's circumference. Measures for capacity (volume) and mass (weight) were to be derived from the unit of

length, thus relating the basic units of

the system to each other and to nature. Furthermore, the larger and smaller

versions of each unit were to be created by multiplying or dividing the basic

units by 10 and its multiples. This fea- ture provided a great convenience to users of the system, by eliminating the need for such calculations as dividing

by 16 (to convert ounces to pounds) or by 12 (to convert inches to feet). Similar calculations in the metric system

could be performed simply by shifting the decimal point. Thus the metric sys- tem is a "base-10" or "decimal" system.

The Commission assigned the name

metre (which we also spell meter) to

the unit of length. This name was de- rived from the Greek word metron,

meaning "a measure." The physical standard representing the meter was to be constructed so that it would equal one ten-millionth of the distance from

the north pole to the equator along the meridian of the earth running near Dunkirk in France and Barcelona in Spain.

The metric unit of mass, called the "gram,'

- was defined as the mass of one

cubic centimeter (a cube that is 1/100

of a meter on each side) of water at its temperature of maximum density. The

cubic decimeter (a cube 1/10 of a meter on each side) was chosen as the unit of

fluid capacity. This measure was given

the name "liter." Although the metric system was not accepted with enthusiasm at first, adoption by other nations occurred steadily

after France made its use compulsory

in 1 840. The standardized character and

decimal features of the metric system

made it well suited to scientific and en- gineering work. Consequently, it is not

surprising that the rapid spread of the

system coincided with an age of

rapid technological development. In the United States, by Act of Congress in 1866, it was made "lawful throughout

the United States of America to employ

the weights and measures of the metric

system in all contracts, dealings or court proceedings."

By the late 1860's, even better metric

standards were needed to keep pace

with scientific advances. In 1875, an in- ternational treaty, the "Treaty of the Meter," set up well-defined metric stand- ards for length and mass, and established permanent machinery to recom- mend and adopt further refinements in the metric system. This treaty, known

as the Metric Convention, was signed by

17 countries, including the United

States. As a result of the Treaty, metric

standards were constructed and distrib- uted to each nation that ratified the Convention. Since 1893, the internationally agreed-to metric standards have

served as the fundamental weights and

measures standards of the United States. By 1900 a total of 35 nations—in- cluding the major nations of continental Europe and most of South America

had officially accepted the metric sys- tem. Today, with the exception of the United States and a few small countries, the entire world is using predominantly

the metric system or is committed to such use. In 1971 the Secretary of Commerce, in transmitting to Congress the

results of a 3-year study authorized by

the Metric Study Act of 1968, recom- mended that the U.S. change to pre- dominant use of the metric system

through a coordinated national pro- gram. The Congress is now considering

this recommendation.

The International Bureau of Weights

and Measures located at Sevres, France,

serves as a permanent secretariat for the Metric Convention, coordinating the ex- change of information about the use and refinement of the metric system. As

measurement science develops more precise and easily reproducible ways of defining the measurement units, the General Conference of Weights and Measures—the diplomatic organization made

up of adherents to the Convention

meets periodically to ratify improve- ments in the system and the standards. In 1960, the General Conference

adopted an extensive revision and simplification of the system. The name Le

Systeme International a" Unites (Inter- national System of Units), with the in- ternational abbreviation SI, was adopted

for this modernized metric system. Further improvements in and additions to SI were made by the General Conference in 1964, 1968, and 1971.
eights and

measures were among the earliest tools invented by man. Primitive

societies needed

rudimentary measures for many tasks: constructing

dwellings of an appropriate size and

shape, fashioning clothing, or bartering

food or raw materials. Man understandably turned first to parts of his body and his natural surroundings for measuring instruments.

Early Babylonian and Egyptian records

and the Bible indicate that length was

first measured with the forearm, hand,

or finger and that time was measured by

the periods of the sun, moon, and other

heavenly bodies. When it was necessary

to compare the capacities of containers

such as gourds or clay or metal vessels,

they were filled with plant seeds which

were then counted to measure the volumes. When means for weighing

were invented, seeds and stones served

as standards. For instance, the "carat,"

still used as a unit for gems, was

derived from the carob seed. As societies evolved, weights and

measures became more complex. The

invention of numbering systems and the science of mathematics made it possible

to create whole systems of weights and

measures suited to trade and commerce,

land division, taxation, or scientific research. For these more sophisticated

uses it was necessary not only to weigh

and measure more complex things—it was also necessary to do it accurately

time after time and in different places.

However, with limited international

exchange of goods and communication

of ideas, it is not surprising that

different systems for the same purpose

developed and became established in

different parts of the world—even in different parts of a single continent.

THE ENGLISH

SYSTEM

The measurement system commonly

used in the United States today is nearly

the same as that brought by the

colonists from England. These measures

had their origins in a variety of

cultures—Babylonian, Egyptian,

Roman, Anglo-Saxon, and Norman

French. The ancient "digit," "palm,"

"span," and "cubit" units

evolved into the "inch,"

"foot," and "yard" through a complicated transformation not

yet fully understood.

Roman contributions include

the use of the number 12 as a base (our foot is divided into 12 inches) and words from

which we derive many of our

present weights and measures

names. For example, the 12

divisions of the Roman "pes," or foot, were called unciae. Our words "inch" and

"ounce" are both derived from

that Latin word.

The "yard" as a measure of length can be traced back to the

early Saxon kings. They wore a sash or girdle around the

waist that could be removed

and used as a convenient

measuring device. Thus the word "yard" comes from the Saxon word "gird" meaning

the circumference of a person's

waist. Standardization of the various units and their combinations into a loosely

related system of weights and

measures sometimes occurred

in fascinating ways. Tradition

holds that King Henry I decreed that the yard should be

the distance from the tip of his nose to the end of his thumb.

The length of a furlong (or

furrow-long) was established by

early Tudor rulers as 220

yards. This led Queen

Elizabeth I to declare, in the 16th century, that henceforth

the traditional Roman mile of

5 000 feet would be replaced

by one of 5 280 feet, making

the mile exactly 8 furlongs and

providing a convenient

relationship between two previously

ill- related measures. Thus, through royal edicts, England

by the 18th century had achieved a greater degree of standardization than

the continental countries. The English

units were well suited to commerce and

trade because they had been developed

and refined to meet commercial needs.

Through colonization and dominance of world commerce during the 17th, 18th,

and 19th centuries, the English system

WITH A CHART OF THE

MODERNIZED

METRIC SYSTEM

fa fa Weights and measures may be

ranked among the necessaries of life to every individual of human society. They

enter into the economical arrangements

and daily concerns of 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 ingenuity 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 war. The knowledge of

them, as in established use, is among

the first elements of education, and is often learned by those who learn

nothing else, not even to read and write. This knowledge is riveted in the memory

by the habitual application of it to the

employments of men throughout life.^fa JOHN QUINCY ADAMS

Report to the Congress, 1821

I

' I'

I

'

I 'I

'

I '

I '

I

of weights and measures was spread to and established in many parts of the world, including the American colonies. However, standards still differed to an extent undesirable for commerce among the 13 colonies. The need for greater uniformity led to clauses in the

Articles of Confederation (ratified by

the original colonies in 1781) and the Constitution of the United States

(ratified in 1790) giving power to the Congress to fix uniform standards for
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weights and measures. Today, standards

supplied to all the States by the National Bureau of Standards assure uniformity throughout the country.

THE METRIC

SYSTEM

The need for a single worldwide

coordinated measurement system was

recognized over 300 years ago. Gabriel

Mouton, Vicar of St. Paul in Lyons,

proposed in 1670 a comprehensive

decimal measurement system based on

the length of one minute of arc of a great circle of the earth. In 1671, Jean

Picard, a French astronomer, proposed

the length of a pendulum beating

seconds as the unit of length. (Such a pendulum would have been fairly easily

reproducible, thus facilitating the widespread distribution of uniform

standards.) Other proposals were made,

but over a century elapsed before any

action was taken.

In 1790 in the midst of the French

Revolution, the National Assembly of France requested the French Academy

of Sciences to "deduce an invariable standard for all the measures and all the weights." The Commission

appointed by the Academy created a system that was, at once, simple and

scientific. The unit of length was to be

a portion of the earth's circumference. Measures for capacity (volume) and

mass (weight) were to be derived from

the unit of length, thus relating the

basic units of the system to each other

and to nature. Furthermore, the larger

and smaller versions of each unit were

to be created by multiplying or

dividing the basic units by 10 and its powers. This feature provided a great

convenience to users of the system, by

eliminating the need for such

calculations as dividing by 16 (to convert ounces to pounds) or by 12

(to convert inches to feet). Similar

calculations in the metric system could

be performed simply by shifting the decimal point. Thus the metric system

is a "base- 10" or "decimal" system.

The Commission assigned the name

metre—meter—to the unit of length.

This name was derived from the Greek

word metron, meaning "a measure."

The physical standard representing the meter was to be constructed so that it would equal one ten-millionth of the distance from the north pole to the equator along the meridian of the earth running near Dunkirk in France and

Barcelona in Spain.

The metric unit of mass, called the "gram," was defined as the mass of one

cubic centimeter (a cube that is 1/100

of a meter on each side) of water at its temperature of maximum density.

The cubic decimeter (a cube 1/10 of a meter on each side) was chosen as the

unit of fluid capacity. This measure was given the name "liter." Although the metric system was not

accepted with enthusiasm at first, adoption by other nations occurred

steadily after France made its use compulsory in 1840. The standardized

character and decimal features of the metric system made it well suited to

scientific and engineering work.

Consequently, it is not surprising that the rapid spread of the system

coincided with an age of rapid

technological development. In the United States, by Act of Congress in 1866, it was made "lawful throughout

the United States of America to employ

the weights and measures of the metric

system in all contracts, dealings or court proceedings."

By the late 1860's, even better metric

standards were needed to keep pace

with scientific advances. In 1875, an

international treaty, the "Treaty of the Meter," set up well-defined metric

standards for length and mass, and

established permanent machinery to

recommend and adopt further refinements in the metric system. This

treaty, known as the Metric

Convention, was signed by 17

countries, including the United States. As a result of the Treaty, metric

standards were constructed and

distributed to each nation that ratified the Convention. Since 1893, the

internationally agreed-to metric

standards have served as the fundamental weights and measures

standards of the United States. By 1900 a total of 35 nations

including the major nations of continental Europe and most of South

America—had officially accepted the metric system. In 1971 the Secretary of Commerce, in transmitting to Congress

the results of a 3-year study authorized

by the Metric Study Act of 1968,

recommended that the U.S. change to predominant use of the metric system

through a coordinated national

program. The Congress responded by

enacting the Metric Conversion Act of 1975. Today, with the exception of a few small countries, the entire world is using the metric system or is changing

to such use. The International Bureau of Weights

and Measures located at Sevres,

France, serves as a permanent

secretariat for the Meter Convention,

coordinating the exchange of information about the use and

refinement of the metric system. As

measurement science develops more

precise and easily reproducible ways of defining the measurement units, the General Conference on Weights and

Measures—the diplomatic organization

made up of adherents to the Convention—meets periodically to

ratify improvements in the system and

the standards.

In 1960, the General Conference

adopted an extensive revision and

simplification of the system. The name

Le Systeme International d'Unites (International System of Units), with

the international abbreviation SI, was

adopted for this modernized metric

system. Further improvements in and

additions to SI were made by the General Conference in 1964, 1968,

1971, 1975, 1979, and 1983.
"Weights and measures may be

ranked among the necessaries of life to every individual of human society. They

enter into the economical arrangements

and daily concerns of 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 ingenuity 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 war. The knowledge of them, as in established use, is among

the first elements of education, and is often learned by those who learn nothing else, not even to read and write. This knowledge is riveted in the memory

by the habitual application of it to the employments of men throughout life."

JOHN QUINCYADAMS

Report to the Congress, 1821

A Brief

HISTORY

of Measurement

Systems

WITH A CHART OF THE

MODERN METRIC SYSTEM

COPY

eighfsflnd A

measles

;_ '& were! jfl

among the

earliest

tools invented by man. Primitive

societies needed rudimentary

measures for many tasks:

constructing dwellings of an

appropriate size and shape,

fashioning clothing, and bartering

food or raw materials.

Man understandably turned

first to parts of his body and his

natural surroundings for measuring

instruments. Early Babylonian and

Egyptian records, and the Bible,

indicate that length was first

measured with the forearm, hand, or

finger and that time was measured

by the periods of the sun, moon, and

other heavenly bodies. When it was

necessary to compare the capacities

of containers such as gourds or clay

or metal vessels, they were filled

with plant seeds that were then

counted to measure the volumes. With the development of scales as a means for weighing, seeds and

stones served as standards. For

instance, the "carat," still used as a mass unit for gems, is derived from

the carob seed.

As societies evolved, measurements became more

complex. The invention of numbering systems and the science

of mathematics made it possible to

create whole systems of measurement units suited to trade and commerce, land division,

taxation, and scientific research. For

these more sophisticated uses, it was

necessary not only to weigh and measure more complex things - it was also necessary to do it

accurately time after time and in

different places. However, with

limited international exchange of goods and communication of ideas, it

is not surprising that different

systems for the same purpose

developed and became established

in different parts of the world - even in different parts of the same country.

THE ENGLISH SYSTEM

The measurement system commonly used in the United States

today is nearly the same as that

brought by the colonists from

England. These measures had their

origins in a variety of cultures -

Babylonian, Egyptian, Roman,

Anglo-Saxon, and Norman French. The ancient "digit," "palm," "span,"

and "cubit" units of length slowly lost

preference to the length units "inch,"

"foot," and "yard."

Roman contributions include

the use of 1 2 as a base number (the

foot is divided into 1 2 inches) and

the words from which we derive many of our present measurement

unit names. For example, the 1

divisions of the Roman "pes," or

foot were called unciae. Our words

"inch" and "ounce" are both derived

from that Latin word.

The "yard" as a measure of

length can be traced back to early

Saxon kings. They wore a sash or

girdle around the waist that could be removed and used as a convenient

measuring device. The word "yard" comes from the Saxon word "gird"

meaning the circumference of a

person's waist

Standardizing various units and

combining them into loosely related

systems of measurement units sometimes occurred in fascinating

ways. Tradition holds that King Henry I decreed that a yard should

be the distance from the tip of his

nose to the end of his outstretched

thumb. The length of a furlong (or

furrow-long) was established by

early Tudor rulers as 220 yards. This

led Queen Elizabeth I to declare, in

the 1 6th century, that henceforth

the traditional Roman mile of 5000

feet would be replaced by one of 5280 feet making the mile exactly

eight furlongs and providing a

convenient relationship between the

furlong and the mile.

Thus, through royal edicts,

England by the 1 8th century had

achieved a greater degree of

standardization than other European

countries. The English units were

well suited to commerce and trade

because they had been developed

and refined to meet commercial

needs. Through English colonization

and its dominance of world

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The meter is the length of the path traveled by light in vacuum during a time interval of 1/299 792 158 of a second.

1 METER

I I

1/299 792 458 OF A SECOND

The SI unit of speed is the meter per second (m/s).

The speed of light in vacuum is 299 792 4. meters per second.

The SI unit of acceleration is the meter

per second per second (m/s1 ).

The SI unit of area is the square meter

(m'). The SI unit of volume is the cubic meter (m1 ). The liter (1 cubic decimeter)

although not an SI unit, is accepted for use

with the SI and is commonly used to

> fluid votur

The second is the duration of 9 1 92 63 1 770 cycles of the radiation associated with a specific transition of the cesium 1 33 atom. The second is realized by

tuning an oscillator to the resonance

frequency associated with the above

definition. Just before entering a microwave

cavity, cesium atoms are forced into the right atomic state by a laser beam. A detector

reglsters a signal only when the oscillator delivers just the right frequency to the microwave cavity causing transitions and

changing the state of the atoms. This change

in state is sensed at the detector.

G

Thi imber of periods or cycles per second

is called frequency. The SI unit for frequency

Is the hertz (Hz). One hertz Is the same as one cycle per second. Standard frequencies and the cornea time are broadcast by radio

stations WWV andWWVB in Colorado, and WWVH in Hawaii. N1ST delivers digital timing signals by telephone and through the

the modern metric system

The International System of Units (SI), the modem version of the metric

system, is established by international agreement. It provides a logical and

interconnected framework for all measurements in science, industry, and commerce. Officially abbreviated SI, the system is built upon a foundation

of seven base units, shown on this chart along with their descriptions. All other SI units are derived from these units. Multiples and submultiples are expressed using a decimal system. Use of metric units was legalized in the United States in 1866. Since 1893 the yard and pound have been defined in terms of the meter and the kilogram.The base units for time, electric current, amount of substance, and luminous intensity are the same in both

the inch-pound and metric systems.

The ampere is that current which, if maintained in each of two infinitely long

parallel wires separated by one meter in fret space, would produce a force between the two wires (due to their magnetic fields) of

2 x 1 0" 7 newtons for each meter of length. r T

The SI unit of electric

potential difference

is the volt (V).

1 V = 1 W/A

The SI unit of

electric resistance

is the ohm (12).

1 fi = 1 V/A

The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10 11 hertz (Hz) and that has a radiant

intensity in that direction of 1/683 v

steradian.

Radiation at frequencies other than 540 x 10 13 Hz is also measured in candelas

in accordance with the standard luminous

efficiency, V( A), curve that peaks at 540 x 10' 1 Hz (yellow-green).

The kelvin is the fraction 1 /273.1 6 of the thermodynamic temperature of the triple point of water. The temperature 0 K is commonly referred to as "absolute zero." On the widely used Celsius temperature

freezes at 0 °C

and boils at about 1 00 °C. One Celsius degree is an

interval of 1 K, and zero

degrees Celsius is 273.15 K. An interval of one Celsius degree corresponds to an

interval of 1.8 Fahrenheit degrees on the Fahrenheit temperature scale.

The standard temperature at the triple point of water is provided by a special cell, an

evacuated glass cylinder containing pure

water. When the cell is cooled enough so

that a mande of

, ice forms around

the reentrant

well, the temperature at the

interface of solid,

liquid, and vapor

s 273.1 6 K. Thermometers to be calibrated

ire placed in the n

;e JL. „ 4

The standard for the unit of mass, the

kilogram, is a cylinder of platinum-iridium alloy kept by the International Bureau ofWeights

and Measures near Paris.A duplic custody of the National Institute of Standards and Technology

serve as the mass standard

for the United States. This is the only base

unit still defined by an artifact.

The SI unit of

force is the newton (N). One newton

is a force that,

applied to a one

kilogram object,

will give the objea

an acceleration of one meter per second

per second.

1 N - U -mis1

The weight of an object is the force exerted on it by gravity. Gravity gives a mass a downward acceleration of about 9.8 m/s1

.

The SI unit for pressure is the pascal (Pa),

1 Pa = 1 N/m2

The SI unit for work and energy of any kind

is the joule (J).

1

J = 1 N-m

The SI unit for power of any kind is the watt (W).

1 W = 1 J/s

The mole is the amount of substance of a system that contains as many elementary

entities as there are atoms in 0.012 kilogram

of carbon 1 2, When the mole is used, the elementary

entities must be specified and may be atoms,

molecules, ions, electrons, other particles, or

specified groups of such particles.

The SI unit of concentration (of amount of

substance) is the mole per cubic meter

(mol/m3

).
commerce during the 1 7th, 1 8th,

and 1 9th centuries, the English

system of measurement units became established in many parts

of the world, including the

American colonies.

However, standards still

differed to an extent undesirable for commerce, even among the 1

3

American colonies. The need for

greater uniformity led to clauses in

the Articles of Confederation

(ratified by the original colonies in

1 781 ) and the Constitution of the

United States (ratified in 1 790) that

gave Congress the power to fix

uniform standards for weights and

measures. Today, standards supplied

to all the states by the National

Institute of Standards and

Technology assure uniformity

throughout the country.

THE METRIC SYSTEM

The need for a single worldwide coordinated measurement system was

recognized over 300 years ago.

Gabriel Mouton.Vicar of Si Paul's Church in Lyons and an astronomer, proposed in 1 670 a comprehensive

decimal measurement system based on the length of one minute of arc

of a great circle of the Earth. Mouton also proposed the swing

length of a pendulum with a

frequency of one beat per second as

the unit of length. A pendulum with

this beat would have been fairly

easily reproducible, thus facilitating

the widespread distribution of

uniform standards. Other proposals were made, but more than a century

elapsed before any action was taken.

In 1 790, in the midst of the

French Revolution, the National Assembly of France requested the

French Academy of Sciences to "deduce an invariable standard for all

the measures and all the weights."

The Commission appointed by the Academy created a system that was,

at once, simple and scientific. The

unit of length was to be a portion of

the Earth's circumference. Measures

for capacity (volume) and mass were

to be derived from the unit of

length, thus relating the basic units of

the system to each other and to

nature. Furthermore, larger and

smaller multiples of each unit were

to be created by multiplying or

dividing the basic units by 1 0 and its

powers. This feature provided a

great convenience to users of the

system, by eliminating the need for

such calculations as dividing by 1

6

(to convert ounces to pounds) or by

1 2 (to convert inches to feet).

Similar calculations in the metric

system could be performed simply

by shifting the decimal point Thus,

the metric system is a "base-10" or

"decimal" system.

The Commission assigned the name metre - meter -- to the unit

of length. This name was derived

from the Greek word metron,

meaning "a measure." The physical

standard representing the meter was

to be constructed so that it would

equal one ten-millionth of the

distance from the North Pole to the

equator along the meridian running

near Dunkirk in France and

Barcelona in Spain.

The initial metric unit of mass,

the "gram," was defined as the mass

of one cubic centimeter (a cube that

is 0.01 meter on each side) of water

at its temperature of maximum

density. The cubic decimeter

(a cube 0.1 meter on each side) was chosen as the unit for capacity. The fluid volume measurement for

the cubic decimeter was given the name "liter."

Although the metric system was not accepted with enthusiasm

at first adoption by other nations

occurred steadily after France made

its use compulsory in 1 840. The

standardized structure and decimal

features of the metric system made

it well suited for scientific and

engineering work. Consequently, it

is not surprising that the rapid

spread of the system coincided with

an age of rapid technological development In the United States,

by Act of Congress in 1 866, it became "lawful throughout the

United States of America to employ

the weights and measures of the

metric system in all contracts,

dealings or court proceedings."

By the late 1 860s, even better

metric standards were needed to

keep pace with scientific advances.

In 1 875, an international agreement known as the Meter Convention, set

up well-defined metric standards for

length and mass and established permanent mechanisms to recommend and adopt further

refinements in the metric system.

This agreement commonly called

the 'Treaty of the Meter" in the

United States, was signed by 1

7

countries, including the United

States. As a result of the Treaty,

metric standards were constructed

and distributed to each nation that

ratified the Convention. Since 1 893,

the internationally adopted-to

metric standards have served as the

fundamental measurement standards

of the United States.

By 1 900 a total of 35 nations

-- including the major nations of

continental Europe and most of

South America -- had officially

accepted the metric system.

In 1 960, the General

Conference on Weights and

Measures, the diplomatic

organization made up of the

signatory nations to the Meter

Convention, adopted an extensive

revision and simplification of the

system. Seven units - the meter

(for length), the kilogram (for mass),

the second (for time), the ampere

(for electric current), the kelvin (for thermodynamic temperature), the mole (for amount of substance),

and the candela (for luminous

intensity) ~ were established as the

base units for the system. The name

Systeme International d'Unites

(International System of Units),

with the international abbreviation

SI, was adopted for this modern

metric system.

In 1971, the U.S. Secretary of Commerce, in transmitting to

Congress the results of a 3-year

study authorized by the Metric

Study Art of 1 968, recommended

that the U.S. change to predominant

use of the metric system through a

coordinated 1 0-year national

program. The Congress responded

by enacting the Metric Conversion Act of 1 975, calling for voluntary

conversion. Amendments to the Act

in 1 988 designated the metric

system as the "preferred system of

weights and measures for United

States trade and commerce."

Measurement science

continues to develop more precise

and easily reproducible ways of

defining measurement units. The

working organizations of the

General Conference on Weights and

Measures coordinate the exchange

of information about the use and

refinement of the metric system and make recommendations concerning improvements in the system and its

related standards. The General

Conference meets periodically to

ratify improvements. Additions and improvements to SI were made by

the General Conference in 1 964, 1967-1968,1971,1975,1979,

1983, 1991, and 1995. Nisr U.S. DEPARTMENT OF COMMERCE

Technology Administration

National Institute of Standards and Technology