International System Of Units
The International System of Units (SI) is a globally recognized measurement standard that emerged during the French Revolution in the late 18th century. It was established to provide a consistent framework for scientific measurement, aligning with the growing precision needed in fields like chemistry and physics. The SI is based on seven fundamental properties: length (meter), mass (kilogram), time (second), electric current (ampere), thermodynamic temperature (kelvin), amount of substance (mole), and luminous intensity (candela). These base units ensure that measurements are universally understood and can be replicated across different contexts.
In 2019, significant updates were made to the definitions of these units, linking them to universal constants rather than physical artifacts, which enhances their stability and precision. The metric system's simplicity, with its base-10 structure, contrasts sharply with the irregularities of systems like the Imperial or US customary measurements. Despite its wide adoption globally, acceptance of the SI in some regions, notably the United States, remains challenging due to entrenched nonmetric practices and cultural resistance. As international trade increases and scientific education evolves, the shift toward the metric system is likely to advance, but it will require ongoing efforts to overcome historical and social barriers.
International System Of Units
Summary
The development of the metric system, which served as the basis of the International System of Units (Le Système International d'Unités, or SI), occurred during the French Revolution in the mid-eighteenth century. This coincided with the beginning of the age of modern science, especially chemistry and physics, as the value of physical measurements in the conduct of those pursuits became apparent. As scientific activities became more precise and founded on sound theory, the common nature of science demanded an equally consistent system of units and measurements. The units in the SI have been defined by international accord to provide consistency in all fields of endeavor. The basic units are defined for only seven fundamental properties of matter. All other consistent units are derived as functions of these seven fundamental units.
Definition and Basic Principles
The International System of Units (SI) is the internationally accepted standard measurement system used worldwide. The SI units are ascribed to seven fundamental physical properties and two supplementary properties—length, mass, time, electric current, thermodynamic temperature, the amount of a substance, luminous intensity, and the magnitude of plane and solid angles.
Length is the extent of some physical structure or boundary in two dimensions, such as the distance from one point to another, how tall someone is, or the distance between nodes of a sinusoidal wave. The SI base unit associated with length is the meter.
Mass refers to the amount of material in a bulk quantity. The term is often used interchangeably with weight, although the mass of any object remains constant while its weight varies according to the strength of the gravitational field to which it is subject. The SI base unit for mass is the kilogram, defined in terms of the Planck constant (h).
Time is a more difficult property to define outside of itself, as it relates to the continuous progression of existence of some state through past, present, and future stages. The SI base unit of time is the second (s).
Electric current is the movement of electronic charge from one point to another, ascribed by the SI base unit of the ampere. One ampere is defined in relation to the elementary charge constant (e) as the movement of one coulomb of charge, as one mole of electrons, for one second.
Thermodynamic temperature refers to the measurement of temperature in relation to absolute zero. The SI base unit used is the kelvin, which is expressed in relation to the Boltzmann constant (k) of 1.380 649 × 10−23. It was previously based on the triple point of water, or the temperature and pressure at which water's three phases are in thermodynamic equilibrium.
The amount of a substance is defined by an SI base unit called the mole. The term is essentially never used outside the context of atoms, molecules, and certain subatomic particles, such as electrons. A mole of any substance is the quantity of that substance containing 6.02214129 × 1023 of these elementary entities. That number is known as the Avogadro constant.
The term luminousintensity refers to the brightness, or the quantity, of light or other forms of electromagnetic energy emitted from a source. Initially, luminous intensity was referred to by comparison to the light of a candle flame, but the variability of such a source is not conducive to standardization. The SI base unit for luminous intensity is the candela. This is about the amount of light emitted by a candle flame, but it has been standardized to mean an electromagnetic field strength of 0.00146 watts.
The derived unit known as the plane angle refers to the angular separation of two lines from a common point in a two-dimensional plane. The SI derived unit for plane angles is the radian. A complete rotation about a point origin is an angular displacement of 2π radians or 360°. The extension of this into three dimensions is known as the solid angle and is measured by the SI derived unit called the steradian.
Measuring any property or quantity is accomplished by comparing the particular amount of the property or quantity to the amount represented by the standard unit. For example, an object that is 3.62 times as long as the distance defined as 1 meter is said to be 3.62 meters long. Similarly, an object proportionately 6,486 times as massive as the quantity defined as 1 kilogram is said to weigh or have a mass of 6,486 kilograms. Using standard reference units such as the SI units ensures that measurements and quantities have the same meaning in all places where that system is used.
Background and History
Traditionally, measurement systems have employed units that relate to various parts or proportions of the human body. Ready and convenient measuring tools in earlier times included the hand, the foot, the thumb, and the pace. Because these human measures tend to vary from person to person, no work that depends on measurements can be repeated exactly, even if the same person has carried it out. Specialization in tasks eventually led to the realization that standard units of measurement would be beneficial. The determination of certain units were established by royal decree as long ago as the signing of the Magna Carta.
The Industrial Revolution in Europe and the growth of science as a common international pursuit drove the need for a unified system of measurement that would be independent of human variability and consistent from place to place. The International System of Units was developed and first used in about 1799. It represents the first real standardized system of measurements. Before this time, a broad variety of measurement systems were in use because many countries had developed their own measuring standards for use internally and in any territories that they held.
A need for a standardized system of measurement had been recognized by various luminaries and was proposed in 1670 by French scientist Gabriel Mouton. The incompatibility and variability of the many measurement systems used at that time often resulted in unfair trade practices and power struggles. In 1790, the French Academy of Sciences was charged with developing a system of measurement that would be fixed and independent of any inconsistencies arising from human intervention. On April 7, 1795, the French National Assembly decreed the use of the meter as the standard unit of length. It was defined as equal to one ten-millionth part of one quarter of the terrestrial meridian, specified by measurements undertaken between Dunkirk and Barcelona. The liter was later defined as being a unit of volume equal to the volume of a cube that is one-tenth of a meter on a side and the kilogram as being equal to the mass of one liter of pure water. Larger and smaller quantities are indicated by increasing and decreasing prefixes, all indicating units with some power of ten times greater or lesser than the base units.
Standard reference models of the metric units were made and kept at the Palais-Royale. It was then required that measuring devices used for trade and commerce be regularly checked for accuracy against the official standard versions. Eventually, as measurement methods became far more precise, it became clear that even these rigorously maintained reference models were susceptible to physical change over time, disrupting consistency. While adjustments were occasionally made, there was a movement to connect the definitions of the base units to more universally stable properties.
How It Works
All measurement practices relate the actual properties and proportions of something to a defined standard that is relevant to that property or proportion. For example, two-dimensional linear quantities are related to a standard of length, and the mass of an object is proportionately related to a standard unit of mass. For example, the standard SI unit for measuring length is the meter. An object determined to be, for example, 6.3 times longer than the base unit of one meter is, therefore, said to be 6.3 meters long. Similarly, an object that is, say, 5.5 times heavier than the base unit of one kilogram is said to weigh 5.5 kilograms, and something that has a volume of 22.4 times that of the base unit of one liter is said to have a volume of 22.4 liters.
The modern metric system is simple, especially when compared to its most common predecessor systems, the British Imperial and the US customary systems. Whereas these predecessor systems use units such as inches, feet, yards, ounces, and pounds that relate to one another irregularly, the metric system has always used the simple relation of factors of ten for all of its units. For example, the decameter is equal to ten meters (10 × 1 meter), while the decimeter is equal to one-tenth of a meter (0.1 × 1 meter). A complete series of prefixes indicates the size of the smaller and larger units used in a measurement. Thus, a decimeter is ten times smaller than a meter, a centimeter is a hundred times smaller than a meter, a millimeter is a thousand times smaller, and so on. Correspondingly, there are ten millimeters in a centimeter, ten centimeters in a decimeter, and ten decimeters in a meter. There are one thousand milliliters in a liter, one hundred liters in a hectoliter, one thousand grams in a kilogram, and one thousand milligrams in a gram. The uniformity of the system readily allows one to visualize and estimate relative sizes and quantities.
The basic units of the SI have always been defined in relation to some universal and unalterable standard. In some cases, the definitions have been changed to relate the unit to a more permanent universal feature or a more accurately known property. For example, as the accuracy of time measurement greatly improved, the definition of the meter was changed from being a specific fraction of the distance between two fixed terrestrial points to "the length of the path traveled by light in vacuum during a time interval of 1/299792458 of a second."
A major turning point along these lines came in November 2018, when sixty countries participating in the General Conference on Weights and Measures unanimously voted to revise the definitions of the SI base units so all seven would be based on universal constants. The meter, candela, and second already conformed to this requirement, but their definitions were amended with more precise language. The kilogram, kelvin, mole, and ampere were officially redefined in terms of constants, effective May 2019. Redefined, the kilogram is based on three criteria—the duration of the second, the length of one meter, and the Planck constant (h) (6.62607015 × 10−34 joule second). The kelvin was further specified based on the exact value 1.380649 × 10−23 joule/kelvin assigned to the Boltzmann constant (kB). The Avogadro constant (NA) was revised to equal 6.02214076 × 1023 mol-1 rather than 1 gram per mole. Widely hailed as a major development in science despite little impact on daily measurements for most people, the decision meant all base units would be tied to the most stable properties known.
The units of measurement of other properties and quantities reflect that those properties and quantities can be viewed as combinations of the seven fundamental properties. That is, for any dimensional property Q, the dimensions of Q are derived from the expression:
dim Q = LαMβTγIδθεNζJη
The dimensional exponents are integers representing the degree of involvement of the corresponding propertylength (L), mass (M), time (T), electric current (I), thermodynamic temperature (θ), amount of material (N), and luminous intensity (J). Remember that any quantity raised to the power 0 has the numerical value of 1.
Applications and Products
Applications and products related to the metric system essentially fall into two categorieseducational devices and training to promote familiarity with the system and metric versions of existing products and the tools required for maintenance.
Educational Devices and Training. The long history of independent measurement systems has served to entrench those systems in common usage. Therefore, a very large body of materials, products, and devices have been constructed using nonmetric measurements. More significantly, those independent systems have been so deeply entrenched in the education systems of many nations, including the United States, that several generations of people have grown up using no other system. The familiarity gained through a lifetime of using a particular system generally allows individuals to visualize and estimate quantities in terms of that measurement system. Changing to a different measurement system (such as the effort in the US to introduce the metric system in the 1970s) represents a paradigm shift that leaves many individuals unable to associate the new measurement units with even the most familiar dimensions. Such a change, however, can be accommodated in several ways as the world continues to adopt the metric system of measurement as its universal standard.
The most basic method of replacing one system with another is to incorporate the new system into the basic education system, teaching it to become the entrenched measurement system as children progress through school. It is, therefore, important not only that teachers are educated in the use of the metric system but also that they actively replace their reliance on any former system that they have used. This guarantees that, in time, the older system is completely displaced from the public lexicon and the metric system becomes the primary measurement system within essentially one generation.
Those who have already left the school system can learn the metric system through training programs. Training in the metric system can be incorporated into professional development programs at the workplace or offered in formal training programs by local educational institutions or third-party providers.
Metric Versions of Existing Devices. Most goods and devices produced in countries that have not adopted the metric standard must be maintained using their original component dimensions because metric and nonmetric values are not generally interchangeable. The fundamental difference between the two bases of measurement ensures that any coincidence of size from one system to the other is exactly thatcoincidental. Therefore, switching to metric goods and devices requires that complete new lines of products be made with metric rather than nonmetric dimensions. This requirement places an odd constraint on the situation because nonmetric parts and tools must still be produced to maintain existing nonmetric devices. At the same time, new devices to replace those that fail are produced in metric dimensions. This means that tradespeople, such as automobile mechanics, maintenance workers, and engineers, must obtain double sets of tools, and supply stores must maintain double sets of components and supplies. In addition, tools for taking measurements must be capable of using metric and nonmetric dimensions. However, the incorporation of electronic capabilities into many tools has greatly minimized the difficulties that arise from this dual requirement.
Careers and Course Work
The International System of Units is not broad enough to provide a field of study or advanced coursework and, by itself, is unlikely to form the basis of a career. However, when converting from a nonmetric measurement system to a metric system, opportunities will arise for those familiar with SI to create and provide training to facilitate understanding and use of the metric system.
Anyone entering a technical or scientific field must become familiar with the metric system, which has been the standard system of measurement in those areas since the SI was developed. As the metric system becomes increasingly widely adopted and accepted in the United States, students and others should expect to carry out measurements and calculations using the appropriate metric system units.
Careers that depend specifically on measurement include quality-control engineering and most branches of scientific research and physical engineering. Specific examples of measurement-based careers include civil engineering, medical and biochemical analysis, analytical chemistry, metrology, mechanical engineering, and industrial chemical engineering.
Social Context and Future Prospects
The 2019 redefinition of the SI base units represented a significant moment in history for the scientific community. It marked the first time that all the measurements within the metric system would be based on fundamental natural properties rather than physical characteristics. While most individuals' use of the metric system was unchanged by the redefinition, it was hailed by scientists for its potential to spark new theoretical and technological innovations. In addition, it was expected to reduce the complexity and cost of calibration for precision devices in laboratories and industrial applications. However, the changes were criticized by some members of the scientific community, who questioned the specifics of the new definitions and even debated the legitimacy of some of the base units.
Efforts continue to make the metric system the standard system of measurement in the United States, as it already is in most of the rest of the world. As international trade and offshore manufacturing increase, the necessity for American industry to adopt the SI increases. In addition, educational systems have increased their focus on science and technology, making metric systems more familiar to children in American schools. However, resistance to abandoning the US customary system of measures remains widespread, and metric adoption is subject to changing political and social attitudes.
In the meantime, the development of other measurement systems and hybrids of measuring systems progresses. For example, the manufacturing community adopted a standard unit called the metric inch, which is the standard inch divided into hundredths, to be used on visual measuring devices. There is some logic to this adoption, as this measurement corresponds with the limits of differentiation of which the human eye is capable to a better degree than the millimeter division of the metric system. However, such developments will likely delay or interfere with adopting the metric system.
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