Metric Units by Graham Nelson

(For Glulx Only)   Version 2

Scientific kinds of value for simulations.

The metric system provides a consistent set of units for scientific measurement of the world. Though often associated with the French Revolution or with Napoleon, the system of metric units only really began to displace existing units in May 1875, when it was made official by an international treaty. In 1960, it was renamed the "Système international d'unités", which is usually abbreviated "SI".

This extension is a kit for writers who want to make realistic simulations, backed up by some quantitative physics. It defines kinds of values for the 25 or so SI units in common usage, and more than 100 notations for them. It also makes sure they multiply correctly. For instance, a mass times an acceleration produces a force, so

say "You feel a force of [2kg times 5 m/ss]."

will produce the text "You feel a force of 10N." The easiest way to see how all these units combine is to run one of the examples below and look at the Kinds index which results.

For each unit, both names and notations are allowed. Thus '2 kilograms' is equivalent to '2kg'. Both English and French spellings of 'meter'/'metre' and 'liter'/'litre' are allowed, but we insist on 'gram' not 'gramme' and 'tonne' not 'ton'. ('Ton' is too easily confused with the Imperial measure, which is not quite the same.) We can print back the same value in a variety of ways. For instance:

say "[m0 in metric units]";
say "[m0 in kg]";
say "[m0 in g]";
say "[m0 in kilograms]";

might produce: '2.04kg', '2.04kg', '2040g', '2.04 kilograms'. The text expansion '... in metric units' prints any value of any of these units in its most natural notation: 2.04kg is thought to be better than 2040g, but 981g would be better than 0.981kg. Or in the case of our variant spellings:

say "[C in metric units]";
say "[C in milliliters]";
say "[C in millilitres]";

might produce '47 ml', '47 milliliters', '47 millilitres'. It's also worth remembering that any value can be rounded:

say "[C to the nearest 5 ml]";

would produce '45 ml', for instance.

For detailed notes on each of the units, consult the Kinds index for any project using this extension.

We haven't included every SI unit. There are hundreds of kinds of value which turn up in physics, and we only include the commonest 25 or so. The missing ones which have named SI units are:

angle (measured in radians), solid angle (measured in steradians), luminous flux (lux), electric capacitance (Farads), electric resistance (Ohms), electric conductance (Siemens), magnetic flux (Webers), magnetic field (Teslas), inductance (Henries), radioactivity (Becquerels), absorbed radioactive dose (Grays), equivalent radioactive dose (Sieverts), chemical quantity (mole), catalytic activity (katals).

As can be seen, we've missed out units for chemistry, electromagnetic effects beyond the basic ones, and radioactivity. It would be easy to add any of these which might be needed:

Electric resistance is a kind of value.

1.0 Ohm (in metric units, in Ohms, singular) or 2 Ohms (in metric units, in Ohms, plural) specifies an electric resistance.

Electric resistance times electric current specifies a voltage.

Similarly, there are many kinds of value which don't have named SI units, but where physicists write them down as compounds. These are also easy to add as needed:

Angular momentum is a kind of value.

1.0 Nms specifies an angular momentum.

Momentum times length specifies an angular momentum.

Besides angular momentum, 'Metric Units' also leaves out:

volumetric flow (cu m/s), jerk (m/sss), snap (m/ssss), angular velocity (rad/s, though Inform would probably use degrees/s), torque (Nm), wavenumber (1/m), specific volume (cu m/kg), molar volume (cu m/mole), molar heat capacity (J/K/mol), molar energy (J/mol), specific energy (J/kg), energy density (J/cu m), surface tension (J/sq m), thermal conductivity (W/m/C), viscosity (sq m/s), conductivity (S/m), permittivity (F/m), permeability (H/m), electric field strength (V/m), magnetic field strength (A/m), resistivity (Ohm metre).

This extension is pretty faithful to SI conventions. It chooses centigrade rather than Kelvin for temperature, but otherwise it's strictly metric, and does not define Imperial measures. See the example below for how to add these.

A	Example Galileo, Galileo Dropping a cannonball or a feather from a variety of heights.

B	Example The Empire Strikes Back Using good old Imperial measures of length and area alongside these Frenchified metric ones.