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.