6 Unit definitions
The conversion information is read from a units data file which is
called `units.dat' and is probably located in the `/usr/local/share'
directory. If you invoke `units' with the `-V' option, it will print
the location of this file. The default file includes definitions for
all familiar units, abbreviations and metric prefixes. It also
includes many obscure or archaic units.
Many constants of nature are defined, including these:
pi ratio of circumference to diameter
c speed of light
e charge on an electron
force acceleration of gravity
mole Avogadro's number
water pressure per unit height of water
Hg pressure per unit height of mercury
au astronomical unit
k Boltzman's constant
mu0 permeability of vacuum
epsilon0 permitivity of vacuum
G Gravitational constant
mach speed of sound
The database includes atomic masses for all of the elements and numerous
other constants. Also included are the densities of various ingredients
used in baking so that `2 cups flour_sifted' can be converted to
`grams'. This is not an exhaustive list. Consult the units data file
to see the complete list, or to see the definitions that are used.
The unit `pound' is a unit of mass. To get force, multiply by the
force conversion unit `force' or use the shorthand `lbf'. (Note that
`g' is already taken as the standard abbreviation for the gram.) The
unit `ounce' is also a unit of mass. The fluid ounce is `fluidounce'
or `floz'. British capacity units that differ from their US
counterparts, such as the British Imperial gallon, are prefixed with
`br'. Currency is prefixed with its country name: `belgiumfranc',
The US Survey foot, yard, and mile can be obtained by using the `US'
prefix. These units differ slightly from the international length
units. They were in general use until 1959, and are still used for
geographic surveys. The acre is officially defined in terms of the US
Survey foot. If you want an acre defined according to the
international foot, use `intacre'. The difference between these units
is about 4 parts per million. The British also used a slightly
different length measure before 1959. These can be obtained with the
When searching for a unit, if the specified string does not appear
exactly as a unit name, then the `units' program will try to remove a
trailing `s' or a trailing `es'. If that fails, `units' will check for
a prefix. All of the standard metric prefixes are defined.
To find out what units and prefixes are available, read the standard
units data file.
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