On 3/15/07, Bruce Feist <bfeist@xxxxxxxxxxxxx> wrote:
The link for a meter, or rather a centimeter, is mathematical rather than physical: a cubic centimeter is a milliliter. I don't know how this relates to the redefinitions -- do they implicitly redefine a liter? What's the dependency -- is a milliliter based on a centimeter, and a gram on a milliliter, or is it reversed, or neither? Bother. I was trying to supply an answer, and instead I've asked more questions.
There are fundamental building blocks of all unit systems: See http://en.wikipedia.org/wiki/SI for the example of the metric system of units. All other units are built up from these or are fractions of these. For instance: the Force unit in SI, Newton or N, is simply kg*m/s^2 ... based on Newton's second law ... F=ma ... (N) = (kg)(m/s^2). One Joule is the energy required to apply a 1 Newton force over one meter ... (J) = (N)(m) = (kg)(m^2/s^2) The properties of water is what usually relates these units together since water is important to humans: 1000 kg of water has a volume of 1000 L = 1 m^3, temperature difference between the melting point of wather and the boiling point is 100 degK (or degC). Similar relations can be found in the Imperial system. *Every* other unit can be broken down into the fundamental units. Those are the units for which we have benchmark references. For instance, both SI and Imperial systems use seconds as the time baseline. It used to be defined by some fraction of the Earth's revolution around the Sun. But that was found to not be invariate enough to use as a baseline for some measurements (like the measurement of how long it takes for the Earth to go around the Sun). So it was rereferenced to some number of vibrations of a certain atomic element. The other units are references and rereferenced as necessary to perform more and more accurate measurements of things. If your "yard-stick" is varying significantly with respect to what you are trying to measure, then you will have a hard time getting your measurement and need to find a more stable yard-stick. There's your metrology lesson for the day. /Mike
