Measurement
Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events. In other words, measurement is a process of determining how large or small a physical quantity is as compared to a basic reference quantity of the same kind.
The scope and application of measurement are dependent on the context and discipline. In natural sciences and engineering, measurements do not apply to nominal properties of objects or events, which is consistent with the guidelines of the International Vocabulary of Metrology published by the International Bureau of Weights and Measures. However, in other fields such as statistics as well as the social and behavioural sciences, measurements can have multiple levels, which would include nominal, ordinal, interval and ratio scales.
Measurement is a cornerstone of trade, science, technology and quantitative research in many disciplines. Historically, many measurement systems existed for the varied fields of human existence to facilitate comparisons in these fields. Often these were achieved by local agreements between trading partners or collaborators. Since the 18th century, developments progressed towards unifying, widely accepted standards that resulted in the modern International System of Units. This system reduces all physical measurements to a mathematical combination of seven base units. Metrology is the science of measurement.
Measurement can also be described as the comparison of an unknown quantity with a known or standard quantity.
History
Methodology
The measurement of a property may be categorised by the following criteria: type, magnitude, unit, and uncertainty. They enable unambiguous comparisons between measurements.- The level of measurement is a taxonomy for the methodological character of a comparison. For example, two states of a property may be compared by ratio, difference, or ordinal preference. The type is commonly not explicitly expressed, but implicit in the definition of a measurement procedure.
- The magnitude is the numerical value of the characterisation, usually obtained with a suitably chosen measuring instrument.
- A unit assigns a mathematical weighting factor to the magnitude that is derived as a ratio to the property of an artefact used as standard or a natural physical quantity.
- An uncertainty represents the random and systemic errors of the measurement procedure; it indicates a confidence level in the measurement. Errors are evaluated by methodically repeating measurements and considering the accuracy and precision of the measuring instrument.
Standardisation of measurement units
The first proposal to tie an SI base unit to an experimental standard independent of a physical artefact reference standard was by Charles Sanders Peirce, who proposed to define the metre in terms of the wavelength of a spectral line. This directly influenced the Michelson–Morley experiment; Michelson and Morley cite Peirce, and improve on his method.
Standards
With the exception of a few fundamental quantum constants, units of measurement are derived from historical agreements. Nothing inherent in nature dictates that an inch has to be a certain length, nor that a mile is a better measure of distance than a kilometre. Over the course of human history, however, first for convenience and then out of necessity, standards of measurement evolved so that communities would have certain common benchmarks. Laws regulating measurement were originally developed to prevent fraud in commerce.Units of measurement are generally defined on a scientific basis, overseen by governmental or independent agencies, and established in international treaties, pre-eminent of which is the General Conference on Weights and Measures, established in 1875 by the Metre Convention, overseeing the International System of Units. For example, the metre was redefined in 1983 by the CGPM in terms of the speed of light, the kilogram was redefined in 2019 in terms of the Planck constant and the international yard was defined in 1960 by the governments of the United States, United Kingdom, Australia and South Africa as being exactly 0.9144 metres.
In the United States, the National Institute of Standards and Technology, a division of the United States Department of Commerce, regulates commercial measurements. In the United Kingdom, the role is performed by the National Physical Laboratory, in Australia by the National Measurement Institute, in South Africa by the Council for Scientific and Industrial Research, and in India by the National Physical Laboratory of India.
Units and systems
A unit is a known or standard quantity in terms of which other physical quantities are measured.File:MetricImperialUSCustomaryUnits.jpg|thumb|upright|A baby bottle that measures in three measurement systems—metric, imperial, and US customaryImperial and US customary systems
Before SI units were widely adopted around the world, the British systems of English units and later imperial units were used in Britain, the Commonwealth and the United States. The system came to be known as U.S. customary units in the United States and is still in use there and in a few Caribbean countries. These various systems of measurement have at times been called foot-pound-second systems after the Imperial units for length, weight and time even though the tons, hundredweights, gallons, and nautical miles, for example, have different values in the U.S. and imperial systems. Many Imperial units remain in use in Britain, which has officially switched to the SI system, with a few exceptions such as road signs, where road distances are shown in miles and speed limits are in miles per hour. Draught beer and cider must be sold by the imperial pint, and milk in returnable bottles can be sold by the imperial pint. Many people measure their height in feet and inches and their weight in stone and pounds, to give just a few examples. Imperial units are used in many other places: for example, in many Commonwealth countries that are considered metricated, land area is measured in acres and floor space in square feet, particularly for commercial transactions. Similarly, gasoline is sold by the gallon in many countries that are considered metricated.Metric system
The metric system is a decimal system of measurement based on its units for length, the metre and for mass, the kilogram. It exists in several variations, with different choices of base units, though these do not affect its day-to-day use. Since the 1960s, the International System of Units is the internationally recognised metric system. Metric units of mass, length, and electricity are widely used around the world for both everyday and scientific purposes.International System of Units
The International System of Units is the modern revision of the metric system. It is the world's most widely used system of units, both in everyday commerce and in science. The SI was developed in 1960 from the metre–kilogram–second system, rather than the centimetre–gram–second system, which, in turn, had many variants. The SI units for the seven base physical quantities are:| Base quantity | Base unit | Symbol | Defining constant |
| time | second | s | hyperfine splitting in caesium-133 |
| length | metre | m | speed of light, c |
| mass | kilogram | kg | Planck constant, h |
| electric current | ampere | A | elementary charge, e |
| temperature | kelvin | K | Boltzmann constant, k |
| amount of substance | mol | mol | Avogadro constant, NA |
| luminous intensity | candela | cd | luminous efficacy of a 540 THz source, Kcd |
In the SI, base units are the simple measurements for time, length, mass, temperature, amount of substance, electric current and light intensity. Derived units are constructed from the base units: for example, the watt, i.e. the unit for power, is defined from the base units as m2·kg·s−3. Other physical properties may be measured in compound units, such as material density, measured in kg·m−3.