ENGINEERING UNITS 1.5

TABLE 1.4 SI Prefixes

Factor Prefix Symbol

1018 exa E1015 peta P1012 tera T109 giga G106 mega M103 kilo k102 hecto* h10 deka* da10�1 deci* d10�2 centi c10�3 milli m10�6 micro �10�9 nano n10�12 pico p10�15 femto f10�18 atto a

* Generally to be avoided.Source: From Rohsenow, Hartnett, and

Ganic.2

Weights and Measures (now the National Bureau of Standards), by executive order,fixed the values of the U.S. yard and pound in terms of the meter and kilogram,respectively, as 1 yd 3600/3937 m and 1 lb � 0.453 592 4277 kg. By agreementin 1959 among the national standards laboratories of the English-speaking nations,the relations in use now are: 1 yd � 0.9144 m, whence 1 in � 25.4 mm exactly;and 1 lb � 0.453 592 37 kg, or 1 lb � 453.59 g (nearly).

4. English Units. The base units for the English engineering units are given inTable 1.6 (third column). The unit of force in English units is the pound force (lbf).However, the use of the pound mass (lb) and pound force in engineering workcauses considerable confusion in the proper use of these two fundamentally differ-ent units. A clear understanding of the units of mass and force can be gained byexamining Newton’s second law of motion. With any system of units, a conversionfactor gc must be introduced into the newtonian dynamics equation so that bothsides of the equation will have the same units. Thus,

maF �

gc

in which the numerical value and units of gc depend on the units chosen for mass,force, length, and time.

The units of pound mass and pound force are related by the standard gravita-tional acceleration, which has a value of 32.174 ft /s2. When a 1-lb mass is held ata location on the earth’s surface where the gravitational acceleration is 32.174ft /s2, the mass weighs 1 lbf. With this system of units, the value of gc is determinedas follows:

(continues on page 1.16)

5-68 CRC Handbook of Engineering Tables

Prefix Names of Multiples and Submultiples of Units

Definitions of the Most Important International System (SI) Units

The ampere (unit of electric current) is the constant current that, if maintained in two straight parallel conductorsof infinite length, of negligible circular sections, and placed 1 meter apart in a vacuum, will produce between theseconductors a force equal to 2 ¥ 10–7 newton per meter of length.

The candela is the luminous intensity, in the direction of the normal, of a blackbody surface 1/600,000 squaremeter in area, at the temperature of solidification of platinum under a pressure of 101,325 newtons per square meter.

The coulomb (unit of quantity of electricity) is the quantity of electricity transported in 1 second by a currentof 1 ampere.

The ephemeris second (unit of time) is exactly 1/31 556 925.974 7 of the tropical year of 1900, January, 0 days,and 12 hours ephemeris time.

The fraud (unit of electric capacitance) is the capacitance of a capacitor between the plates of which there appearsa difference of potential of 1 volt when it is charged by a quantity of electricity equal to 1 coulomb.

The henry (unit of electric inductance) is the inductance of a closed circuit in which an electromotive force of 1volt is produced when the electric current in the circuit varies uniformly at a rate of 1 ampere per second.

The International Practical Kelvin Temperature Scale of 1960 and the International Practical Celsius TemperatureScale of 1960 are defined by a set of interpolation equations based on the following reference temperatures:

The joule (unit of energy) is the work done when the point of application of 1 newton is displaced a distance of1 meter in the direction of the force.

The kelvin (unit of thermodynamic temperature) is the fraction 1/273.16 of the thermodynamic temperature ofthe triple point of water. The decision was made at the 13th General Conference on Weights and Measures onOctober 13, 1967, that the name of the unit of thermodynamic temperature would be changed from degree Kelvin(symbol: ˚K) to kelvin (symbol: K). The name (kelvin) and symbol (K) are to be used for expressing temperatureintervals. The former convention that expressed a temperature interval in degrees Kelvin or, abbreviated, deg K isdropped. However, the old designations are acceptable temporarily as alternatives to the new ones. One may alsoexpress temperature intervals in degrees Celsius.

International System (SI) Metric Units (continued)

Decimal Equivalent Prefix Pronunciation Symbol Exponential Expression

1,000,000,000,000 tera ter¢a· T 10+12

1,000,000,000 giga ji¢ga· G 10+9

1,000,000 mega meg¢a· M 10+6

1,000 kilo kil¢o– k 10+3

100 hecto hek¢to– h 10+2

10 deka dek¢a· da 100.1 deci des¢i d 10–1

0.01 centi sent¢i c 10–2

0.001 milli mil¢i m 10–3

0.000 001 micro m–i¢kro– m 10–6

0.000 000 001 nano nan¢o– n 10–9

0.000 000 000 001 pico pe¢ko– p 10–12

0.000 000 000 000 001 femto fem¢to– f 10–15

0.000 000 000 000 000 001 atto at¢to– a 10–18

K Deg C

Oxygen, liquid-gas equilibrium 90.18 –182.97Water, solid-liquid equilibrium 273.15 0.00Water, solid-liquid-gas equilibrium 273.16 0.01Water, liquid-gas equilibrium 373.15 100.00Zinc, solid-liquid equilibrium 692.655 419.505Sulfur, liquid-gas equilibrium 717.75 444.6Silver, solid-liquid equilibrium 1233.95 960.8Gold, solid-liquid equilibrium 1336.15 1063.0

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