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