APPENDIX III THE METRIC SYSTEM The  metric  system  was  developed  by  French scientists in 1790 and was specifically designed to be an easily used system of weights and measures to benefit science,   industry,   and   commerce.   Soon   after development, scientists the world over adopted it for their work. Early in the nineteenth century many European countries adopted the new system for engineering and commerce.  It  was  possible  for  these  countries  to  trade manufactured  goods  with  one  another  and  not  be concerned with having to buy special wrenches and tools  to  repair  the  machinery  received  in  trade. Countries  could  buy  and  sell  machine  tools  and precision  instruments  without  having  to  modify  or  alter them. Today, with the exception of the United States and a  few  small  countries,  the  entire  world  is  using predominantly the metric system or is committed to its use. It becomes a matter of time until the United States adopts the International System of Units (SI), which is the  formal  name  for  the  metric  system.  Exactly  when the United States will adopt this system or how long it will take to change from the use of non-SI units to SI units is unknown. Meanwhile, the use of SI units is sure to spread and is expected to become universal soon. Much of the equipment in the United States Navy is already measured in SI units. Certain of its weapons are sized  in  “metric,”   such  as  20  millimeters  and  40 millimeters.  Existing  maps  and  charts  may  show distances  in  meters  (instead  of  yards)  and  kilometers (instead  of  miles). Some  SI  units  are  base  units;  that  is,  metric standards  defined  and  adopted  by  international  treaty. Other SI units are derived from the base units and are either expressed in terms of the base unit or are specially named. The base unit for measuring distance, the meter, is defined as one ten-millionth of the distance from the Equator to the North Pole. The metric standard for weight, the gram, is defined as the weight of one cubic centimeter of pure water. Other SI standards include the second (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 base unit.  Derived  units  having  special  names  include  the hertz   (frequency),   watt   (power),   and   farad (capacitance),  volt  (electromotive  force),  and  ohm (electric  resistance). The  metric  system  is  a  base-10  (decimal)  number system. It is convenient and easy to use because one unit of measure is converted to smaller and larger units of measure  by  dividing  and  multiplying  by  powers  often or by shifting the decimal point. For example, 12.3 millimeters  convert  to  1.23  centimeters.  Calculations, such as dividing by 16 (to convert ounces to pounds) and multiplying  by  12  (to  convert  feet  to  inches),  are eliminated. The result of multiplying a base unit by a power of ten is referred to as a multiple; the result of dividing by a power of ten, a submultiple. Names of multiples and submultiple of the base unit are formed by adding prefixes to the name of the base unit. The already mentioned  millimeter,  centimeter,  and  kilometer  are examples. It is rather simple to relate SI units to non-SI units. Compared to the yard, the meter is a little longer (about 0.6 mile). The basic unit of volume, the liter, is a little larger than a quart (about 1.06 qt). The weight of a liter of pure water is 1 kilogram, which is a little more than 2 pounds (about 2.2 lb). The SI unit for measuring power,  the  kilowatt,  is  somewhat  bigger  than  one horsepower (about 1.3 hp). In working non-SI units and SI units, it helps to have a  table  of  common  equivalent  weights  and  measures, such as the one that follows. This table also gives the factor you multiply by in order to convert a non-SI unit to 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, refer to the correspondence course "The Metric System," NAVEDTRA  475-01-00-79.  This  course  is  available  to you through your ESO office. AIII-1