Units and Standards

One characteristic of all of the above units is that they have been selected arbitrarily. The committee that established the metric system could, for example, have defined the meter as one one-hundredth the distance between Paris and Sèvres. It was completely free to choose any standard it wished.

Some measurements, however, suggested "natural" units. In the field of electricity, for example, the charge carried by a single electron would appear to be a natural unit of measurement. That quantity is known as the elementary charge (e) and has the value of 1.6021892 × 10^-19 coulomb. Other natural units of measurement include the speed of light (c: 2.99792458 × 10⁸ m/s), the Planck constant (6.626176 × 10^-34 joule per hertz), the mass of an electron (m_e: 0.9109534 × 10^-30 kg), and the mass of a proton (m_p: 1.6726485 × 10^-27 kg). As you can see, each of these natural units can be expressed in terms of SI units, but they are often used as basic units in specialized fields of science.