A unit of measurement is some specific
quantity that has been chosen as the standard against which other
measurements of the same kind are made. For example, the meter is the
unit of measurement for length in the metric system. When an object is
said to be 4 meters long, that means that the object is four times as
long as the unit standard (1 meter).
The term standard refers to the physical
object on which the unit of measurement is based. For example, for many
years the standard used in measuring length in the metric system was the
distance between two scratches on a platinum-iridium bar kept at the
Bureau of Standards in Sèvres, France. A standard serves as a norm
against which other measuring devices of the same kind are made. The
meter stick in a school classroom or home is thought to be exactly one
meter long because it was made from a permanent model kept at the
manufacturing plant that was originally copied from the standard meter
in France.
All measurements consist of two parts: a
scalar (numerical) quantity and the unit designation. In the measurement
8.5 meters, the scalar quantity is 8.5 and the unit designation is
meters.
**History**
The need for units and standards developed
at a point in human history when people needed to know how much of
something they were buying, selling, or exchanging. A farmer might want
to sell a bushel of wheat, for example, for ten dollars, but he or she
could do so only if the unit "bushel" were known to potential
buyers. Furthermore, the unit "bushel" had to have the same
meaning for everyone who used the term.
The measuring system that most Americans
know best is the British system, with units including the foot, yard,
second, pound, and gallon. The British system grew up informally and in
a disorganized way over many centuries. The first units of measurement
probably came into use shortly after the year 1215. These units were
tied to easily obtained or produced standards. The yard, for example,
was defined as the distance from King Henry II's nose to the thumb of
his outstretched hand. (Henry II of England reigned from 1154 to 1189.)
The British system of measurement consists
of a complex, irrational (meaning, in this case, not sensibly organized)
collection of units whose only advantage is its familiarity. As an
example of the problems it poses, the British system has three different
units known as the quart. These are the British quart, the U.S. dry
quart, and the U.S. liquid quart. The exact size of each of these
"quarts" differs.
In addition, a number of different units
are in use for specific purposes. Among the units of volume in use in
the British system (in addition to those mentioned above) are the bag,
barrel (of which there are three types—British and U.S. dry, U.S.
liquid, and U.S. petroleum), bushel, butt, cord, drachm, firkin, gill,
hogshead, kilderkin, last, noggin, peck, perch, pint, and quarter.
**The
metric system**
In an effort to bring some rationality to
systems of measurement, the French National Assembly established a
committee in 1790 to propose a new system of measurement with new units
and new standards. That system has come to be known as the metric system
and is now the only system of measurement used by all scientists and in
every country of the world except the United States and the Myanmar
Republic. The units of measurement chosen for the metric system were the
gram (abbreviated g) for mass, the liter (L) for volume, the meter (m)
for length, and the second (s) for time.
**Words
to Know**
**British
system:** A
collection of measuring units that has developed haphazardly over many
centuries and is now used almost exclusively in the United States and
for certain specialized types of measurements.
**Derived
units:**
Units of measurements that can be obtained by multiplying or dividing
various combinations of the nine basic SI units.
**Metric
system:** A
system of measurement developed in France in the 1790s.
**Natural
units:**
Units of measurement that are based on some obvious natural standard,
such as the mass of an electron.
**SI
system:** An
abbreviation for Le Système International d'Unités, a system of
weights and measures adopted in 1960 by the General Conference on
Weights and Measures.
A specific standard was chosen for each of
these basic units. The meter was originally defined as one ten-millionth
the distance from the North Pole to the equator along the prime
meridian. As a definition, this standard is perfectly acceptable, but it
has one major disadvantage: a person who wants to make a meter stick
would have difficulty using that standard to construct a meter stick of
his or her own.
As a result, new and more suitable
standards were selected over time. One improvement was to construct the
platinum-iridium bar standard mentioned above. Manufacturers of
measuring devices could ask for copies of the fundamental standard kept
in France and then make their own copies from those. As you can imagine,
the more copies of copies that had to be made, the less accurate the
final measuring device would be.
The most recent standard adopted for the
meter solves this problem. In 1983, the International Conference on
Weights and Measures defined the meter as the distance that light
travels in ^{1}/_{299,792,458} second. The standard is
useful because it depends on the most accurate physical measurement
known—the second—and because anyone in the world is able, given the
proper equipment, to determine the true length of a meter.
**Le
Système International d'Unités (the SI system)**
In 1960, the metric system was modified
somewhat with the adoption of new units of measurement. The modification
was given the name of Le Système International d'Unités, or the
International System of Units. This system is more commonly known as the
SI system.
Nine fundamental units make up the SI
system. These are the meter (abbreviated m) for length, the kilogram
(kg) for mass, the second (s) for time, the ampere (A) for electric
current, the kelvin (K) for temperature, the candela (cd) for light
intensity, the mole (mol) for quantity of a substance, the radian (rad)
for plane angles, and the steradian (sr) for solid angles.
**Derived
units**
Many physical phenomena are measured in
units that are derived from SI units. For example, frequency is measured
in a unit known as the hertz (Hz). The hertz is the number of vibrations
made by a wave in a second. It can be expressed in terms of the basic SI
unit as s^{−1}. Pressure is another derived unit. Pressure
is defined as the force per unit area. In the SI system, the unit of
pressure is the pascal (Pa) and can be expressed as kilograms per meter
per second squared, or kg/m/s^{2}. Even units that appear to
have little or no relationship to the nine fundamental units can,
nonetheless, be expressed in these terms. The absorbed dose, for
example, indicates that amount of radiation received by a person or
object. In the metric system, the unit for this measurement is the gray.
One gray can be defined in terms of the fundamental units as meters
squared per second squared, or m^{2}/s^{2}.
Many other commonly used units can also be
expressed in terms of the nine fundamental units. Some of the most
familiar are the units for area (square meter: m^{2}), volume
(cubic meter: m^{3}), velocity (meters per second: m/s),
concentration (moles per cubic meter: mol/m^{3}), density
(kilogram per cubic meter: kg/m^{3}), luminance (candela per
square meter: cd/m^{2}), and magnetic field strength (amperes
per meter: A/m).
A set of prefixes is available that makes it possible to use the
fundamental SI units to express larger or smaller amounts of the same
quantity. Among the most commonly used prefixes are milli- (m) for
one-thousandth; centi- (c) for one-hundredth; micro- (*μ*)
for one-millionth; kilo- (k) for one thousand times; and mega- (M) for
one million times. Thus, any volume can be expressed by using some
combination of the fundamental unit (liter) and the appropriate prefix.
One million liters, using this system, would be a megaliter (ML), and
one millionth of a liter would be a microliter (*μ*L).
**Natural
units**
One characteristic of all of the above
units is that they have been selected arbitrarily (by individual
preference or convenience rather than by law). The committee that
established the metric system could, for example, have defined the meter
as one one-hundredth the distance between Paris and Sèvres. It was
completely free to choose any standard it wanted to.
Some measurements, however, suggest
"natural" units. In the field of electricity, for example, the
charge carried by a single electron would appear to be a natural unit of
measurement. That quantity is known as the elementary charge (e) and has
the value of 1.6021892 × 10^{−19} coulomb. Other natural
units of measurement include the speed of light (c: 2.99792458 × 10^{8}
m/s), the Planck constant (ℏ: 6.626176 × 10^{−34}
joule per hertz), the mass of an electron (m_{e}: 0.9109534 ×
10^{−30} kg), and the mass of a proton (m_{p}:
1.6726485 × 10^{−27} kg). As you can see, each of these
natural units can be expressed in terms of SI units, but they often are
used as basic units in specialized fields of science.
**Unit
conversions between systems**
For many years, an effort has been made to
have the metric system, including SI units, adopted worldwide. As early
as 1866, the U.S. Congress legalized the use of the metric system. More
than a hundred years later, in 1976, Congress adopted the Metric
Conversion Act, declaring it the policy of the nation to increase the
use of the metric system in the United States.
In fact, little progress has been made in
that direction. Indeed, elements of the British system of measurement
continue in use for specialized purposes throughout the world. All
flight navigation, for example, is expressed in terms of feet, not
meters. As a consequence, it is still necessary for an educated person
to be able to convert from one system of measurement to the other.
In
1959, English-speaking countries around the world met to adopt standard
conversion factors between British and metric systems. To convert from
the pound to the kilogram, for example, it is necessary to multiply the
given quantity (in pounds) by the factor 0.45359237. A conversion in the
reverse direction, from kilograms to pounds, involves multiplying the
given quantity (in kilograms) by the factor 2.2046226. Other relevant
conversion factors are 1 inch equals 2.54 centimeters and 1 yard equals
0.9144 meter.
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