Standard Units of Measurement: the SI System
No matter what field of science you enter, you will need to take measurements, understand them, communicate them to others, and be able to repeat them. In other words, we all have to speak the same basic language. Whether you are a chemist, a physicist, a biologist, an engineer, or even a medical doctor, you will need a consistent way of communicating size, mass, shape, temperature, time, amount, energy, power, and speed.
Consider the screen on which you're reading this text right now. It might be an LCD screen, in which case a very specific orientation of very specific molecules is aligned and realigned under very specific electric fields at precisely defined places on the screen. This alignment controls the way that light travels through the molecules. These alignments are generated by various computer algorithms but are universally accepted and defined to closely reproduce color, appearance, intensity, etc.
Though different in many ways, these various fields of science also have a great deal in common -- and it is all based on measurements. The chemist developing his or her specific formulation for a liquid crystal has to communicate meaningful information to an engineer about the physical properties required for manufacture and synthesis, and the engineer has to be able to communicate with other engineers, physicists, and chemists to design circuit boards, display screens, and electronic interfaces. If all don't speak the same language, the enterprise will never get off the ground.
The International System of Units (abbreviated SI, from the French Système international d'unités) is the metric system used in science, industry, and medicine and in much commerce worldwide . Depending on your age and geographic location, you might be very familiar with the "Imperial" system. Imperial units include gallons, feet, miles, and pounds, and the system remains in common use in many places for "everyday" measurements. In much of Europe, and in all scientific circles, however, the SI, or metric, system is in common use.
Metric units include:
- the kilogram (kg), for mass
- the second (s), for time
- the Kelvin (K), for temperature
- the ampere (A), for electric current
- the mole (mol), for the amount of a substance
- the candela (cd), for luminous intensity
- the meter (m), for distance
These are the seven basic units in the SI system, illustrated nicely in .
There is much interesting history regarding the origins of the units and the measurements themselves -- they evolved over time with use and were defined for clarity and simplicity, essential requirements of a standard measurement.
- The meter, or metre (m), was originally defined as 1/10,000,000 of the distance from the Earth's equator to the North Pole, measured on the circumference through Paris. It is defined in modern terms as the length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second.
- The kilogram (kg) was defined as the mass of one thousandth of a cubic meter -- which is a liter -- of water. It is currently defined as the mass of a platinum-iridium kilogram sample maintained by the Bureau International des Poids et Mesures in Sevres, France.
- The second (s) was originally based on a "standard day" of 24 hours, with each hour divided in 60 minutes and each minute divided in 60 seconds. Currently, a second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.
- The ampere (A) was originally defined electrochemically as the current required to deposit 1.118 milligrams of silver per second from a solution of silver nitrate. At present, the ampere is the constant current that, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 \cdot 10−7 newton per meter of length.
- The kelvin (K) is the thermodynamic unit of temperature, with its origin at 0 K. The incremental size of the Kelvin is the same as the degree Celsius (centigrade), but the unit of the thermodynamic scale is the Kelvin. The kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water, which is 0.01 degrees Celsius.
- The mole (mol) is a number relating molecular or atomic mass to a constant number of particles. It is defined as the amount of a substance that contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12.
- The candela (cd) was originally "candlepower," back in the days when candles were common sources of illumination and their properties were standardized. In more modern terms, with the rise of incandescent and fluorescent light sources, the candela is the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 540 =cdot 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
It should be apparent that the move into modern times has greatly refined the conditions of measurement for each basic unit in the SI system, making the measurement of, for example, the luminous intensity of a light source a standard measurement in every laboratory in the world. A light source made to produce 20 cd will be the same regardless of whether it is made in the United States, in the Russian Federation, in the UK, or anywhere else. The use of the SI system provides all scientists and engineers with a common language of measurement.