Geologia

Appendix A: SI Units
` ´ ‘‘SI’’ is an abbreviation for Le Systeme International d’Unites, an international system of units adopted by many national and international authorities, associations, professional societies, and agencies. SI is closely related to but not identical with the former cgs, mks, and mksa systems of metric units. Official information, development history, and more detail on SIcan be found in Bureau of Standards Special Publication 330 1974 and in the SEG Subcommittee on Metrification 1981 publication. SI is based on seven base units listed in Table 1 and two supplementary units the last two . SI derived units are formed by combining the base and supplementary units. Some of the derived units are listed in Table 2. Table 3 shows how larger or smaller units can be made byadding prefixes. When prefixes are used, the first syllable is accented. Note that k and M stand for 10 3 and 10 6 , whereas M and MM or m and mm are sometimes used in the oil industry for designating thousands and millions of gas volumes . Prefixes are raised to the power of the unit employed; for example, km3 means cubic kilometers, not thousands of cubic meters. Prefixes are not compounded GWrather than kMW . SEG allows the forms in Table 4 in addition to those in Tables 1 to 3. Table 5 relates cgs electromagnetic and electrostatic units to SI units; see also Figure E-8. Figure M-1 relates cgs and SI magnetic units.

Rules about writing units Symbols are written in Roman not italics type. They are never pluralized. Unit names, including prefixes, are not capitalized except at thebeginning of a sentence or in titles. Unit names are pluralized in the usual manner, as 100 meters, 70 henries, except for lux, hertz, and siemens. Fractional values require the singular form. Periods are not used after symbols, that is, symbols are not abbreviations. Symbols are lower case except when named for a person exception: L for liter . A space separates a numerical value and the unit symbolexcept for °C ; thus, 10 m, 0.112 s, 1.5 g/cm3, 20°C. A hyphen separates value and symbol when used as an adjective; thus, 35-mm film. No space separates a prefix and the symbol; thus, ms for milliseconds, kW for kilowatt. The symbols ‘‘/’’ or ‘‘.’’ are used to indicate the compounding of symbols for example, km/s or N.m for kilometers per second and newton-meter , but are not used when units arewritten out. Where symbols are compounded, parentheses should be used to avoid ambiguity, as W/ m.k . ‘‘P’’ is not acceptable as an abbreviation for ‘‘per.’’ ‘‘Per’’ should not be compounded; thus, ‘‘meters per second squared,’’ not ‘‘meters per second per second.’’ Use rather than • for products of numbers; thus 6.2 5, not 6.2.5. A space should be used on each side of symbols for multiplication,addition, subtraction, convolution , , , * and for the division symbol but not for /. Numbers with many decimal places should be grouped by threes separated by a space rather than by a comma which Europeans read as a decimal point ; thus, 4 720 525 or 0.528 75. For numbers smaller than one, a zero should be shown in the units place. A space is not necessary for four-digit numbers.

403

Appendix A404

Table 1. SI base and supplementary units. Quantity Length Mass Time Electric current Thermodynamic temperature Amount of substance Luminous intensity Plane angle Solid angle SI unit meter or metre kilogram second ampere kelvin mole candela radian steradian symbol m kg s A K mol cd rad sr

Note that the kilogram is not a unit of force (weight). The word ‘‘weight’’ is often ambiguous andits use should be avoided. The temperature unit kelvin is not ‘‘degree kelvin.’’

‘‘Squared’’ or ‘‘cubed’’ should follow unit names except for areas and volumes; thus, meter per second squared, square meter, watt per cubic meter. The spellings ‘‘metre’’ and ‘‘litre’’ are preferred but ‘‘meter’’ and ‘‘liter’’ are the official U.S. forms of spelling. The use of liter as a cubic decimeter is...