Why was the metric system invented?
As science began to develop a few hundred years ago, scientists had trouble with measurements. Standards varied from nation to nation and even within one country. So during the 1700's scientists argued for a sensible system of measurement that could be accepted all over the world.
Such a system was invented in France in 1791. The French had other reasons for doing it, too. They were in the middle of a revolution at the time. The leaders of the revolution wanted to get away from all reminders of their hated past. They were, therefore, willing to set up a new system of measurement.
They began with length. They decided to establish the "meter" (from a Latin word meaning "measure") as a standard. Because of this, the entire system of measurement is called the metric system. Originally they tried to make the meter exactly 1/40,000,000 of the circumference of the earth. But when calculations turned out to be wrong about the earth's circumference, the meter was taken to be the distance between two marks on a platinum-iridium bar. All units of measurement in this system—length, capacity, mass—are linked in some way to the meter.
Actually, the metric system is easy to remember and easy to use. At first, though, people didn't want to change over. In 1840 the French Government had to insist that the people use the metric system or be punished.
Other nations gradually adopted the metric system, and today almost the whole world uses it. The United Kingdom is "phasing in" the system today.