In the late 1700's, scientists in France decided to create a new system of units to replace the "English" system. Part of this had to do with the general desire to replace old things during the French Revolution, but mostly it was just an attempt to make a system of units that were easier to understand and work with. Over the years, more and more countries have adopted the Metric System for both scientific and "everyday" use. The US is the last "holdout" in this regard, although American scientists use the Metric System exclusively.
The basic units in the metric system have nothing to do with the size of a queen's foot, or the length of a clothier's arm:
The basic unit of length in the metric system is the meter, defined as the distance of a ceratin number of wavelengths in the light wave given off by a certain kind of atom.
The basic unit of time is the second, just as in the "English" system. It is defined as the time for a certain type of atom to oscillate a certain number of times, hence the term "atomic clock."
The basic unit of mass is the gram. A cube-shaped "drop" of pure water 1/100 of a meter on a side has a mass of one gram.
The Metric System measures temperature on the Kelvin temperature scale, although people use the Celsius scale for everyday life. John Thompson, Lord Kelvin, invented the Kelvin scale because he needed a scale that never became zero. Multiplying or dividing by zero in a scientific equation can have interesting results! A Kelvin is exactly the same size as a degree Celsius. Zero Kelvins (0 K = 273 degrees below zero Celsius) is colder than anything can get. Room temperature is about 300 K.
There are other metric units, such as the Coulomb, a unit of electrical charge. We will discuss compund units, such as liters, km/hr, Newtons, and Amperes in a later section.
While these basic units in the Metric System are useful, they are not always the best units to use. To take an example from the "English" system, would you measure your height in miles? No, because your height is such a small fraction of a mile that the number wouldn't mean much. Inches and feet are better units. Similarly, you would not measure the distance between cities in inches, because the number would then be impossibly large. The "English" system solves this problem with its bizarre conversion factors; the Metric System uses the more manageable Powers of Ten notation, described on the next page.
Questions? Ask the designer, Jim Heath
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