Since the dawn of civilisation, humans have had the necessity to give value and quantity to physical measurements made in the natural world for ease of communication and understanding. For example, the passing of time can be quantifiable by assuming that it is constant, or an object can be ‘weighed’ by its mass relative to another object. For each of these quantifiable aspects of our world, we have created a unit which is used to describe the magnitude or quantity of that measurement. We are currently aware of seven quantities which we are able to measure, and from which we can derive the measurements for all other quantities.
Time – The fundamental unit for time is the second (s). It was originally based on the amount of time in a day of 24 hours, where each hour is divided into 60 minutes and each minute is divided into 60 seconds. However, we now know that a complete rotation of the Earth actually takes 23 hours, 56 minutes, and 4.1 seconds, so we quantify the second by defining it as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom. Confusing, huh? I think I’ll just stick with the second.
Mass – The fundamental unit of mass is the kilogram (kg). It was originally based on the mass of one litre of water. It is currently defined as the mass of a platinum-iridium kilogram sample which is maintained by the Bureau International des Poids et Mesures in Sevres, France along with six identical sister copies in a secure vault which is rarely opened.
Length – The fundamental unit of length is the metre (m). It was originally defined as 1/10,000,000 of the distance from the Earth’s equator to the North Pole as measured on the circumference through Paris. It is currently defined as the distance travelled by light in a vacuum over a time interval of 1/299,892,458 of a second.
Electric current – The fundamental unit of electric current is the ampere (A), which is a measure of the amount of electric charge passing a point in an electric circuit in a given time. The original standard ampere was defined as the current required to deposit 1.118 milligrams of silver per second from a solution of silver nitrate. Compared to the currently accepted quantity, it differs by 0.015%, which is fairly significant. The current definition of the ampere (pardon the pun) is a constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2×10-7 newtons per metre of length.
Temperature – The fundamental unit of temperature is the kelvin (K). This used to use the same scale as Celsius, with one increment being one degree change in temperature. 0K is absolute zero, the lowest temperature at which anything can exist (about -273.16 degrees Celsius). You can therefore convert between Celsius and kelvin by adding or subtracting 273 from the value. The modern definition of the kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water, which is the temperature and pressure at which the three phases (gas, liquid, and solid) of water coexist in thermodynamic equilibrium.
Amount of substance – The fundamental unit of the amount of substance is the mole (mol), and it is a number that relates molecular or atomic mass to a constant number of particles. It was originally defined as the molecular mass of a substance in grams. It is now defined as the amount of a substance that contains as many elementary particles as there are atoms in 0.012 kg of carbon-12.
Luminous intensity – The fundamental unit of luminous intensity is the candela (cd). In the past when candles were the most common form of illumination, it was referred to as ‘candlepower’ and was based on the brightness of candles. Now, with the prevalence of incandescent and fluorescent light sources (Let There Be Fluorescent Light…), the candela is defined as the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 5.4 ×1014 Hz and that has a radiant intensity in that direction of 1/683 watts per steradian.
These seven fundamental units make up the basis of the International System of Units (abbreviated SI, from the French système international d’unités), which is the standard unit system used worldwide in science, industry and medicine. All other SI units can be derived from these fundamental units alone. Scientists are ever striving to make their field more and more precise; the current unit derivations are too dependent on physical values. Many proposals have been made to gradually change all of the fundamental SI units so that they are derived from universal constants, like hoq length is based on the speed of light. For the time being, however, I think we can cope with counting the seconds in our heads. One elephant… two elephants… three elephants…