Metric System

I

Introduction

Metric System, a decimal system of physical units based on a unit of length known as the meter (Greek metron, “measure”). Introduced and adopted by law in France in the 1790s, a majority of countries subsequently adopted the metric system as a common system of weights and measures. Scientists in all countries use the metric system in their work.

The meter (m), which is approximately 39.37 in, was originally defined as one ten-millionth of the distance from the equator to the North Pole on a line running through Paris. From 1792 to 1799, French scientists measured part of this distance. Treating the Earth as a perfect sphere, they then estimated the total distance and divided it into ten-millionths. Later, after it was discovered that the Earth is not a perfect sphere, the standard meter was defined as the distance between two fine lines marked on a bar of platinum-iridium alloy. In 1960 the meter was redefined as 1,650,763.73 wavelengths of the reddish-orange light given off by a form of the element krypton. The measurements of modern science required still greater precision, however, and in 1983 the meter was defined as the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second.

The United States uses inches, feet, miles, pounds, tons, and gallons as units of length, weight, and volume for common measurements. Today, however, within the framework of the International System of Units, these English-system units are legally based on metric standards.

II

Using the Metric System

The metric system is known for its simplicity. All units of measurement in the metric system are based on decimals—that is, units that increase or decrease by multiples of ten. A series of Greek decimal prefixes is used to express units of ten or greater; a similar series of Latin decimal prefixes is used to express fractions. For example, deca equals ten, hecto equals one hundred, kilo equals one thousand, mega equals one million, giga equals one billion, and tera equals one trillion. For units below one, deci equals one-tenth, centi equals one-hundredth, milli equals one-thousandth, micro equals one-millionth, nano equals one-billionth, and pico equals one-trillionth. For conversion of metric system units to English-system units, see Weights and Measures.

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The simplicity of the metric system is evident in how easily one unit of measure can be changed into another. To change units in the metric system, simply move the decimal point to the right or the left, depending on whether the unit of measurement is increasing or decreasing by ten or one hundred and so on. For example, 1,672,928 millimeters equals 167,292.8 centimeters, which equals 1,672.928 meters, which equals 1.672928 kilometers. Compare the ease of this conversion with English units by trying to convert 1,672,928 inches into feet, rods, furlongs, and miles.

A

Length

People who were taught the English system of measurements in schools in the United States often have difficulty visualizing metric units. One way to visualize a meter is to think of the distance from the floor to the top of a doorknob, or the distance from the edge of an adult’s shoulder to the end of the opposite outstretched arm. Smaller things are measured in centimeters and millimeters. A millimeter is quite small, about the thickness of a dime. A centimeter is ten times bigger, about the height of a stack of ten dimes.

Millimeters are primarily used to measure very small items such as letters in a book, while centimeters are used to measure items ranging from the size of a book to the height of a person. Thereafter meters are used until one reaches distances about the length of five city blocks when kilometers are used. One kilometer is the approximate distance that an adult can walk in 12 minutes in a straight line and on a level road. Kilometers are used to measure long distances within cities or between cities.

B

Volume

Small volumes, such as the contents of a drinking glass, are measured in cubic centimeters, and large volumes, such as the contents of industrial fuel tanks, are measured in liters. A liter is 1,000 cubic centimeters—also known as 1,000 milliliters. A normal-sized drinking glass contains about 300 milliliters. The size of a liter is increasingly well known in the United States because of the many one-liter beverage bottles found in grocery stores or supermarkets. Very large volumes are measured in cubic meters. Freight containers used in the shipping industry, including railroad, truck, and ocean shipping, have a capacity of about 70 cubic meters.

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