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  Measurement is the determination of the dimensions, extent, or quantity of something. A unit is a standard quantity in relation to which other quantities are measured. There have been many systems of units. Some ancient units, such as the day, the foot, and the pound, are still in use. SI units, the latest version of the metric system, are widely used in science.  
  A scale is an indexing system that shows the order of magnitude of a physical quantity through a one-to-one correspondence between a number and the physical quantity. Physical quantities like temperature and sound are measured using scales.  
  Systems of Measurement  
  imperial system  
  The imperial system is a traditional system of units developed in the U.K., based largely on the foot, pound, and second (f.p.s.) system. The imperial system (also known as the U.S. Customary system) is still in common usage in the United States, despite the fact that SI units have been adopted by the U.S. scientific community.  
imperial system
  The foot, yard and inch owe their length to Henry I of England. He decreed that one yard was the length of his extended arm, measured from nose to fingertip.  


  c.g.s. system  
  The c.g.s. system of units is based on the centimeter, gram, and second, as units of length, mass, and time, respectively.  
  m.k.s. system  
  In the m.k.s. system the base units of the meter, kilogram, and second replace the centimeter, gram, and second of the c.g.s. system. As in the c.g.s. system, there is no standard unit for volume.  
  metric system  
  The metric system of weights and measures was developed in France in the 18th century and recognized by other countries in the 19th century. Based  
Units in the Imperial System
1 foot = 12 inches
1 yard = 3 feet
1 rod = 5 1/2 yards (=161/2 feet)
1 chain = 4 rods (= 22 yards)
1 furlong = 10 chains (= 220 yards)
1 mile = 5,280 feet
1 mile = 1,760 yards
1 mile = 8 furlongs

1 fathom = 6 feet
1 cable length = 120 fathoms
1 nautical mile = 6,076.11549 feet

1 square foot = 144 square inches
1 square yard = 9 square feet
1 square rod = 301/4 square yards
1 acre = 4 roods
1 acre = 4,840 square yards
1 square mile = 640 acres

1 cubic foot = 1,728 cubic inches
1 cubic yard = 27 cubic feet
1 bulk barrel = 5.8 cubic feet

1 register ton = 100 cubic feet


  (table continued on next page)  




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  (table continued from previous page)  
1 fluid ounce = 8 fluid drams
1 gill = 5 fluid ounces
1 pint = 4 gills
1 quart = 2 pints
1 gallon = 4 quarts
1 peck = 2 gallons
1 bushel = 4 pecks
1 quarter = 8 bushels
1 bulk barrel = 36 gallons
Weight (avoirdupois)  
1 ounce = 4371/2 grains
1 ounce = 16 drams
1 pound = 16 ounces
1 stone = 14 pounds
1 quarter = 28 pounds
1 hundredweight = 4 quarters
1 ton = 20 hundredweight


  largely on the m.k.s. system, the metric system uses the meter as the base unit of length, the kilogram as the base unit of mass, the second as the base unit of time, and the liter as the base unit of volume.  
  The metric system has been replaced for scientific work by the SI units to avoid inconsistencies in definition of the thermal calorie and electrical quantities.  
  The metric system was made legal for most purposes in the United States and U.K. in the 19th century. A metric act was passed in the United States in 1975, but metric measurement has not been readily adopted, except by the scientific community.  
  SI units  
  SI units (French Système International d'Unités) comprise a standard system of scientific units used by scientists worldwide. In 1960 an international conference  
Units in the Metric System
  1 centimeter  
= 10 millimeters  
  1 decimeter  
= 10 centimeters = 100 millimeters
  1 meter  
= 10 decimeters = 1,000 millimeters
  1 dekameter  
= 10 meters  
  1 hectometer  
= 10 dekameters = 100 meters
  1 kilometer  
= 10 hectometers = 1,000 meters

  1 square centimeter  
= 100 square millimeters  
  1 square meter  
= 10,000 square centimeters = 1,000,000 square millimeters
  1 are  
= 100 square meters  

Mass (avoirdupois)
  1 hectare  
= 100 ares = 10,000 square meters
  1 square kilometer  
= 100 hectares = 1,000,000 square meters
  1 centigram  
= 10 milligrams  
  1 decigram  
= 10 centigrams = 100 milligrams
  1 gram  
= 10 decigrams = 1,000 milligrams
  1 dekagram  
= 10 grams  
  1 hectogram  
= 10 dekagrams = 100 grams
  1 kilogram  
= 10 hectograms = 1,000 grams
  1 metric ton  
= 1,000 kilograms  

  1 cubic centimeter  
= 1,000 cubic millimeters  
  1 cubic decimeter  
= 1,000 cubic centimeters = 1,000,000 cubic millimeters
  1 cubic meter  
= 1,000 cubic decimeters = 1,000,000,000 cubic millimeters

  1 centiliter  
= 10 milliliters  
  1 deciliter  
= 10 centiliters = 100 milliliters
  1 liter  
= 10 deciliters = 1,000 milliliters
  1 decaliter  
= 10 liters  
  1 hectoliter  
= 10 decaliters = 100 liters
  1 kiloliter  
= 10 hectoliters = 1,000 liters





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Imperial and Metric Conversion Factors
  To convert from
imperial to metric
Multiply by Multiply by To convert from
metric to imperial
25.4 0.0393701 millimeters
0.3048 3.28084 meters
0.9144 1.09361 meters
0.201168 4.97097 kilometers
1.609344 0.621371 kilometers
  square inches  
6.4516 0.1550 square centimeters
  square feet  
0.092903 10.7639 square meters
  square yards  
0.836127 1.19599 square meters
  square miles  
2.589988 0.386102 square kilometers
4046.856422 0.000247 square meters
0.404866 2.469955 hectares
  cubic inches  
16.387064 0.061024 cubic centimeters
  cubic feet  
0.028317 35.3147 cubic meters
  cubic yards  
0.764555 1.30795 cubic meters
  cubic miles  
4.1682 0.239912 cubic kilometers
  fluid ounces (U.S.)  
29.5735 0.033814 milliliters
  fluid ounces (imperial)  
28.413063 0.035195 milliliters
  pints (U.S.)  
0.473176 2.113377 liters
  pints (imperial)  
0.568261 1.759754 liters
  quarts (U.S.)  
0.946353 1.056688 liters
  quarts (imperial)  
1.136523 0.879877 liters
  gallons (U.S.)  
3.785412 0.364172 liters
  gallons (imperial)  
4.54609 0.219969 liters
28.349523 0.035274 grams
0.453592 2.20462 kilograms
  tons (U.S.)  
907.18474 0.001102 kilograms
  tons (imperial)  
1016.046909 0.000984 kilograms
  tons (U.S.)  
0.907185 1.10231 metric tons
  tons (imperial)  
1.016047 0.984207 metric tons
  miles per hour  
1.609344 0.621371 kilometers per hour
  feet per second  
0.3048 3.28084 meters per second
  pound force  
4.44822 0.224809 newton
  kilogram force  
9.80665 0.101972 newton
  pound-force per square inch  
6.89476 0.145038 kilopascals
  tons-force per square inch (imperial)  
15.4443 0.064779 megapascals
10.1325 0.098692 newtons per square centimeter
14.695942 0.068948 pounds-force per square inch
4.1868 0.238846 joule
  watt hour  
3,600 0.000278 joule
0.7457 1.34102 kilowatts
Fuel consumption      
  miles per gallon (U.S.)  
0.4251 2.3521 kilometers per liter
  miles per gallon (imperial)  
0.3540 2.824859 kilometers per liter
  gallons per mile (U.S.)  
2.3521 0.4251 liters per kilometer
  gallons per mile (imperial)  
2.824859 0.3540 liters per kilometer





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  on weights and measures recommended the universal adoption of a revised international (or SI) system with seven prescribed "base units": the meter (m) for length, kilogram (kg) for mass, second (s) for time, ampere (A) for electric current, kelvin (K) for thermodynamic temperature, candela (cd) for luminous intensity, and mole (mol) for quantity of matter.  
SI Units
Quantity SI unit Symbol
absorbed radiation dose gray Gy
amount of substance mole1 mol
electric capacitance farad F
electric charge coulomb C
electric conductance siemens S
electric current ampere1 A
energy or work joule J
force newton N
frequency hertz Hz
illuminance lux Ix
inductance henry H
length meter1 m
luminous flux lumen Im
luminous intensity candela1 cd
magnetic flux weber Wb
magnetic flux density tesla T
mass kilogram1 kg
plane angle radian rad
potential difference volt V
power watt W
pressure pascal Pa
radiation dose equivalent sievert Sv
radiation exposure roentgen R
radioactivity becquerel Bq
resistance ohm W
solid angle steradian sr
sound intensity decibel dB
temperature ºCelsius °C
temperature, thermodynamic kelvin1 K
time second1 s
  1 SI base unit.  


  Two supplementary units are included in the SI system—the radian (rad) and steradian (sr)—used to measure plane and solid angles. In addition, there are recognized derived units that can be expressed as simple products or divisions of powers of the basic units, with no other integers appearing in the expression; for example, the watt.  
  Some non-SI units, well established and internationally recognized, remain in use in conjunction with the SI system: minute, hour, and day in measuring time; multiples or submultiples of base or derived units which have long-established names, such as ton for mass, the liter for volume; and specialist measures such as the metric carat for gemstones.  
  Scientific Notation  
  Scientific notation is a method of writing numbers often used by scientists, particularly for very large or very small numbers. The numbers are written with one digit before the decimal point and multiplied by a power of 10. The number of digits given after the decimal point depends on the accuracy required. For example, the speed of light is 2.9979 x 108 m/ 1.8628 x 105 mi per second.  
  Types of Measurement  
  length and distance  
  meter The meter (symbol m) is the SI unit of length, equivalent to 1.093 yards. It is defined by scientists as the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second.  
  The meter was originally (in 1791) defined as one ten-millionth of the distance from the North Pole to the equator, on a line through Paris.  


  foot The foot (symbol ft) is the imperial unit of length, equivalent to 0.3048 m, used in Britain since Anglo-Saxon times. It originally represented the length of a human foot. One foot contains 12 inches and is one-third of a yard.  
  yard A yard (symbol yd) is equivalent to 3 feet (0.9144 m). It is a commonly used unit of length in the United States, where it is also sometimes used to denote a cubic yard (0.7646 cubic meters), as of topsoil.  
  Area is the size of a surface. It is measured in square units, such as square inches, square feet, or square kilometers (km2). Surface area is the area of the outer surface of a solid.  
  hectare A hectare (symbol ha) is the metric unit of area equal to 100 ares or 10,000 square meters (2.47 acres). Trafalgar Square, London's only metric square, was laid out as one hectare.  
  acre The traditional English land measure, the acre, is equal to 4,840 square yards (4,047 sq m/0.405 ha).  




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SI Prefixes
Multiple Prefix Symbol Example
1,000,000,000,000,000,000 (1018) exa- E Eg (exagram)
1,000,000,000,000,000 (1015) peta- P PJ (petajoule)
1,000,000,000,000 (1012) tera- T TV (teravolt)
1,000,000,000 (109) giga- G GW (gigawatt)
1,000,000 (106) mega- M MHz (megahertz)
1,000 (103) kilo- k kg (kilogram)
100 (102) hecto- h hm (hectometer)
10 (101) deka- da daN (dekanewton)
1/10 (10–1) deci- d dC (decicoulomb)
1/100 (10–2) centi- c cm (centimeter)
1/1,000 (10–3) milli- m mm (millimeter)
1/1,000,000 (10–6) micro- ( (F (microfarad)
1/1,000,000,000 (10–9) nano- n nm (nanometer)
1/1,000,000,000,000 (10–12) pico- p ps (picosecond)
1/1,000,000,000,000,000 (10–15) femto- f frad (femtoradian)
1/1,000,000,000,000,000,000 (10–18) atto- a aT (attotesla)


  Originally meaning a field, it was the size that a yoke of oxen could plow in a day.  
  As early as Edward I's reign (1272–1307), the acre was standardized by statute for official use, although local variations in Ireland, Scotland, and some English counties continued. It may be subdivided into 160 square rods (one square rod equaling 25.29 sq m/ 30.25 sq yd).  
  volume and capacity  
  The space occupied by a three-dimensional solid object is called volume. Capacity, the alternative term for volume, is generally used to refer to the amount of liquid or gas that may be held in a container. Units of capacity include the liter and milliliter (metric) and the pint and gallon (imperial).  
  cubic measure Cubic measure is the measure of volume, indicated either by the word "cubic" followed by a linear measure, as in "cubic foot," or the word "cubed" after a linear measure, as in "meter cubed" (m3). A cubic decimeter (symbol dm3) corresponds to the volume of a cube whose edges are all 1 dm (10 cm) long; it is equivalent to a capacity of one liter.  
  liter The liter (symbol l) is the metric unit of volume and capacity, equal to one cubic decimeter (2.11 pints). It was formerly defined as the volume occupied by one kilogram of pure water at 4°C at standard pressure, but this is slightly larger than one cubic decimeter.  
  gallon The imperial liquid or dry measure of capacity, the gallon, is equal to 3.785 liters and is subdivided into four quarts or eight pints. The U.K. gallon is equivalent to 4.546 liters.  
  pint The pint is the imperial liquid measure of capacity equal to 16 fluid ounces, half a quart, one-eighth of a gallon, or 0.473 liter. (A dry pint is equal to 0.550 liter.) In the U.K. a dry or liquid pint is equal to 0.568 liter.  
  weight and mass  
  Weight is the force exerted on an object by gravity. The weight of an object depends on its mass—the amount of material in it—and the strength of earth's gravitational pull, which decreases with height. Consequently, an object weighs less at the top of a mountain than at sea level. On the moon, an object has only one-sixth of its weight on earth, because the pull of the moon's gravity is one-sixth that of the earth's.  
  If the mass of a body is m kilograms and the gravitational field strength is g newtons per kilogram, its weight W in newtons is given by W = mg.  
  Mass is the quantity of matter in a body as measured by its inertia. In the SI system, the base unit of mass is the kilogram. At a given place, equal masses experience equal gravitational forces, which are known as the weights of the bodies. Masses may, therefore, be compared by comparing the weights of bodies at the same place. The standard unit of mass to which all other masses are compared is a platinum-iridium  




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  cylinder of 1 kg, which is kept at the International Bureau of Weights and Measures in Sèvres, France.  
  kilogram SI unit (symbol kg) of mass equal to 1,000 grams (2.24 lb). It is defined as a mass equal to that of the international prototype held at the International Bureau of Weights and Measures in Sèvres, France.  
  pound The imperial unit of mass is the pound (abbreviation lb). The commonly used avoirdupois pound, also called the imperial standard pound (7,000 grains/0.45 kg), differs from the pound troy (5,760 grains/0.37 kg), which is used for weighing precious metals. It derives from the Roman libra, which weighed 0.327 kg.  
  ton and metric ton A ton is an imperial unit of mass. The short ton, used in the United States, is 907 kg/2,000 lb. The long ton, used in the U.K., is 1,016 kg/ 2,240 lb.  
  In shipping, the ton is a unit of volume equal to 2.83 cubic meters/100 cubic feet. Gross tonnage is the total internal volume of a ship in tons; net register tonnage is the volume used for carrying cargo or passengers. Displacement tonnage is the weight of the vessel, in terms of the number of imperial tons of seawater displaced when the ship is loaded to its load line; it is used to describe warships.  
  The metric ton (U.K. "tonne") of 1,000 kg/2,204.6 lb is equivalent to 0.9842 of an imperial ton.  
  mole A mole (symbol mol) is the SI unit of the amount of a substance. It is defined as the amount of a substance that contains as many elementary entities (atoms, molecules, and so on) as there are atoms in 12 g of the isotope carbon-12.  
  One mole of an element that exists as single atoms weighs as many grams as its atomic number (so one mole of carbon weighs 12 g), and it contains 6.022045 x 1023 atoms, which is Avogadro's number.  
  carat The carat is the unit for measuring the mass of precious stones; it is equal to 0.2 g/0.00705 oz, and is part of the c0016-01.giftroy system of weights used for precious metals and gems. The carat is also the unit of purity in gold (U.S. "karat"). Pure gold is 24-karat; 22-karat (the purest used in jewelry) is 22 parts gold and two parts alloy (to give greater strength). Originally, one karat was the weight of a carob seed (Arabic quirrat "seed").  
  The continuous passage of existence is recorded by division into hours, minutes, and seconds. Formerly the measurement of time was based on the earth's rotation on its axis, but this was found to be irregular. Therefore the second, the standard SI unit of time, was redefined in 1956 in terms of the earth's annual orbit of the sun, and in 1967 in terms of a radiation pattern of the element cesium.  
  Designed by the National Institute of Standards and Technology (NIST) Physics Laboratory to provide an historic understanding of the evolution of time measurement. There are sections on Ancient Calendars, Early Clocks, World Time Scales, the Atomic Age, and the Revolution in Timekeeping.  
  year A year is a large unit of time measurement based on the orbital period of the earth around the sun. The tropical year, from one spring equinox to the next, lasts 365.2422 days. It governs the occurrence of the seasons, and is the period on which the calendar year is based. The sidereal year is the time taken for the earth to complete one orbit relative to the fixed stars, and lasts 365.2564 days (about 20 minutes longer than a tropical year). The difference is due to the effect of precession, which slowly moves the position of the equinoxes. The calendar year consists of 365 days, with an extra day added at the end of February each leap year. Leap years occur in every year that is divisible by four, except that a century year is not a leap year unless it is divisible by 400. Hence 1900 was not a leap year, but 2000 will be.  
  second The second (symbol sec or s) is the basic SI unit of time, one-sixtieth of a minute. It is defined as the duration of 9,192,631,770 cycles of regulation (periods of the radiation corresponding to the transition between two hyperfine levels of the ground state) of the cesium-133 isotope.  
  minute A minute is a unit of time consisting of 60 seconds; 60 minutes make an hour.  
  hour An hour is a period of time comprising 60 minutes; 24 hours make one calendar day.  
  day A day is the time taken for the earth to rotate once on its axis. The solar day is the time that the earth takes to rotate once relative to the sun. It is divided into 24 hours, and is the basis of our calendar day. The sidereal day is the time that the earth takes to rotate once relative to the stars. It is 3 minutes 56 seconds shorter than the solar day, because the sun's position against the background of stars as seen from earth changes as the earth orbits it.  




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  month A month is a unit of time based on the motion of the moon around the earth. The time from one new or full moon to the next (the synodic or lunar month) is 29.53 days. The time for the moon to complete one orbit around the earth relative to the stars (the sidereal month) is 27.32 days. The solar month equals 30.44 days, and is exactly one-twelfth of the solar or tropical year, the time taken for the earth to orbit the sun. The calendar month is a human invention, devised to fit the calendar year.  
  Celsius scale Previously called centigrade, the Celsius scale of temperature ranges from freezing to boiling of water and is divided into 100 degrees, freezing point being 0 degrees and boiling point 100 degrees.  
  The degree centigrade (°C) was officially renamed Celsius in 1948 to avoid confusion with the angular measure known as the centigrade (one-hundredth of a grade). The Celsius scale is named for the Swedish astronomer Anders Celsius who devised it in 1742 but in reverse (freezing point was 100°; boiling point 0°).  
  Brief but comprehensive description of how Swedish astronomer Anders Celsius devised his temperature scale.  
  Fahrenheit scale The Fahrenheit temperature scale was invented in 1714 by the Polish-born Dutch physicist Gabriel Fahrenheit and was commonly used in English-speaking countries until the 1970s, after which the Celsius scale was generally adopted, though the Fahrenheit scale remains in use in the United States. In the Fahrenheit scale, intervals are measured in degrees (°F); °F = (°C x 9/5) + 32.  
  Fahrenheit took as the zero point the lowest temperature he could achieve anywhere in the laboratory, and, as the other fixed point, body temperature, which he set at 96°F. On this scale, water freezes at 32°F and boils at 212°F.  
  Kelvin scale Introduced in the 19th century by the Irish physicist William Thomson Kelvin, the Kelvin scale of absolute temperature is used by scientists. It begins at absolute zero (–273.15°C) and increases by the same degree intervals as the Celsius scale; that is, 0°C is the same as 273.15 K and 100°C is 373.15 K.  
  Loudness is the subjective judgment of the level or power of sound reaching the ear. The human ear cannot give an absolute value to the loudness of a single sound, but can only make comparisons between two different sounds. The precise measure of the power of a sound wave at a particular point is called its intensity.  
  Accurate comparisons of sound levels may be made using sound-level meters, which are calibrated in units called decibels.  
  decibel scale The decibel scale is used for audibility measurements, as one decibel unit (symbol dB), representing an increase of about 25%, is about the smallest change the human ear can detect. A whisper has an intensity of 20 dB; 140 dB (a jet aircraft taking off nearby) is the threshold of pain.  
  Used originally to compare sound intensities, the decibel was used subsequently to measure electrical or electronic power outputs and is now also used to compare voltages. An increase of 10 dB is equivalent to a 10-fold increase in intensity or power, and a 20-fold increase in voltage. The difference in decibels between two levels of intensity (or power) L1 and L2 is 10 log10(L1/L2); a difference of 1 dB thus corresponds to a change of about 25%. For two voltages V1 and V2, the difference in decibels is 20 log10(V1/V2); 1 dB corresponding in this case to a change of about 12%.  
  Decibel measurements of noise are often "Aweighted" to take into account the fact that some sound wavelengths are perceived as being particularly loud.  
  ampere The SI unit of electrical current is the ampere (symbol A). Electrical current is measured in a similar  
Decibel Scale
Typical sound
threshold of hearing
rustle of leaves in gentle breeze
quiet whisper
average whisper
quiet conversation
hotel; theater (between performances)
loud conversation
traffic on busy street
factory (light/medium work)
heavy traffic
jet aircraft at takeoff
threshold of pain
space rocket at takeoff





Page 8
Table of Equivalent Temperatures
Celsius and Fahrenheit temperatures can be interconverted as follows: C =  (F – 32) × 100/180; F = (C × 180/100) + 32.


  way to water current, in terms of an amount per unit of time; one ampere represents a flow of about 6.28 × 1018 electrons per second, or a rate of flow of charge of one coulomb per second.  
  The ampere is defined as the current that produces a specific magnetic force between two long, straight, parallel conductors placed 1 m/3.3 ft apart in a vacuum. It is named for the French physicist and mathematician André Ampère (1775–1836).  
  volt The SI unit of electromotive force or electric potential is the volt (symbol V). A small battery has a potential of 1.5 volts, while a high-tension transmission line may carry up to 765,000 volts. The domestic electricity supply is 110 volts in the United States. In the U.K. it is 230 volts (lowered from 240 volts in 1995).  
  The absolute volt is defined as the potential difference necessary to produce a current of one ampere through an electric circuit with a resistance of one ohm (SI unit of electrical resistance). It can also be defined as the potential difference that requires one joule of work to move a positive charge of one coulomb from the lower to the higher potential. It is named for the Italian physicist Alessandro Volta (1745–1827).  
  watt The watt (symbol W) is the SI unit of power (the rate of expenditure or consumption of energy), defined as one joule (SI unit of work and energy, see below) per second. A light bulb, for example, may use 40, 60, 100, or 150 watts of power; an electric heater will use several kilowatts (thousands of watts). The watt is named for the Scottish engineer James Watt (1736–1819).  
  The absolute watt is defined as the power used when one joule of work is done in one second. In electrical terms, the flow of one ampere of current through a  




Page 9
  conductor whose ends are at a potential difference of one volt uses one watt of power (watts = volts x amperes).  
  joule The joule (symbol J) is the SI unit of work and energy, replacing the calorie (one joule equals 4.2 calories). It is defined as the work done (energy transferred) by a force of one newton (SI unit of force) acting over one meter. It can also be expressed as the work done in one second by a current of one ampere at a potential difference of one volt. One watt is equal to one joule per second.  
  calorie The c.g.s. unit of heat, the calorie, is defined as the amount of heat required to raise the temperature of one gram of water by 1°C. It is now replaced by the joule (one calorie is approximately 4.2 joules). In dietetics, the Calorie or kilocalorie is equal to 1,000 calories.  
  The kilocalorie measures the energy value of food in terms of its heat output: 28 g/1 oz of protein yields 120 kilocalories; the same amount of carbohydrate yields 110 kilocalories; fat, 270 kilocalories; and alcohol, 200 kilocalories.  
U.S. Weights and Measures
  A metric act was passed in the United States in 1975, but metric measurement has not been readily adopted, except by the scientific community.  
  Customary U.S. weights and measures  
  1 township  
= 6 miles square1
Length     = 36 sections (of land)
  1 foot  
= 12 inches   = 36 square miles1
  1 yard  
= 3 feet Cubic measure  
  1 rod (pole or perch)  
= 5c0035-03.gif yards
  1 cubic foot  
= 1,728 cubic inches
  = 16c0035-03.gif feet
  1 cubic yard  
= 27 cubic feet
  1 chain  
= 4 rods   = 46,656 cubic inches
  = 22 yards Volume (dry measure)  
  1 furlong  
= 10 chains    
  = 220 yards
  1 quart  
= 2 pints
  = 40 rods   = 67.2006 cubic inches
  = 660 feet
  1 peck  
= 8 quarts
  1 mile  
= 8 furlongs   = 16 pints
  = 1,760 yards   = 537.605 cubic inches
  = 5,280 feet
  1 bushel  
= 4 pecks
  1 league  
= 3 miles   = 32 quarts
  = 5,280 yards   = 2,150.42 cubic inches
Nautical = 15,840 feet Volume (liquid measure) and apothecaries' fluid measures
  1 fathom  
= 6 feet
  1 fluid dram  
= 60 minims
  1 cable length  
= 120 fathoms   = 0.2256 cubic inches
  = 720 feet
  1 fluid ounce  
= 8 fluid drams
  1 international nautical mile  
= 6,076.11549 feet   = 1.8047 cubic inches
  1 cup  
= 64 fluid drams
Area     = 8 fluid ounces
  1 square foot  
= 144 square inches   = 14.438 cubic inches
  1 square yard  
= 9 square feet
  1 pint  
= 128 fluid drams
  = 1,296 square inches   = 16 fluid ounces
  1 square rod  
= 30c0035-02.gif square yards   = 2 cups
  = 272c0035-02.gif square feet   = 28.875 cubic inches
  1 square mile  
= 640
  1 quart  
= 256 fluid drams
    = 32 fluid ounces
  1 section (of land)  
= 1 mile square1   = 4 cups
  1 Square miles are not the same as miles square.  


  (table continued on next page)  




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  (table continued from previous page)  
Volume (liquid measure) and apothecaries' fluid measures Avoirdupois weight  
  1 apothecaries' ounce  
= 480 grains
  = 2 pints   = 24 scruples
  = 57.75 cubic inches   = 8 drams
  1 gallon  
= 1,024 fluid drams
  1 apothecaries' pound  
= 5,760 grains
  = 128 fluid ounces   = 288 scruples
  = 16 cups   = 96 apothecaries' drams
  = 8 pints Apothecaries' weights  
  = 4 quarts   = 12 apothecaries' ounces
  =231 cubic inches Troy weights  
  1 pennyweight  
= 24 grains
Avoirdupois weight  
  1 troy ounce  
= 480 grains
  1 dram  
= 27c0039-06.gif grains   = 240 pennyweights
  1 ounce  
= 437 c0035-03.gif grains
  1 troy pound  
= 5,760 grains
  = 16 drams   = 240 pennyweights
  1 pound  
= 7,000 grains   = 12 toy ounces
  = 256 drams Gunter's or surveyor's chain measures
  = 16 ounces
  1 link  
= 7.92 inches
  1 chain  
= 100 links
Avoirdupois weight     = 4 rods
  1 hundredweight2  
= 100 pounds   = 66 feet
  1 gross (or long)  
  1 statute mile  
= 88 chains
= 112 pounds   = 320 rods
  1 ton2  
= 2,000 pounds   = 5,280 feet
  = 20 hundredweights Units of circular measure
  1 gross (or long ton)2  
= 2,240 pounds
  = 20 gross (or long ton)
= 60 seconds
= 60 minutes
  1 scruple  
= 20 grains
  Right angle  
= 90 degrees
  1 dram  
= 60 grains
  Straight angle  
= 180 degrees
  = 3 scruples
= 360 degrees
  2 The terms ''hundredweight" and "ton" are normally used to mean the 100-pound hundredweight and the 2,000-pound ton. These units are sometimes designated the terms "net" or "short" to differentiate them from the corresponding "gross" or "long" hundredweight and ton.  


Miscellaneous Units
Unit Definition
acoustic ohm c.g.s. unit of acoustic impedance (the ratio of sound pressure on a surface to sound flux through the surface)
acre-foot unit sometimes used to measure large volumes of water such as reservoirs; 1 acre-foot = 1,233.5 cu m/ 43,560 cu ft
astronomical unit unit (symbol AU) equal to the mean distance of the earth from the sun: 149,597,870 km/92,955,808 mi
atmosphere unit of pressure (abbreviation atm); 1 standard atmosphere = 101,325 Pa
barn unit of area, especially the cross-sectional area of an atomic nucleus; 1 barn = 10–28 sq m
barrel unit of liquid capacity; the volume of a barrel depends on the liquid being measured and the country and state laws. In the United States, 1 barrel of oil = 42 gal (159 l/34.97 imperial gal), but for federal taxing of fermented liquor (such as beer), 1 barrel = 31 gal (117.35 l/25.81 imperial gal). Many states fix a 36-gallon barrel for cistern measurement and federal law uses a 40-gallon barrel to measure "proof spirits." 1 barrel of beer in the U.K. = 163.66 l (43.23 U.S. gal/36 imperial gal)
base box imperial unit of area used in metal plating; 1 base box = 20.232 sq m/31,360 sq in
baud unit of electrical signaling speed equal to 1 pulse per second
brewster unit (symbol B) for measuring reaction of optical materials to stress
British thermal unit imperial unit of heat (symbol Btu); 1 Btu = approximately 1,055 J


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  (table continued from previous page)  
Unit Definition
bushel measure of dry and (in the UK) liquid volume. 1 bushel (struck measure) = 8 dry U.S. gallons (64 dry U.S. pt/35.239 l/2,150.42 cu in). 1 heaped U.S. bushel = 1,278 bushels, struck measure (81.78 dry pt/45.027 l/2,747.715 cu in), often referred to a 1 1/4 bushels, struck measure. In the U.K., 1 bushel = 8 imperial gallons (64 imperial pt); 1 U.K. bushel = 1.03 U.S. bushels
cable unit of length used on ships, taken as 1/10 of a nautical mile (185.2 m/607.6 ft)
carcel obsolete unit of luminous intensity
cental name for the short hundredweight; 1 cental = 45.36 kg/100 lb
chaldron obsolete unit measuring capacity; 1 chaldron = 1.309 cu m/46.237 cu ft
clausius in engineering, a unit of entropy; defined as the ratio of energy to temperature above absolute zero
cleanliness unit unit for measuring air pollution; equal to the number of particles greater than 0.5 mm in diameter per cu ft of air
clo unit of thermal insulation of clothing; standard clothes have insulation of about 1 clo, the warmest have about 4 clo per 2.5 cm/1 in of thickness
clusec unit for measuring the power of a vacuum pump
condensation number in physics, the ratio of the number of molecules condensing on a surface to the number of molecules touching that surface
cord unit for measuring the volume of wood cut for fuel; 1 cord = 3.62 cu m/128 cu ft, or a stack 2.4 m/8 ft long, 1.2 m/4 ft wide and 1.2 m/4 ft high
crith unit of mass for weighing gases; 1 crith = the mass of 1 liter of hydrogen gas at standard temperature and pressure
cubit earliest known unit of length; 1 cubit = approximately 45.7 cm/18 in, the length of the human forearm from the tip of the middle finger to the elbow
curie former unit of radioactivity (symbol Ci); 1 curie = 3.7 × 1010 becquerels
dalton international atomic mass unit, equivalent to 1/12 of the mass of a neutral carbon-12 atom
darcy c.g.s. unit (symbol D) of permeability, used mainly in geology to describe the permeability of rock
darwin unit of measurement of evolutionary rate of change
decontamination factor unit measuring the effectiveness of radiological decontamination; the ratio of original contamination to the radiation remaining
demal unit measuring concentration; 1 demal = 1 gram-equivalent of solute in 1 cu dm of solvent
denier unit used to measure the fineness of yarns; 9,000 m of 15 denier nylon weighs 15 g/0.5 oz
diopter optical unit measuring the power of a lens; the reciprocal of the focal length in meters
dram unit of apothecaries' measure; 1 dram = 60 grains/3.888 g
dyne c.g.s. unit of force; 105 dynes = 1 N
einstein unit unit for measuring photoenergy in atomic physics
eotvos unit unit (symbol E) for measuring small changes in the intensity of the earth's gravity with horizontal distance
erg c.g.s. unit of work; equal to the work done by a force of 1 dyne moving through 1 cm
erlang unit for measuring telephone traffic intensity; for example, 90 minutes of carried traffic measured over 60 minutes = 1.5 erlangs ("carried traffic" refers to the total duration of completed calls made within a specified period)
fathom unit of depth measurement in mining and seafaring; 1 fathom = 1.83 m/6 ft
finsen unit unit (symbol FU) for measuring intensity of ultraviolet light
fluid ounce measure of capacity; equivalent in the United States to 1/16 of a pint (1/20 of a pint in the U.K. and Canada)
foot-candle unit of illuminance, replaced by the lux; 1 foot-candle = 10.76391 lux
foot-pound imperial unit of energy (symbol ft-lb); 1 ft-lb = 1.356 joule
frigorie unit (symbol fg) used in refrigeration engineering to measure heat energy, equal to a rate of heat extraction of 1 kilocalorie per hour
furlong unit of measurement, originating in Anglo-Saxon England, equivalent to 201.168 m/220 yd
galileo unit (symbol Gal) of acceleration; 1 galileo = 10–2 m s–2
gauss c.g.s. unit (symbol Gs) of magnetic flux density, replaced by the tesla; 1 gauss = 1 × 10–4 tesla
gill imperial unit of volume for liquid measure; equal to 1/4 of a pint (in the United States, 4 fl oz/0.118 l; in the U.K., 5 fl oz/0.142 l)
grain smallest unit of mass in the three English systems of measurement (avoirdupois, troy, apothecaries' weights) used in the United States and U.K.; 1 grain = 0.0648 g
hand unit used in measuring the height of a horse from front hoof to shoulder (withers); 1 hand = 10.2 cm/4 in


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Unit Definition
hardness number unit measuring hardness of materials. There are many different hardness scales: Brinell, Rockwell, and Vickers scales measure the degree of indentation or impression of materials; Mohs' scale measures resistance to scratching against a standard set of minerals
hartree atomic unit of energy, equivalent to atomic unit of charge divided by atomic unit of length; 1 hartree = 4.850 × 10–18 J
haze factor unit of visibility in mist or fog; the ratio of brightness of mist compared with that of the object
Hehner number unit measuring concentration of fatty acids in oils; a Hehner number of 1 = 1 kg of fatty acid in 100 kg of oil or fat
hide unit of measurement used in the 12th century to measure land; 1 hide = 60–120 acres/25–50 ha
horsepower imperial unit (abbreviation hp) of power; 1 horsepower = 746 W
hundredweight imperial unit (abbreviation cwt) of mass; 1 cwt = 45.36 kg/100 lb in the United States and 50.80 kg/112 lb in the U.K.
inferno unit used in astrophysics for describing the temperature inside a star; 1 inferno = 1 billion K (degrees Kelvin)
iodine number unit measuring the percentage of iodine absorbed in a substance, expressed as grams of iodine absorbed by 100 grams of material
jansky unit used in radio astronomy to measure radio emissions or flux densities from space; 1 jansky = 10–26 Wm–2 Hz–1. Flux density is the energy in a beam of radiation which passes through an area normal to the beam in a single unit of time. A jansky is a measurement of the energy received from a cosmic radio source per unit area of detector in a single time unit
kayser unit used in spectroscopy to measure wave number (number of waves in a unit length); a wavelength of 1.0 cm has a wave number of 1 kayser
knot unit used in navigation to measure a ship's speed; 1 knot = 1 nautical mile per hour, or about 1.15 miles per hour
league obsolete imperial unit of length; 1 league = 3 nautical mi/5.56 km or 3 statute mi/4.83 km
light-year unit used in astronomy to measure distance; the distance traveled by light in one year, approximately 9.46 × 1012 km/5.88 × 1012 mi
mache obsolete unit of radioactive concentration; 1 mache = 3.7 × 10–7 curies of radioactive material per cu m of a medium
maxwell c.g.s. unit (symbol Mx) of magnetic flux, the strength of a magnetic field in an area multiplied by the area; 1 maxwell = 10–8 weber
megaton measurement of the explosive power of a nuclear weapon; 1 megaton = 1 million tons of trinitrotoluene (TNT)
millimeter of mercury unit of pressure (symbol mmHg) used in medicine for measuring blood pressure
morgan arbitrary unit used in genetics; 1 morgan is the distance along the chromosome in a gene that gives a recombination frequency of 1%
nautical mile unit of distance used in navigation, equal to the average length of 1 minute of arc on a great circle of the earth; 1 international nautical mile = 1.852 km/6,076 ft
neper unit used in telecommunications; gives the attenuation of amplitudes of currents or powers as the natural logarithm of the ratio of the voltage between two points or the current between two points
oersted c.g.s. unit (symbol Oe) of magnetic field strength, now replaced by amperes per meter (1 Oe = 79.58 amp per m)
parsec unit (symbol pc) used in astronomy for distances to stars and galaxies; 1 pc = 3.262 light-years, 2.063 × 105 astronomical units, or 3.086 × 1013 km
peck obsolete unit of dry measure, equal to 8 imperial quarts or 1 quarter bushel (8.1 l in the United States or 9.1 l in the U.K.)
pennyweight imperial unit of mass; 1 pennyweight = 24 grains = 1.555 × 10–3 kg
perch obsolete imperial unit of length; 1 perch = 5 1/2 yards = 5.029 m, also called the rod or pole
point metric unit of mass used in relation to gemstones; 1 point = 0.01 metric carat = 2 × 10–3 g
poise c.g.s. unit of dynamic viscosity; 1 poise = 1 dyne-second per sq cm
poundal imperial unit (abbreviation pdl) of force; 1 poundal = 0.1383 newton
quart imperial liquid or dry measure; in the United States, 1 liquid quart = 0.946 l, while 1 dry quart = 1.101 l; in the U.K., 1 quart =2 pt/1.137 l
rad unit of absorbed radiation dose, replaced in the SI system by the gray; 1 rad = 0.01 joule of radiation absorbed by 1 kg of matter


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  (table continued from previous page)  
Unit Definition
relative biological relative damage caused to living tissue by different types of radiation
rood imperial unit of area; 1 rood = 1/4 acre = 1,011.7 sq m
roentgen unit (symbol R) of radiation exposure, used for X- and gamma rays
rydberg atomic unit of energy; 1 rydberg = 2.425 × 10–18 J
sabin unit of sound absorption, used in acoustical engineering; 1 sabin = absorption of 1 sq ft (0.093 sq m) of a perfectly absorbing surface
scruple imperial unit of apothecaries' measure; 1 scruple = 20 grains = 1.3 × 10–3 kg
shackle unit of length used at sea for measuring cable or chain; 1 shackle = 15 fathoms (27 m/90 ft)
slug obsolete imperial unit of mass; 1 slug = 14.59 kg/32.17 lb
snellen unit expressing the visual power of the eye
sone unit of subjective loudness
standard volume in physics, the volume occupied by 1 kilogram molecule (molecular mass in kilograms) of any gas at standard temperature and pressure; approximately 22.414 cu m
stokes c.g.s. unit (symbol St) of kinematic viscosity; 1 stokes = 10–4 m2 s–1
stone imperial unit (abbreviation st) of mass; 1 stone = 6.35 kg/14 lb
strontium unit measures concentration of strontium-90 in an organic medium relative to the concentration of calcium
tex metric unit of line density; 1 tex is the line density of a thread with a mass of 1 gram and a length of 1 kilometer
tog measure of thermal insulation of a fabric, garment, or quilt; the tog value is equivalent to 10 times the temperature difference (in °C) between the two faces of the article, when the flow of heat across it is equal to 1 W per sq m


  Measurement Chronology  
Measurement Chronology
c. 3500 B.C. The gnomon—the first clock—is invented, probably in Egypt. It consists of a vertical stick or pillar inserted in the ground, the length of its shadow giving an idea of the time.
c. 3000 B.C. The cubit, the length of the arm from the elbow to the extended finger tips, is devised in Egypt as the standard unit of linear measure. A royal cubit of black granite serves as the standard for all other cubit sticks.
c. 2400 B.C. Sumerian scribes develop a calendar consisting of twelve 30-day months (360 days).
1800 B.C. The Babylonian Empire standardizes the year by adopting the lunar calendar of the Sumerian sacred city of Nippur. Previously, each city inserted intercalated months according to its own needs.
c. 1000 B.C. The Hindu calendar is developed in India. It is based on a solar year of 360 days divided into 12 lunar months of 27 or 28 days with a leap month intercalated every 60 months to bring it into line with the true solar year.
c. 975 B.C. The Gezer Calendar is devised. Based on a lunar cycle of 12 months and 354 days, it is tied into the solar year and forms the basis of the Hebrew calendar.
738 B.C. Romulus, traditionally the founder of Rome, devises a lunar calendar with 10 months, 6 of 30 days and 4 of 31 days. The year begins in March and ends in December and is followed by an uncounted winter gap.
c. 600 B.C. The Roman king Tarquinius Priscus introduces the Roman Republican calendar. It consists of 12 months with a total of 355 days. An intercalated month is added between February step with the seasons. Intercalations, however, are made irregularly and it becomes hopelessly confused. The calendar forms the basis of the Gregorian calendar.
587 B.C. The Babylonians introduce their calendar to Jerusalem after their conquest of the city. It provides the Jews with a finite calendar with a New Year's day. The Babylonian names continue to be used.
c. 300 B.C. The Greek scholar Dicaerachus places the first orientation line on a map of the world; it runs through Gibraltar and Rhodes. The idea eventually leads to the system of parallels and meridians, and methods of projecting them.
237 B.C. The Egyptian king Ptolemy III improves the Egyptian calendar by introducing an extra day every four years to the basic 356-day calendar.
c. 140 B.C. The Greek Stoic philosopher Crates of Mallus, in Anatolia, makes the first globe.





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105 B.C. The Chinese historian Sima Qian reforms the Chinese calendar.
46 B.C. The Roman consul and dictator Julius Caesar instructs Alexandrian astronomer Sosigenes to bring the Roman Republican calendar into line with the solar year. He creates the Julian calendar in which the year is 365 days long and begins January 1. An extra day is inserted between February 23 and 24 every four years. The year 46 B.C. is 445 days long to bring it into line with the solar year.
c. 12 B.C. A geographical commentary written by the Roman co-ruler (with Augustus) Marcus Vipsanius Agrippa, based on surveys on Rome's military roads, is used to produce a map of the world.
9 B.C. The first caliper rule for measuring the thickness of objects is made by an anonymous Chinese inventor who inscribes the date of manufacture on it.
c. 800 Harun ar-Rashid presents an elaborate astronomical water-clock, built by the Arab engineer al-Jazari, to the Frankish emperor Charlemagne. The mechanism is driven by falling water, and the clock sounds the time by dropping bronze balls.
1120 The French-born Prior Walcher of Malvern Abbey, England, introduces the measurement of latitude and longitude in degrees, minutes, and seconds.
1326 The English astronomer Richard of Wallingford writes Canones de instrumento, which describes the astronomical clock he built at St. Albans, England, in 1320.
1340 The avoirdupois weight system in introduced in England.
Dec 15, 1582 The Gregorian calendar is adopted in the United Netherlands; the date advances by ten days.
1670 Gabriel Menton, the vicar of the Church of Saint-Paul, Lyon, France, invents the meter as a scientific measure of distance, based on a minute of arc on the earth's surface.
1671 Respected French scientist Jean Picard is among several scientist who take up French clergyman Gabriel Menton's proposal of the meter as a standard measure of distance.
1700 The new Gregorian calendar is introduced in Germany and other Protestant European states, replacing the older, less accurate, Julian calendar.
Dec 10, 1799 The metric system of measurement is officially adopted in France.
1822 The German scientist Friedrich Mohs introduces a scale for specifying the hardness of minerals.
1827 King George IV of Great Britain and Ireland standardizes the acre at 43,560 sq ft (1/640 of a square mile).
1864 The metric system of measurement is officially adopted in England, though it does not supplant the imperial system until the 1970s.
1866 The U.S. government legalizes the metric system of measurement.
May 20, 1875 The Internatinal Bureau of Weights and Measures is established in France by a treaty signed in Paris. Located at Sèvres, its purpose is to unify system of measurement, and to establish standards by providing a prototype meter and kilogram as the basis for all scientific and other measure.
Jan 1, 1876 The International System of Weights and Measures comes into effect in France.
1916 The Royal Society sponsors a Board of Scientific Societies to promote cooperation in pure and applied science and to promote the application of science for the service of Britain.
1927 The International Temperature Scale is adopted by countries subscribing to the International Bureau of Weights and Measures.
1947 The British government sets up the Advisory Committee on Scientific Policy.
1949 The Chinese Academy of Sciences is founded in Shanghai.
Oct 1960 The 11th general Conference on Weights and Measures replace the metric system with the International System (SI) of weights and measure. It redefines the seven basic units of measurement, form which all others are derived, in atomic terms. The meter, for instance is redefine as 1,650,763.73 wavelengths of the orange-red line in the krypton-86 spectrum.
1967 The 13th General Committee on Weights and Measures redefines the second in terms of the resonant frequency of the cesium atom.
1971 In its report of Congress, A Metric America: A Decision Whose Time has Come, the U.S. National Bureau of Standards recommends that the United States change to the International Metric System over a period of ten years.
Dec 11, 1975 The U.S. Congress passes legislation calling for the voluntary conversion to the metric system in ten years.
Oct 20, 1983 The General Conference on Weights and Measures at Sèvres, France, redefines the meter as the distance that light travels through a vacuum in 1/299,792,458 seconds.





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  Angström, Anders Jonas (1814–1874) Swedish astrophysicist who worked in spectroscopy and solar physics. His Recherches sur le spectre solaire (1868) presented an atlas of the solar spectrum with measurements of 1,000 spectral lines expressed in units of one-ten-millionth of a millimeter, the unit which later became the angstrom.  
  Arnold, John (1736–1799) English horologist. He first made his name as the maker of a very small half-quarter repeater for King George III, but later became known for his chronometers.  
  Atwater, Wilbur Olin (1844–1907) U.S. agricultural chemist and educator. As professor of chemistry at Wesleyan University from 1873, he directed the first agricultural experiment station. He was also involved in fertilizer testing and the investigation of nutritional and calorimetric standards.  
  Banneker, Benjamin (1731–1806) American astronomer, surveyor, and mathematician who published almanacs 1792–97. In 1753, having studied only a pocket watch, Banneker constructed a striking clock, the first of its kind in America. He took part in the survey that prepared the establishment of the U.S. capital, Washington, D.C.  
  Bürgi, Jost (1552–1632) Swiss-born clockmaker and mathematician. One of the first clockmakers to use second hands, Bürgi also introduced a mechanism for providing the escapement with a constant driving force. In mathematics, he developed a comprehensive system of logarithms (about 30 years before the Scottish mathematician John Napier), but his work has been largely ignored.  
  Celsius, Anders (1701–1744) Swedish astronomer, physicist, and mathematician who introduced the Celsius temperature scale. In 1742 he presented a proposal to the Swedish Academy of Sciences that all scientific measurements of temperature should be made on a fixed scale based on two invariable (generally speaking) and naturally occurring points. His scale defined 0° as the temperature at which water boils, and 100° as that at which water freezes. This scale, in an inverted form devised eight years later by his pupil Martin Strömer, has since been used in almost all scientific work.  
  Edgeworth, Richard Lovell (1744–1817) Anglo-Irish inventor. He produced an early form of visual telegraphy, the velocipede, the perambulator land-measuring wheel, and various forms of carriage, including a phaeton and a sailpropelled version.  
  Fahrenheit, Gabriel Daniel (1686–1736) Polish-born Dutch physicist who invented the first accurate thermometer in 1724 and devised the Fahrenheit temperature scale. Using his thermometer, Fahrenheit was able to determine the boiling points of liquids and found that they vary with atmospheric pressure. His first thermometers contained a column of alcohol which expanded and contracted directly, as originally devised in 1701 by the Danish astronomer Ole Römer (1644–1710). Fahrenheit substituted mercury for alcohol because its rate of expansion, although less than that of alcohol, is more constant and could be used over a much wider temperature range. To reflect the greater sensitivity of his thermometer, Fahrenheit expanded Romer's scale so that blood heat was 90° and an ice-salt mixture was 0°; on this scale freezing point was 30°. Fahrenheit later adjusted the scale to ignore body temperature as a fixed point so that the boiling point of water came to 212° and freezing point was 32°. This is the Fahrenheit scale that is still in use today.  
  Harrison, John (1693–1776) English horologist and instrumentmaker. He made the first chronometers that were accurate enough to allow the precise determination of longitude at sea, and so permit reliable and safe navigation over long distances.  
  Ohm, Georg Simon (1789–1854) German physicist who studied electricity and discovered the fundamental law that bears his name. The SI unit of electrical resistance, the ohm, is named for him, and the unit of conductance (the inverse of resistance) was formerly called the mho, which is "ohm" spelled backward.  
  Vernier, Pierre (c. 1580–1637) French engineer and instrumentmaker who invented a means of making very precise measurements with what is now called the vernier scale. He realized the need for a more accurate way of reading angles on the surveying instruments he used in mapmaking while working as a military engineer. In 1631 he published The Construction, Uses and Properties of a New Mathematical Quadrant, in which he explained his method.  
  absolute weight
the weight of a body considered apart from all modifying influences such as the atmosphere. To determine its absolute weight, the body must, therefore, be weighed in a vacuum or allowance must be made for buoyancy.
apparatus, either mechanical or electromechanical, for measuring acceleration or deceleration—that is, the rate of increase or decrease in the velocity of a moving object.
  acoustic ohm
c.g.s. unit of acoustic impedance (the ratio of the sound pressure on a surface to the sound flux through the surface). It is analogous to the ohm as the unit of electrical impedance.
unit sometimes used to measure large volumes of water, such as the capacity of a reservoir (equal to its area in acres multiplied by its average depth in feet). One acre foot equals 1,233.5 cu m/43,560 cu ft or the amount of water covering one acre to a depth of one foot.
measurement of height, usually given in meters above sea level.




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unit (symbol Å) of length equal to 10
–10 meters or one-ten-millionth of a millimeter, used for atomic measurements and the wavelengths of electromagnetic radiation. It is named for the Swedish astrophysicist Anders Ångström.
abbreviation for American National Standards Institute, a U.S. national standards body.
  apothecaries' weights
obsolete units of mass, formerly used in pharmacy: 20 grains equal one scruple; three scruples equal one dram; eight drams equal an apothecary's ounce (oz apoth.), and 12 such ounces equal an apothecary's pound (lb apoth.). There are 7,000 grains in one pound avoirdupois (0.454 kg).
metric unit of area, equal to 100 square meters (119.6 sq yd); 100 ares make one hectare.
  astronomical unit
(symbol AU) unit equal to the mean distance of the earth from the sun: 149,597,870 km/ 92,955,800 mi. It is used to describe planetary distances. Light travels this distance in approximately 8.3 minutes.
  atmosphere, or standard atmosphere,
in physics, a unit (symbol atm) of pressure equal to 760 torr, 1013.25 millibars, or 1.01325 × 10
5 newtons per square meter. The actual pressure exerted by the atmosphere fluctuates around this value, which is assumed to be standard at sea level and 0°C/32°F, and is used when dealing with very high pressures.
  atomic time
time as given by atomic clocks, which are regulated by natural resonance frequencies of particular atoms, and display a continuous count of seconds.
  Avogadro's number, or Avogadro's constant,
the number of carbon atoms in 12 g of the carbon-12 isotope (6.022045 × 10
23). The relative atomic mass of any element, expressed in grams, contains this number of atoms. It is named for the Italian physicist Amedeo Avogadro (1776–1856).
system of units of mass based on the pound (0.45 kg), which consists of 16 ounces (each of 16 drams) or 7,000 grains (each equal to 65 mg).
unit of pressure equal to 10
5 pascals or 106 dynes/cm2, approximately 750 mmHg or 0.987 atm. Its diminutive, the millibar (one-thousandth of a bar), is commonly used by meteorologists.
unit of liquid capacity, the value of which depends on the liquid being measured. It is used for petroleum, a barrel of which contains 159 liters/58 U.S. gallons; a barrel of alcohol contains 189 liters/69 U.S. gallons.
in engineering, a unit of electrical signaling speed equal to one pulse per second, measuring the rate at which signals are sent between electronic devices such as telegraphs and computers; 300 baud is about 300 words a minute.
SI unit (symbol Bq) of radioactivity, equal to one radioactive disintegration (change in the nucleus of an atom when a particle or ray is given off) per second.
unit of sound measurement equal to ten decibels. It is named for the Scottish-born U.S. scientist Alexander Graham Bell (1847–1922).
nautical term applied to half-hours of watch. A day is divided into seven watches, five of four hours each and two, called dogwatches, of two hours. Each half-hour of each watch is indicated by the striking of a bell, eight bells signaling the end of the watch.
sensitive thermometer that measures the energy of radiation by registering the change in electrical resistance of a fine wire when it is exposed to heat or light. It was devised in 1880 by the U.S. astronomer Samuel Langley (1834–1906) for measuring radiation from stars.
unit (symbol B) for measuring the reaction of optical materials to stress, defined in terms of the slowing down of light passing through the material when it is stretched or compressed.
  British Standards Institution (BSI)
U.K. national standards body. Although government funded, the institution is independent. The BSI interprets international technical standards for the U.K., and also sets its own.
  British thermal unit
imperial unit (symbol Btu) of heat, now replaced in the SI system by the c0016-01.gifjoule (one British thermal unit is approximately 1,055 joules). Burning one cubic foot of natural gas releases about 1,000 Btu of heat.
abbreviation for c0016-01.gifBritish Standards Institution.
dry or liquid measure equal to eight gallons or four pecks (2,219.36 cu in/36.37 liters) in the U.K.; some U.S. states have different standards according to the goods measured.
symbol for degrees Celsius, sometimes called centigrade.
unit of length, used on ships, originally the length of a ship's anchor cable or 120 fathoms (219 m/720 ft), but now taken as one-tenth of a nautical mile (185.3 m/608 ft).
division of the year into months, weeks, and days and the method of ordering the years. From year one, an assumed date of the birth of Jesus, dates are calculated backwards (
B.C. "before Christ" or B.C.E. "before common era") and forwards (A.D., Latin anno Domini "in the year of the Lord," or C.E. "common era''). The lunar month (period between one new Moon and the next) naturally averages 29.5 days, but the Western calendar uses for convenience a calendar month with a complete number of days, 30 or 31 (February has 28). For adjustments, since there are slightly fewer than six extra hours a year left over, they are added to February as a 29th day every fourth year (leap year), century years being excepted unless they are divisible by 400. For example, 1896 was a leap year; 1900 was not.
SI unit (symbol cd) of luminous intensity, which replaced the old units of candle and standard candle. It measures the brightness of a light itself rather than the amount of light falling on an object, which is called illuminance and measured in c0016-01.giflux.
symbol for cubic centimeter.
former name for the Celsius temperature scale.
instrument for measuring time precisely, originally used at sea. It is designed to remain accurate through all conditions of temperature and pressure. The first accurate marine chronometer, capable of an accuracy of half a minute a year, was made in 1761 by the English horologist and instrumentmaker John Harrison.




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in engineering, a unit of entropy (the loss of energy as heat in any physical process). It is defined as the ratio of energy to temperature above absolute zero.
  cleanliness unit
unit for measuring air pollution: the number of particles greater than 0.5 micrometers in diameter per cubic foot of air. A more usual measure is the weight of contaminants per cubic meter of air.
symbol for centimeter.
  comfort index
estimate of how tolerable conditions are for humans in hot climates. It is calculated as the temperature in degrees Fahrenheit plus a quarter of the relative humidity, expressed as a percentage. If the sum is less than 95, conditions are tolerable for those unacclimatized to the tropics.
  condensation number
in physics, the ratio of the number of molecules condensing on a surface to the total number of molecules touching that surface.
unit for measuring the volume of wood cut for fuel. One cord equals 128 cubic feet (3.456 cubic meters), or a stack 8 feet (2.4 m) long, 4 feet (1.2 m) wide, and 4 feet high.
SI unit (symbol C) of electrical charge. One coulomb is the quantity of electricity conveyed by a current of one ampere in one second.
abbreviation for cubic (measure).
  cubic decimeter
metric measure (symbol dm
3) of volume corresponding to the volume of a cube whose edges are all 1 dm (10 cm) long; it is equivalent to a capacity of one liter.
  cubit earliest known unit of length, which originated between 2800 and 2300 B.C.. It is approximately 50.5 cm/20.6 in long, which is about the length of the human forearm measured from the tip of the middle finger to the elbow.  
  curie former unit (symbol Ci) of radioactivity, equal to 3.7 × 1010 c0016-01.gifbecquerels. One gram of radium has a radioactivity of about one curie. It was named for the French physicist Pierre Curie (1859–1906).  
symbol for c0016-01.gifhundredweight, a unit of weight equal to 112 pounds (50.802 kg); 100 lb (45.36 kg) in the United States.
  decontamination factor
in radiological protection, a measure of the effectiveness of a decontamination process. It is the ratio of the original contamination to the remaining radiation after decontamination: 1,000 and above is excellent; 10 and below is poor.
in mathematics, a unit (symbol °) of measurement of an angle or arc. A circle or complete rotation is divided into 360º. A degree may be subdivided into 60 minutes (symbol '), and each minute may be subdivided in turn into 60 seconds (symbol "). Temperature is also measured in degrees, which are divided on a decimal scale.
unit used in measuring the fineness of yarns, equal to the mass in grams of 9,000 meters of yarn. Thus 9,000 meters of 15 denier nylon, used in nylon stockings, weighs 15 g/0.5 oz, and in this case the thickness of thread would be 0.00425 mm/0.0017 in. The term is derived from the French silk industry; the denier was an old French silver coin.
(abbreviation for Deutsches Institut für Normung) German national standards body, which has set internationally accepted standards for (among other things) paper sizes and electrical connectors.
optical unit in which the power of a lens is expressed as the reciprocal of its focal length in meters. The usual convention is that convergent lenses are positive and divergent lenses negative. Short-sighted people need lenses of power about –0.7 diopter; a typical value for long sight is about +1.5 diopter.
abbreviation for Daylight Saving Time.
c.g.s. unit (symbol dyn) of force. 10
5 dynes make one c0016-01.gifnewton. The dyne is defined as the force that will accelerate a mass of one gram by one centimeter per second per second.
  electron volt
unit (symbol eV) for measuring the energy of a charged particle (ion or electron) in terms of the energy of motion an electron would gain from a potential difference of one volt. Because it is so small, more usual units are mega(million) and giga- (billion) electron volts (MeV and GeV).
  eotvos unit
unit (symbol E) for measuring small changes in the intensity of the earth's gravity with horizontal distance.
c.g.s. unit of work, replaced in the SI system by the c0016-01.gifjoule. One erg of work is done by a force of one dyne moving through one centimeter.
abbreviation for Eastern Standard Time.
symbol for degrees Fahrenheit.
SI unit (symbol F) of electrical capacitance (how much electric charge a capacitor can store for a given voltage). One farad is a capacitance of one c0016-01.gifcoulomb per volt. For practical purposes the microfarad (one millionth of a farad, symbol (F) is more commonly used.
unit of electrical charge equal to the charge on one mole of electrons. Its value is 9.648 × 10
4 c0016-01.gifcoulombs.
unit of depth measurement (1.83 m/6 ft) that is approximately the distance between an adult man's hands when the arms are outstretched.
SI unit of time. It is 10
–15 seconds (one millionth of a billionth).
unit of length equal to 10
–15m, used in atomic and nuclear physics. The unit is named for the Italian-born U.S. physicist Enrico Fermi (1901–1954).
  finsen unit
unit (symbol FU) for measuring the intensity of ultraviolet (UV) light; for instance, UV light of 2 FUs causes sunburn in 15 minutes.
unit of illuminance, now replaced by the c0016-01.giflux. One foot-candle is the illumination received at a distance of one foot from an international candle. It is equal to 10.764 lux.
imperial unit of energy (ft-lb), defined as the work done when a force of one pound moves through a distance of one foot. It has been superseded for scientific work by the c0016-01.gifjoule: one foot-pound equals 1.356 joule.
  f.p.s. system
system of units based on the foot, pound, and second as units of length, mass, and time, respectively. It has now been replaced for scientific work by the SI system.
symbol for c0016-01.giffoot, a measure of distance.




Page 18
unit of measurement, originating in Anglo-Saxon England, equivalent to 220 yd (201.168 m).
symbol for gram.
symbol for gallon, galileo.
unit (symbol gal) of acceleration, used in geological surveying. One galileo is 10
–2 meters per second per second. The earth's gravitational field often differs by several milligals (thousandths of gals) in different places, because of the varying densities of the rocks beneath the surface.
any scientific measuring instrument—for example, a wire gauge or a pressure gauge. The term is also applied to the width of a railroad or tramway track.
c.g.s. unit (symbol Gs) of magnetic induction or magnetic flux density, replaced by the SI unit the c0016-01.giftesla, but still commonly used. It is equal to one line of magnetic flux per square centimeter. The earth's magnetic field is about 0.5 Gs, and changes over time are measured in gammas (one gamma equals 10
–5 gauss).
prefix signifying multiplication by 10
9 (1,000,000,000 or 1 billion), as in gigahertz, a unit of frequency equivalent to 1 billion hertz.
imperial unit of volume for liquid measure, equal to onehalf cup or one-quarter of a pint or 4 fluid ounces. It is used in selling alcoholic drinks. In the U.K., the gill is equal to 5 fluid ounces (0.142 liters).
abbreviation for c0016-01.gifGreenwich Mean Time.
smallest unit of mass in the three English systems (avoirdupois, troy, and apothecaries' weights) used in the United States and U.K., equal to 0.0648 g. It was reputedly the weight of a grain of wheat. One pound avoirdupois equals 7,000 grains; one pound troy or apothecaries' weight equals 5,760 grains.
SI unit (symbol Gy) of absorbed radiation dose. It replaces the c0016-01.gifrad (1 Gy equals 100 rad), and is defined as the dose absorbed when one kilogram of matter absorbs one joule of ionizing radiation. Different types of radiation cause different amounts of damage for the same absorbed dose; the SI unit of dose equivalent is the c0016-01.gifsievert.
  Greenwich Mean Time (GMT)
local time on the zero line of longitude (the Greenwich meridian), which passes through the Old Royal Observatory at Greenwich, London. It was replaced in 1986 by coordinated universal time (UTC), but continued to be used to measure longitudes and the world's standard time zones.
scale for measuring force by comparing it with the force due to gravity (g), often called g-force.
symbol for hectare.
unit used in measuring the height of a horse from front hoof to shoulder (withers). One hand equals 10.2 cm/4 in.
  haze factor
unit of visibility in mist or fog. It is the ratio of the brightness of the mist compared with that of the object.
SI unit (symbol H) of inductance (the reaction of an electric current against the magnetic field that surrounds it). One henry is the inductance of a circuit that produces an opposing voltage of one volt when the current changes at one ampere per second.
SI unit (symbol Hz) of frequency (the number of repetitions of a regular occurrence in one second). Radio waves are often measured in megahertz (MHz), millions of hertz, and the clock rate of a computer is usually measured in megahertz. The unit is named for the German physicist Heinrich Hertz (1857–1894).
  hide or hyde,
Anglo-Saxon unit of measurement used to measure the extent of arable land; it varied from about 296 ha/120 acres in the east of England to as little as 99 ha/40 acres in Wessex. One hide was regarded as sufficient to support a peasant and his household; it was the area that could be plowed in a season by one plow and one team of oxen.
(hp) imperial unit (abbreviation hp) of power, now usually replaced by the watt.
imperial unit (abbreviation cwt) of mass, equal to 112 lb (50.8 kg). It is sometimes called the long hundredweight, to distinguish it from the short hundredweight, or cental, equal to 100 lb (45.4 kg).
abbreviation for Institute of Electrical and Electronic Engineers, the U.S. institute which sets technical standards for electrical equipment and computer data exchange.
  inch (in)
imperial unit of linear measure, a twelfth of a foot, equal to 2.54 centimeters.
in astrophysics, a unit for describing the temperature inside a star. One inferno is I billion K, or approximately 1 billion °C.
  international biological standards
drugs (such as penicillin and insulin) of which the activity for a specific mass (called the international unit, or IU), prepared and stored under specific conditions, serves as a standard for measuring doses. For penicillin, one IU is the activity of 0.0006 mg of the sodium salt of penicillin, so a dose of a million units would be 0.6 g.
  International Organization for Standardization (ISO)
international organization founded in 1947 to standardize technical terms, specifications, units, and so on. Its head-quarters are in Geneva, Switzerland.
unit of radiation received from outer space, used in radio astronomy. It is equal to 10
–26 watts per square meter per hertz, and is named for the U.S. radio engineer Karl Jansky (1905–1950).
SI unit (symbol J) of work and energy, replacing the calorie (one joule equals 4.2 calories)
symbol for kilo-, as in kg (kilogram) and km (kilometer).
symbol for kelvin, a scale of temperature.
unit of wave number (number of waves in a unit length), used in spectroscopy. It is expressed as waves per centimeter, and is the reciprocal of the wavelength. A wavelength of 0.1 cm has a wave number of 10 kaysers.
symbol for kilocalorie.
prefix denoting multiplication by 1,000, as in kilohertz, a unit of frequency equal to 1,000 hertz.
unit (symbol kW) of power equal to 1,000 watts or about 1.34 horsepower.
commercial unit of electrical energy (symbol kWh), defined as the work done by a power of 1,000 watts in one hour and equal to 3.6 megajoules. It is used to




Page 19
  calculate the cost of electrical energy taken from the domestic supply.  
in navigation, unit by which a ship's speed is measured, equivalent to one c0016-01.gifnautical mile per hour (one knot equals about 1.15 miles per hour). It is also sometimes used in aviation.
  kph, or km/h,
symbol for kilometers per hour.
symbol for c0016-01.gifkilowatt.
symbol for liter, a measure of liquid volume.
unit of luminance (the light shining from a surface), equal to one c0016-01.giflumen per square centimeter. In scientific work the c0016-01.gifcandela per square meter is preferred.
  light watt
unit of radiant power (brightness of light). One light watt is the power required to produce a perceived brightness equal to that of light at a wavelength of 550 nanometers and 680 lumens.
SI unit (symbol lm) of luminous flux (the amount of light passing through an area per second).
SI unit (symbol lx) of illuminance or illumination (the light falling on an object). It is equivalent to one c0016-01.giflumen per square meter or to the illuminance of a surface one meter distant from a point source of one c0016-01.gifcandela.
c.g.s. unit (symbol Mx) of magnetic flux (the strength of a magnetic field in an area multiplied by the area). It is now replaced by the SI unit, the c0016-01.gifweber (one maxwell equals 10
–8 weber).
prefix denoting multiplication by a million. For example, a megawatt (MW) is equivalent to a million watts.
one million (10
6) tons. Used with reference to the explosive power of a nuclear weapon, it is equivalent to the explosive force of one million tons of trinitrotoluene (TNT).
symbol for milligram.
SI unit of electrical conductance, now called the c0016-01.gifsiemens; equivalent to a reciprocal ohm.
symbol for mile.
prefix (symbol p) denoting a one-millionth part (10
–6). For example, a micrometer, µm, is one-millionth of a meter.
one-millionth of a meter (symbol µm).
name for the micrometer, one-millionth of a meter.
prefix (symbol m) denoting a one-thousandth part (10
–3). For example, a millimeter, mm, is one-thousandth of a meter.
unit of pressure, equal to one-thousandth of a c0016-01.gifbar.
one-thousandth of a liter (ml), equivalent to one cubic centimeter (cc).
  millimeter of mercury
unit of pressure (symbol mmHg), used in medicine for measuring blood pressure defined as the pressure exerted by a column of mercury one millimeter high, under the action of gravity.
symbol for c0016-01.gifmilliliter.
symbol for c0016-01.gifmillimeter of mercury.
  molar volume
volume occupied by one mole (the molecular mass in grams) of any gas at standard temperature and pressure, equal to 2.24136 x 10
–2 m3.
prefix used in SI units of measurement, equivalent to a one-billionth part (10
–9). For example, a nanosecond is onebillionth of a second.
  nautical mile
unit of distance used in navigation, an internationally agreed-on standard (since 1959) equaling the average length of one minute of arc on a great circle of the earth, or 1,852 m/6,076.12 ft. The term formerly applied to various units of distance used in navigation.
SI unit (symbol N) of force. One newton is the force needed to accelerate an object with mass of one kilogram by one meter per second per second. The weight of a medium size (100 g/3 oz) apple is one newton.
c.g.s. unit (symbol Oe) of magnetic field strength, now replaced by the SI unit ampere per meter. The earth's magnetic field is about 0.5 oersted; the field near the poles of a small bar magnet is several hundred oersteds; and a powerful electromagnet can have a field strength of 30,000 oersteds.
SI unit (symbol
W) of electrical resistance (the property of a conductor that restricts the flow of electrons through it).
(oz) unit of mass, one-sixteenth of a pound avoirdupois, equal to 437.5 grains (28.35 g); also one-twelfth of a pound troy, equal to 480 grains.
standard European sizes for paper, designated by a letter (A, B, or C) and a number (0–6). The letter indicates the size of the basic sheet at manufacture; the number is how many times it has been folded. A4 is obtained by folding an A3 sheet, which is half an A2 sheet, in half, and so on.
in astronomy, a unit (symbol pc) used for distances to stars and galaxies. One parsec is equal to 3.2616 light-years, 2.063 × 10
5 c0016-01.gifastronomical units, and 3.086 × 1013 km.
SI unit (symbol Pa) of pressure, equal to one c0016-01.gifnewton per square meter. It replaces c0016-01.gifbars and millibars (10
5 Pa equals one bar). It is named for the French mathematician Blaise Pascal (1623–1662).
small portable instrument for counting the number of steps taken, and measuring the approximate distance covered by a person walking. Each step taken by the walker sets in motion a swinging weight within the instrument, causing the mechanism to rotate, and the number of rotations are registered on the instrument face.
unit of loudness, equal to the value in decibels of an equally loud tone with frequency 1,000 Hz. The higher the frequency, the louder a noise sounds for the same decibel value; thus an 80-decibel tone with a frequency of 20 Hz sounds as loud as 20 decibels at 1,000 Hz, and the phon value of both tones is 20. An aircraft engine has a loudness of around 140 phons.
  Planck's constant
in physics, a fundamental constant (symbol h) that relates the energy (E) of one quantum of electromagnetic radiation (the smallest possible "packet" of energy; see quantum theory) to the frequency (f) of its radiation by E = hf. Its value is 6.6261 x 10
–34 joule seconds.




Page 20
simple integrating instrument for measuring the area of a regular or irregular plane surface. It consists of two hinged arms: one is kept fixed and the other is traced around the boundary of the area. This actuates a small graduated wheel; the area is calculated from the wheel's change in position.
c.g.s. unit (symbol P) of dynamic viscosity (the property of liquids that determines how readily they flow). It is equal to one dyne-second per square centimeter. For most liquids the centipoise (one-hundredth of a poise) is used. Water at 20°C/68°F has a viscosity of 1.002 centipoise.
imperial unit (abbreviation pdl) of force, now replaced in the SI system by the c0016-01.gifnewton. One poundal equals 0.1383 newtons.
symbol for pint.
imperial liquid or dry measure, equal to two pints. In the United States, a liquid quart is equal to 0.946 liter or 32 ounces, while a dry quart is equal to 1.101 liters. In the U.K., the liquid or dry quart is equal to 1.136 liters.
unit of absorbed radiation dose, now replaced in the SI system by the c0016-01.gifgray (one rad equals 0.01 gray), but still commonly used. It is defined as the dose when one kilogram of matter absorbs 0.01 joule of radiation energy (formerly, as the dose when one gram absorbs 100 ergs).
SI unit (symbol rad) of plane angles, an alternative unit to the c0016-01.gifdegree. It is the angle at the center of a circle when the center is joined to the two ends of an arc (part of the circumference) equal in length to the radius of the circle.
  radiation units
units of measurement for radioactivity and radiation doses. In SI units, the activity of a radioactive source is measured in becquerels (symbol Bq), where one becquerel is equal to one nuclear disintegration per second (an older unit is the curie). The exposure is measured in coulombs per kilogram (C kg
–1); the amount of ionizing radiation (X-rays or gamma rays) which produces one coulomb of charge in one kilogram of dry air (replacing the roentgen). The absorbed dose of ionizing radiation is measured in grays (symbol Gy) where one gray is equal to one joule of energy being imparted to one kilogram of matter (the rad is the previously used unit). The dose equivalent, which is a measure of the effects of radiation on living organisms, is the absorbed dose multiplied by a suitable factor which depends upon the type of radiation. It is measured in sieverts (symbol Sv), where one sievert is a dose equivalent of one joule per kilogram (an older unit is the rem).
  rationalized units
units for which the defining equations conform to the geometry of the system. Equations involving circular symmetry contain the factor 2
p; those involving spherical symmetry 4p. SI units are rationalized, c.g.s. units are not.
  relative biological effectiveness (RBE)
the relative damage caused to living tissue by different types of radiation. Some radiations do much more damage than others; alpha particles, for example, cause 20 times as much destruction as electrons (beta particles).
acronym of roentgen equivalent man unit of radiation dose equivalent.
unit of fluidity equal to the reciprocal of the c0016-01.gifpoise.
abbreviation for roentgen–hour–meter, the unit of effective strength of a radioactive source that produces gamma rays. It is used for substances for which it is difficult to establish radioactive disintegration rates.
  roentgen, or röntgen,
unit (symbol R) of radiation exposure, used for X-rays and gamma rays. It is defined in terms of the number of ions produced in one cubic centimeter of air by the radiation. Exposure to 1,000 roentgens gives rise to an absorbed dose of about 870 rads (8.7 grays), which is a dose equivalent of 870 rems (8.7 sieverts).
  Rydberg constant
in physics, a constant that relates atomic spectra to the spectrum of hydrogen. Its value is 1.0977 × 10
7 per meter.
unit of sound absorption, used in acoustical engineering. One sabin is the absorption of one square foot (0.093 square meter) of a perfectly absorbing surface (such as an open window).
  Saffir-Simpson damage-potential scale
scale of potential damage from wind and sea when a hurricane is in progress: 1 is minimal damage, 5 is catastrophic.
  sec or s
abbreviation for second, a unit of time.
obsolete unit of length, used at sea for measuring cable or chain. One shackle is 15 fathoms (27 m/90 ft).
abbreviation for Système International d'Unités (French "International System of Metric Units").
SI unit (symbol S) of electrical conductance, the reciprocal of the resistance of an electrical circuit. One siemens equals one ampere per volt. It was formerly called the mho or reciprocal ohm.
SI unit (symbol Sv) of radiation dose equivalent. It replaces the rem (1 Sv equals 100 rem). Some types of radiation do more damage than others for the same absorbed dose—for example, an absorbed dose of alpha radiation causes 20 times as much biological damage as the same dose of beta radiation. The equivalent dose in sieverts is equal to the absorbed dose of radiation in grays multiplied by the relative biological effectiveness. Humans can absorb up to 0.25 Sv without immediate ill effects; 1 Sv may produce radiation sickness; and more than 8 Sv causes death.
  speed of light
speed at which light and other electromagnetic waves travel through empty space. Its value is 299,792,458 m/186,281 mi per second. The speed of light is the highest speed possible, according to the theory of relativity, and its value is independent of the motion of its source and of the observer. It is impossible to accelerate any material body to this speed because it would require an infinite amount of energy.
  standard atmosphere
alternative term for c0016-01.gifatmosphere, a unit of pressure.
  standard gravity
acceleration due to gravity, generally taken as 9.81274 m/32.38204 ft per second per second. See also c0016-01.gifg-scale.
  standard illuminant
any of three standard light intensities, A, B, and C, used for illumination when phenomena involving color are measured. A is the light from a filament at 2,848K (2,575°C/4,667°F), B is noon sunlight, and C is normal daylight. B and C are defined with respect to A.




Page 21
  Standardization is necessary because colors appear different when viewed in different lights.  
volume in physics, the volume occupied by one kilogram molecule (the molecular mass in kilograms) of any gas at standard temperature and pressure. Its value is approximately 22.414 cubic meters.
SI unit (symbol sr) of measure of solid (threedimensional) angles, the three-dimensional equivalent of the c0016-01.gifradian. One steradian is the angle at the center of a sphere when an area on the surface of the sphere equal to the square of the sphere's radius is joined to the center.
c.g.s. unit (symbol St) of kinematic viscosity (a liquid's resistance to flow).
  Système International d'Unités
official French name for SI units.
symbol for metric ton and ton.
SI unit (symbol T) of magnetic flux density. One tesla represents a flux density of one c0016-01.gifweber per square meter, or 10
4 c0016-01.gifgauss. It is named for the Yugoslavian-born U.S. physicist and electrical engineer Nikola Tesla (1856–1943).
unit of energy defined as 10
5 British thermal units; equivalent to 1.055 × 108 J. It is no longer in scientific use.
unit of pressure equal to 1/760 of an c0016-01.gifatmosphere, used mainly in high-vacuum technology.
system system of units used for precious metals and gems. The pound troy (0.37 kg) consists of 12 ounces (each of 120 carats) or 5,760 grains (each equal to 65 mg).
abbreviation for coordinated universal time, the standard measurement of time.
device for taking readings on a graduated scale to a fraction of a division. It consists of a short divided scale that carries an index or pointer and is slid along a main scale. It was invented by the French engineer and instrumentmaker Pierre Vernier.
SI unit (symbol Wb) of magnetic flux (the magnetic field strength multiplied by the area through which the field passes). It is named for the German chemist Wilhelm Weber (1804–1891). One weber equals 10
8 c0016-01.gifmaxwells.
  wind-chill factor, or wind-chill index,
estimate of how much colder it feels when a wind is blowing. It is the sum of the temperature (in °F below zero) and the wind speed (in miles per hour). So for a wind of 15 mph at an air temperature of –5°F, the wind-chill factor is 20.
  Zhubov scale
scale for measuring ice coverage, developed in the USSR. The unit is the ball; one ball is 10% coverage, two balls 20%, and so on.
  Further Reading  
  Adam, Barbara E. Time and Social Theory (1990)  
  Cooper, Frank Sundials (1969)  
  Fraser, J. T. Time the Familiar Stranger (1987)  
  Gell, Alfred The Anthropology of Time: Cultural Construction of Temporal Maps and Images (1992)  
  Hall, A. Rupert The Revolution in Science 1500–1750 (1983)  
  Howse, Derek Greenwich Time and the Discovery of Longitude (1980)  
  Mann, Wilfrid Basil Radioactivity and Its Measurement (1966)  
  Mendelssohn, K. The Quest for Absolute Zero: Meaning of Low Temperature Physics (1977)  
  O'Neil, W. M. Time and the Calendars (1975)  
  Ronan, Colin The Cambridge Illustrated History of the World's Science (1983)  
  Sethares, William A. Tuning, Timbre, Spectrum, Scale (1998)  
  Smith, C., and Wise, M. N. Energy and Empire (1989)  
  Taylor, E. G. R. The Haven Finding Art A History of Navigation from Odysseus to Captain Cook (1971)