Defining Units
When trying to describe the quantity or quality of something, whether a chemical
substance or physical phenomenon, it is helpful to have some standard measure to
refer to. A unit is nothing more than a standard by which a
measured value
can be described. For instance, in the old English system, a foot was just
that--the length of a man's foot. This was helpful in that when a
distance
between, say, a house and a water well was described in feet, one who had never
seen the dwelling in question would have an instant idea of how far away the
well was, based solely on a verbal description. The concept today hasn't
changed, although our standards make better scientific sense and are more exact.
Systems of Units
There are two different sets of units used in scientific measurements:
British Engineering units and the metric system (also called
the Standard International (SI) system). Both are based on standards,
though those underlying the British system can be shockingly
inscrutable. The metric system is based on a far less arbitrary system of tens,
and is far more rational, intelligent, and easier to use. In general, people
tend to stick with the system with which they are most familiar, and have the
most intuitive feeling for. The United States uses the British system; most of
the rest of the world (including Britain) uses the metric system. In terms of
science, the metric system is almost always used. For Americans, it is
therefore often necessary to make conversions between the two.
Conversion tables reside in the "Reference" pull-down menu at the top
of most SparkNotes pages.
Metric Units (SI units)
The standards metric terms for commonly measured characteristics are listed in
below.
Temperature in the table above is expressed in both Kelvin (K) and
Celsius (C). One kelvin is exactly the same as one C, except that the
Kelvin scale starts at absolute zero, or -273.15 C, the lowest
temperature possible, instead of the freezing point of water.
Therefore water freezes at 273.15 K or 0 C.
Prefixes for Units
In addition to the basic metric units described above, there are prefixes to
indicate larger or smaller quantities. For example, a meter refers to a
standard metric measure of length. A millimeter refers to a measure that is one
thousandth the size of a meter (one thousand millimeters fit into a meter); a
kilometer refers to a distance one thousand times longer than a meter (one
thousand meters fit into a kilometer). By wisely using these prefixes, you can
avoid having to use huge numbers or having to resort to scientific
notation. The table below shows
the most commonly encountered prefixes:
