Metric system: The dominant system of measurement in science and the world.

Historically, people chose units of measure related to everyday life (the “foot” is one example). Scientists continued this tradition, developing units such as “horsepower” to measure power.

The French challenged this philosophy of measurement during their Revolution, when they decided to give measurement a more scientific foundation. Instead of basing their system on things that change − the length of a person’s foot changes during her lifetime, for example − the French based their system on what they viewed as constant. To accomplish this, they created units such as the meter, which they defined as a certain fraction of the Earth’s circumference. (To be specific: one ten-millionth of the meridian passing through Paris from the equator to the North Pole. It turns out that the distance from the equator to the North Pole does vary, but the metric system’s intent of consistency and measurability was exactly on target.)

The metric system is also based on another inspired idea: units of measurement should be based on powers of 10. This differs from the British system, which provides more variety: 12 inches in a foot, 5280 feet to a mile and so forth.

The metric system makes conversions much simpler to perform. For example, in order to calculate the number of inches in a mile, you would typically multiply by 5280 (for feet in a mile) and then by 12 (for inches in a foot). However, in the metric system, to convert between units, you typically multiply by a power of 10. For instance, to convert from kilometers to meters, you multiply by 1000. The prefix “kilo” means 1000.

The revolutionaries were a little extreme (as revolutionaries tend to be) and they held onto their position of power for only a decade or so. While some of their legacy (including their political art, rather mediocre as is much political art) has been forgotten, their clever and sensible metric system endures. Most scientists, and most countries, use the metric system today.

Scientists continue to update and refine the metric system. This expanded and updated system of measurement used today is called the Système International d’Unités, or SI. We typically use SI units in this textbook; several times, though, we refer to different units that may be better known to you or are commonly used in the sciences. We will discuss some of the SI units further in this chapter.

Over the years, scientists have refined measurement systems, making the definition of units ever more precise. For example, instead of being based on the Earth’s circumference, the meter is now defined as the distance light travels in a vacuum during the time interval of 1/299,792,458 of a second. Although perhaps not as memorable as the initial standard, this definition is important because it is constant, precise, indestructible, and can be reproduced in laboratories around the world.

In addition to using meters for length, the Système International uses seconds (time), kilograms (mass), amperes (electric current), kelvins (temperature), moles (amount of substance) and candelas (luminous intensity). Many other derived units are based on these fundamental units. For instance, a newton measures force and is equal to kilograms times meters per second squared. On Earth, the force of gravity on a small apple is about one newton.

At the risk of drowning you in terminology, we should point out that you might also encounter references to the MKS (meter/kilogram/second) and CGS (centimeter/gram/second) systems. These systems are named for the units they use for length, mass and time.