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

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.)

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