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.

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.

C.

Weight

The basic unit of weight in the metric system is called a gram, and it is equal to the weight of one cubic centimeter of water. This is a very small amount, but it is easy to comprehend. Just pick up a U.S. dollar bill (or any bank note), and its weight is one gram. Because the gram is too light to be a convenient standard of weight, a larger unit has been chosen. This unit is 1,000 grams. Following the regular pattern of metric naming, it is called one kilogram. One thousand grams of water occupy a volume of 1,000 cubic centimeters or one liter. So a person need only pick up a plastic one-liter bottle of water to understand how heavy a kilogram is. Very heavy objects are weighed in tons of 1,000 kilograms each. One thousand kilograms is equal to one metric ton and is not the same as the usual American ton of 907.2 kg.

Many other metric units exist, such as those for measuring area, pressure, and energy. However, these units are rarely encountered in everyday use.

III.

History

Until the metric system was adopted in the 1790s, there was no common system of measurements in the world. In Europe the measuring system of the ancient Romans, some 2,000 years old, was still in use in the 1700s. But because standardization was not dependable, Roman feet and inches differed somewhat in size from one town to another. In addition to feet and inches, there were many local units such as the Italian cantarello for weight, the German Metze for volume, or the English tod for weighing wool. The French, in fact, had more than 1,000 units of measurement by the late 1700s with approximately 250,000 variations in size from one town to another. To straighten out this confusion, committees of French scientists during the French Revolution of the 1790s created the metric system.

The international Treaty of the Meter of 1875, of which the United States was an original signer, created permanent international committees to continually refine the metric system’s accuracy based on the latest scientific knowledge. The headquarters of these committees are located today in a suburb of Paris.

All metric units were originally derived from the meter, but by 1900 the metric system began to be based on the mks (meter-kilogram-second) system. Under this system the unit of mass (the weight of an object at sea level on Earth), was redefined as the kilogram, and the unit of time, the second, was added. Later a unit of the electromagnetic system, the ampere, was added to form the mksa (meter-kilogram-second-ampere) system. Because scientists needed ever-smaller units, the cgs (centimeter-gram-second) system also came into use. The unit of volume, the liter, was originally defined as 1 cubic decimeter (dm³), but in 1901 it was redefined as the volume occupied by a kilogram of water at 4°C and 760 mm of mercury; in 1964 the original definition (dm³) was restored.

In 1960 major revisions to the metric system resulted in a “new” metric system known as SI after the initial letters of its French name Système International d’Unités. The revisions were adopted at a General Conference of Weights and Measures held by countries using the metric system and resulted in the creation of the International System of Units.

In the United States several attempts were made to bring the metric system into general use. In 1821 Secretary of State John Quincy Adams, in a report to Congress, advocated the adoption of the metric system. In 1866 Congress legalized the use of the metric system, and the system was increasingly adopted, notably in medicine and science, as well as in certain sports, such as track and field. In 1893 the Office of Weights and Measures (now the National Institute of Standards and Technology) of the United States adopted the metric system in legally defining the yard and the pound.

In 1965 the United Kingdom became the first of the English-speaking countries to begin an organized effort to abandon the older units of measurement. Canada, Australia, New Zealand, and South Africa quickly followed and adopted the changeover more rapidly than the United Kingdom.

In 1971, after an extensive study, the U.S. secretary of commerce recommended that the United States convert to metric units under a ten-year voluntary plan. In 1975 President Gerald R. Ford signed the Metric Conversion Act. It defines the metric system as being the International System of Units as interpreted in the United States by the secretary of commerce. The act called for voluntary adoption of the metric system. In 1988 a provision in new federal legislation called for all federal agencies to use the metric system in business transactions starting in 1992, but this was never implemented. Lack of public interest and support has prevented the metric system from being adopted in the United States.

For further information on the metric system based on small units, see CGS System.