The British Empire, however, did not adapt the metric system. Since the
primary trading partners of the United States were Great Britain and
Canada, the U.S. kept the "English" standards even though the U.S. had
gone to a decimal system of coinage in 1786. In 1816, President
Madison suggested going to the metric system, but the U.S. stayed with
the English system.
The metric system continued to gain in acceptance throughout the world.
In 1866, the metric system was made legal in the United States.
Eventually, the U.S. defined its "English" units in terms of the metric
system. For example, one inch is defined as being equal to exactly 2.54
Many scientists believed that the metric system should be based upon
natural standards of even greater permanence and greater precision. In
1960, the metric system underwent revision to become the International
System of Units, usually called the SI (Systme International). Among
the changes made was that the meter was defined in wavelengths of a
certain type of light. In 1983, the meter was again redefined to improve
its accuracy. Now the meter is defined as the distance light in vacuum
travels in 1/299,792,458 seconds. Although the technical definition has
changed, the actual length of a meter remained unchanged.
How far would light in a vacuum travel in exactly one second?
Currently, the United States is the only major country in the world to use
the old "English" system (now usually called the United States
Customary System) instead of the SI standard. The Metric Conversion
Act of 1975 passed by the United States Congress states that "the
policy of the United States shall be to coordinate and plan the increasing
use of the metric system in the United States." The United States
continues its conversion to the metric (SI) system (liter bottles of soft
drinks replacing quart bottles, car engine displacement measured in
liters instead of cubic inches, etc.).
The United States Customary System units are defined based on