Predefined Units
================

Since the exact definitions of many kinds of units have evolved over
the years, and since certain countries sometimes have local
differences in their definitions, it is a good idea to examine Calc's
definition of a unit before depending on its exact value.  For
example, there are three different units for gallons, corresponding to
the US (`gal'), Canadian (`galC'), and British (`galUK') definitions.
Also, note that `oz' is a standard ounce of mass, `ozt' is a Troy
ounce, and `ozfl' is a fluid ounce.

The temperature units corresponding to degrees Kelvin and Centigrade
(Celsius) are the same in this table, since most units commands treat
temperatures as being relative.  The `calc-convert-temperature'
command has special rules for handling the different absolute
magnitudes of the various temperature scales.

The unit of volume "liters" can be referred to by either the
lower-case `l' or the upper-case `L'.

The unit `A' stands for Amperes; the name `Ang' is used for Angstroms.

The unit `pt' stands for pints; the name `point' stands for a
typographical point, defined by `72 point = 1 in'.  There is also
`tpt', which stands for a printer's point as defined by the TeX
typesetting system: `72.27 tpt = 1 in'.

The unit `e' stands for the elementary (electron) unit of charge;
because algebra command could mistake this for the special constant
`e', Calc provides the alternate unit name `ech' which is preferable
to `e'.

The name `g' stands for one gram of mass; there is also `gf', one gram
of force.  (Likewise for `lb', pounds, and `lbf'.)  Meanwhile, one
"`g'" of acceleration is denoted `ga'.

The unit `ton' is a U.S. ton of `2000 lb', and `t' is a metric ton of
`1000 kg'.

The names `s' (or `sec') and `min' refer to units of time; `arcsec'
and `arcmin' are units of angle.

Some "units" are really physical constants; for example, `c'
represents the speed of light, and `h' represents Planck's constant.
You can use these just like other units: converting `.5 c' to `m/s'
expresses one-half the speed of light in meters per second.  You can
also use this merely as a handy reference; the `u g' command gets the
definition of one of these constants in its normal terms, and `u b'
expresses the definition in base units.

Two units, `pi' and `fsc' (the fine structure constant, approximately
1/137) are dimensionless.  The units simplification commands simply
treat these names as equivalent to their corresponding values.
However you can, for example, use `u c' to convert a pure number into
multiples of the fine structure constant, or `u b' to convert this
back into a pure number.  (When `u c' prompts for the "old units,"
just enter a blank line to signify that the value really is unitless.)