APPENDIX IIITHE METRIC SYSTEMThe metric system was developed by Frenchscientists in 1790 and was specifically designed to be aneasily used system of weights and measures to benefitscience, industry, and commerce. Soon afterdevelopment, scientists the world over adopted it fortheir work.Early in the nineteenth century many Europeancountries adopted the new system for engineering andcommerce. It was possible for these countries to trademanufactured goods with one another and not beconcerned with having to buy special wrenches andtools to repair the machinery received in trade.Countries could buy and sell machine tools andprecision instruments without having to modify or alterthem.Today, with the exception of the United States anda few small countries, the entire world is usingpredominantly the metric system or is committed to itsuse. It becomes a matter of time until the United Statesadopts the International System of Units (SI), which isthe formal name for the metric system. Exactly whenthe United States will adopt this system or how long itwill take to change from the use of non-SI units to SIunits is unknown. Meanwhile, the use of SI units is sureto spread and is expected to become universal soon.Much of the equipment in the United States Navy isalready measured in SI units. Certain of its weapons aresized in “metric,” such as 20 millimeters and 40millimeters. Existing maps and charts may showdistances in meters (instead of yards) and kilometers(instead of miles).Some SI units are base units; that is, metricstandards defined and adopted by international treaty.Other SI units are derived from the base units and areeither expressed in terms of the base unit or are speciallynamed. The base unit for measuring distance, the meter,is defined as one ten-millionth of the distance from theEquator to the North Pole. The metric standard forweight, the gram, is defined as the weight of one cubiccentimeter of pure water. Other SI standards include thesecond (time) and the degree Celsius (temperature),which was formerly called centigrade. The square meter(area), cubic meter (volume), and meter per second(speed) are derived units expressed in terms of the baseunit. Derived units having special names include thehertz (frequency), watt (power), and farad(capacitance), volt (electromotive force), and ohm(electric resistance).The metric system is a base-10 (decimal) numbersystem. It is convenient and easy to use because one unitof measure is converted to smaller and larger units ofmeasure by dividing and multiplying by powers oftenor by shifting the decimal point. For example, 12.3millimeters convert to 1.23 centimeters. Calculations,such as dividing by 16 (to convert ounces to pounds) andmultiplying by 12 (to convert feet to inches), areeliminated.The result of multiplying a base unit by a power often is referred to as a multiple; the result of dividing bya power of ten, a submultiple. Names of multiples andsubmultiple of the base unit are formed by addingprefixes to the name of the base unit. The alreadymentioned millimeter, centimeter, and kilometer areexamples.It is rather simple to relate SI units to non-SI units.Compared to the yard, the meter is a little longer (about0.6 mile). The basic unit of volume, the liter, is a littlelarger than a quart (about 1.06 qt). The weight of a literof pure water is 1 kilogram, which is a little more than2 pounds (about 2.2 lb). The SI unit for measuringpower, the kilowatt, is somewhat bigger than onehorsepower (about 1.3 hp).In working non-SI units and SI units, it helps to havea table of common equivalent weights and measures,such as the one that follows. This table also gives thefactor you multiply by in order to convert a non-SI unitto an SI unit, or vice versa. For example:3 inches = 3 x 25.4 or 76.2 mm (exact)5 kilometers = 5 x 0.6 or 3 miles (approximate)For a more detailed study of the metric system, referto the correspondence course "The Metric System,"NAVEDTRA 475-01-00-79. This course is available toyou through your ESO office.AIII-1

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