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GNU Emacs Calc 2.02 Manual

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12.3 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.)


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