Metric System

The SI starts by defining seven basic units: one each for length, mass, time, electric current, temperature, amount of substance and luminous intensity. ("Amount of substance" refers to the number of elementary particles in a sample of matter. Luminous intensity has to do with the brightness of a light source.) But only four of these seven basic quantities are in everyday use by nonscientists: length, mass, time, and temperature. Their defined SI units are the meter for length, the kilogram for mass, the second for time and the degree Celsius for temperature. (The other three basic units are the ampere for electric current, the mole for amount of substance, and the candela for luminous intensity.) Almost all other units can be derived from the basic seven. For example, area is a product of two lengths: meters squared, or square meters. Velocity or speed is a combination of a length and a time: kilometers per hour.

The meter was originally defined in terms of Earth's size; it was supposed to be one ten-millionth of the distance from the equator to the North Pole, going straight through Paris. The modern meter, therefore, is defined in terms of how far light will travel in a given amount of time when traveling at—naturally—the speed of light. The speed of light in a vacuum is considered to be a fundamental constant of nature that is invariable, no matter how the continents drift. The standard meter turns out to be 39.3701 inches.

The kilogram is the metric unit of mass, not weight. Mass is the fundamental measure of the amount of matter in an object. The mass of a baseball won't change if you hit it from the earth to the moon, but it will weigh less—have less weight—when it lands on the moon because the moon's smaller gravitational force is pulling it down less strongly. Astronauts can be weightless in space, but they can lose mass only by dieting. As long as we don't leave the earth, though, we can speak loosely about mass and weight as if they were the same thing. So you can feel free to "weigh" yourself (not "mass" yourself) in kilograms. Unfortunately, no absolutely unchangeable standard of mass has yet been found to standardize the kilogram on Earth. The kilogram is therefore defined as the mass of a certain bar of platinum-iridium alloy that has been kept (very carefully) since 1889 at the International The first provisional standard scale of a meter. Made of copper in 1793, the scale is housed in the Archives Nationales de France, Paris. Photograph by Thomas Ernsting. Stock Market. Reproduced by permission. Bureau of Weights and Measures in Sèvres, France. The kilogram turns out to be 2.2046 pounds.

The metric unit of time is the same second that has always been used, except that it is now defined in a more precise way. It no longer depends on the wobbly rotation of Earth (1/86,400th of a day), because Earth is slowing down; her days keep getting a little longer as rotation slows. So the second is now defined in terms of the vibrations of a certain kind of atom known as cesium-133. One second is defined as the amount of time it takes for a cesium-133 atom to vibrate in a particular way 9,192,631,770 times. This may sound like a strange definition, but it is a superbly accurate way of fixing the standard size of the second, because the vibrations of atoms depend only on the nature of the atoms themselves, and cesium atoms will presumably continue to behave exactly like cesium atoms forever. The exact number of cesium vibrations was chosen to come out as close as possible to what was previously the most accurate value of the second.

The metric unit of temperature is the degree Celsius (^oC), which replaces the English system's degree Fahrenheit (^oF). In the scientists' SI, the fundamental unit of temperature is actually the kelvin (K). But the kelvin and the degree Celsius are exactly the same size: 1.8 times as large as the degree Fahrenheit. One cannot convert between Celsius and Fahrenheit simply by multiplying or dividing by 1.8, however, because the scales start at different places. That is, their zero-degree marks have been set at different temperatures.