In science, measurement is the process of estimating or determining the magnitude of a quantity, such as length or mass, relative to a unit of measurement A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of measurement, such as a metre or a kilogram. The term measurement can also be used to refer to a specific result obtained from the measurement process.

History

The word "measurement" is derived from the Greek word "metron," which means a limited proportion.

The history of measurements is a topic within the history of science and technology The history of science and technology is a field of history which examines how humanity's understanding of the natural world (science) and ability to manipulate it (technology) have changed over the millennia. This academic discipline also studies the cultural, economic, and political impacts of scientific innovation.

Standards

With the exception of a few seemingly fundamental quantum In physics, a quantum is the minimum unit of any physical entity involved in an interaction. Behind this, one finds the fundamental notion that a physical property may be "quantized," referred to as "the hypothesis of quantization". This means that the magnitude can take on only certain discrete numerical values, rather than constants, units of measurement are essentially arbitrary; in other words, people make them up and then agree to use them. Nothing inherent in nature dictates that an inch An inch is the name of a unit of length in a number of different systems, including Imperial units, and United States customary units. There are 36 inches in a yard and 12 inches in a foot. A corresponding unit of area is the square inch and a corresponding unit of volume is the cubic inch. The inch is usually the universal unit of measurement in has to be a certain length, or that a mile A mile is a unit of length in a number of different systems. In contemporary English, a mile most commonly refers to the statute mile of 5,280 feet , the survey mile of 5,280 survey feet (1,609.3472 metres (5,280.01 ft)) or the nautical mile of 1,852 metres (6,076.12 ft). It is about a third of the old measurement, the league is a better measure of distance than a kilometer The kilometre , symbol km is a unit of length in the metric system, equal to one thousand metres and is therefore exactly equal to the distance travelled by light in free space in 1⁄ 299,792.458 of a second. It is the conventionally used measurement unit for expressing distances between geographical places in most of the world; notable. Over the course of human history, however, first for convenience and then for necessity, standards of measurement evolved so that communities would have certain common benchmarks. Laws regulating measurement were originally developed to prevent fraud The specific legal definition varies by legal jurisdiction. Fraud is a crime, and also a civil law violation. Defrauding people or entities of money or valuables is a common purpose of fraud, but there have also been fraudulent "discoveries", e.g. in science, to gain prestige rather than immediate monetary gain in commerce. Today, units of measurement are generally defined on a scientific basis, overseen by governmental or supra-governmental agencies, and established in international treaties. The metre, for example, was redefined in 1983 as the distance traveled by light in free space in 1⁄299,792,458 of a second. In the United States ^ b. English is the de facto language of American government and the sole language spoken at home by 80% of Americans age five and older. Spanish is the second most commonly spoken language, the National Institute of Standards and Technology (NIST The National Institute of Standards and Technology , known between 1901 and 1988 as the National Bureau of Standards (NBS), is a measurement standards laboratory which is a non-regulatory agency of the United States Department of Commerce. The institute's official mission is to:), a division of the United States Department of Commerce The United States Department of Commerce is the Cabinet department of the United States government concerned with promoting economic growth. It was originally created as the United States Department of Commerce and Labor on February 14, 1903. It was subsequently renamed to the Department of Commerce on March 4, 1913, and its bureaus and agencies, regulates commercial measurements. In the United Kingdom, the role is performed by the National Physical Laboratory (NPL).

Units and systems

Imperial system

Before SI units The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. It is the world's most widely used system of measurement, both in everyday commerce and in science were widely adopted around the world, the British systems of English units Within the United States, the same term is commonly used to refer to the United States Customary System, which retains some unit names but with different values, as well as to the imperial units and later imperial units Imperial units or the imperial system is a system of units, first defined in the British Weights and Measures Act of 1824, later refined and reduced. The system came into official use across the British Empire. By the late 20th century most nations of the former empire had officially adopted the metric system as their main system of measurement were used in Britain, the Commonwealth The Commonwealth of Nations, normally referred to as the Commonwealth and previously as the British Commonwealth, is an intergovernmental organisation of fifty-four independent member states. All but two of these countries were formerly part of the British Empire and the United States. The system came to be known as U.S. customary units The United States customary system is the most commonly used system of measurement in the United States. It is similar but not identical to the British Imperial units. The U.S. is the only industrialized nation that does not mainly use the metric system in its commercial and standards activities, although the International System of Units (SI, in the United States and is still in use there and in a few Caribbean The Caribbean is a region consisting of the Caribbean Sea, its islands , and the surrounding coasts. The region is located southeast of the Gulf of Mexico and North America, east of Central America, and to the north of South America countries. These various systems of measurement have at times been called foot-pound-second systems after the Imperial units for distance, weight and time even though the tons, hundredweights, gallons, and nautical miles, for example, are different for the U.S. units. Many Imperial units remain in use in Britain despite the fact that it has officially switched to the SI system. Road signs are still in miles, yards, miles per hour, and so on, people tend to measure their own height in feet and inches and milk is sold in pints, 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 (rather than government statistics). Similarly, gasoline is sold by the gallon in many countries that are considered metricated.

Metric system

The metric system The metric system is an international decimalised system of measurement, first adopted by France in 1791, that is the common system of measuring units used by most of the world. It exists in several variations, with different choices of fundamental units, though the choice of base units does not affect its day-to-day use. Over the last two is a decimalized system of measurement A system of measurement is a set of units which can be used to specify anything which can be measured and were historically important, regulated and defined because of trade and internal commerce. Scientifically, when later analyzed, some quantities are designated as fundamental units meaning all other needed units can be derived from them, based on the metre and the gram The gram , (Greek/Latin root grámma); symbol g, is a unit of mass. It exists in several variations, with different choices of base units A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of measurement, though these do not affect its day-to-day use. Since the 1960s, the International System of Units (SI) The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. It is the world's most widely used system of measurement, both in everyday commerce and in science, explained further below, is the internationally recognized standard metric system. Metric units of mass, length, and electricity are widely used around the world for both everyday and scientific purposes. The main advantage of the metric system is that it has a single base unit for each physical quantity. All other units are powers of ten Powers of Ten is a 1977 short documentary film written and directed by Ray Eames and her husband, Charles Eames. The film depicts the relative scale of the Universe in factors of ten . The film is an adaptation of the 1957 book Cosmic View by Kees Boeke, and more recently is the basis of a new book version. Both adaptations, film and book, follow or multiples of ten of this base unit. Unit conversions are always simple because they will be in the ratio of ten, one hundred, one thousand, etc. All lengths and distances, for example, are measured in metres, or thousandths of a metre (millimetres), or thousands of metres (kilometres), and so on. There is no profusion of different units with different conversion factors as in the Imperial system (e.g. inches An inch is the name of a unit of length in a number of different systems, including Imperial units, and United States customary units. There are 36 inches in a yard and 12 inches in a foot. A corresponding unit of area is the square inch and a corresponding unit of volume is the cubic inch. The inch is usually the universal unit of measurement in, feet, yards A yard is a unit of length in several different systems, including English units, Imperial units, and United States customary units. It is equal to 3 feet or 36 inches, although its length in SI units varied slightly from system to system. The most commonly used yard today is the international yard, which is defined to be exactly 0.9144 metre, fathoms A fathom is a unit of length in the Imperial system (and the derived U.S. customary units), used especially for measuring the depth of water, rods The rod is a unit of length equal to 5.5 yards, 5.0292 metres, 16.5 feet, or 1⁄320 of a statute mile. A rod is the same length as a perch and a pole. The lengths of the perch and chain (four rods) were standardized in 1607 by Edmund Gunter. In old English, the term lug is also used). Multiples and submultiples are related to the fundamental unit by factors of powers of ten, so that one can convert by simply moving the decimal place: 1.234 metres is 1234 millimetres or 0.001234 kilometres. The use of fractions A fraction is a number that can represent part of a whole, such as 2/5 of a metre, is not prohibited, but uncommon.

SI

The International System of Units The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. It is the world's most widely used system of measurement, both in everyday commerce and in science (abbreviated SI from the French language French is a Romance language spoken as a first language by about 136 million people worldwide. Around 190 million people speak French as a second language, and an additional 200 million speak it as an acquired foreign language. French speaking communities are present in 57 countries and territories. Most native speakers of the language live in name Système International d'Unités) is the modern, revised form of the metric system The metric system is an international decimalised system of measurement, first adopted by France in 1791, that is the common system of measuring units used by most of the world. It exists in several variations, with different choices of fundamental units, though the choice of base units does not affect its day-to-day use. Over the last two. It is the world's most widely used system of units A system of measurement is a set of units which can be used to specify anything which can be measured and were historically important, regulated and defined because of trade and internal commerce. Scientifically, when later analyzed, some quantities are designated as fundamental units meaning all other needed units can be derived from them,, both in everyday commerce Commerce is a division of trade or production which deals with the exchange of goods and services from producer to final consumer OR commerce is the exchange of goods and services from the point of production to the point of consumption to satisfy human wants. It comprises the trading of something of economic value such as goods, services, and in science Science is, in its broadest sense, any systematic knowledge that is capable of resulting in a correct prediction or reliable outcome. In this sense, science may refer to a highly skilled technique, technology, or practice. The SI was developed in 1960 from the metre-kilogram-second The second , sometimes abbreviated sec., is the name of a unit of time, and is the International System of Units (SI) base unit of time. It may be measured using a clock (MKS) system, rather than the centimetre-gram-second The centimetre-gram-second system is a metric system of physical units based on centimetre as the unit of length, gram as a unit of mass, and second as a unit of time. All CGS mechanical units are unambiguously derived from these three base units, but there are several different ways of extending the CGS system to cover electromagnetism (CGS) system, which, in turn, had many variants. At its development the SI also introduced several newly named units that were previously not a part of the metric system. The SI units for the four basic physical quantities: length, time, mass, and temperature are:

1. metre (m) :SI unit of length
2. second (s) :SI unit of time
3. kilogram (kg) :SI unit of mass
4. kelvin (K) :SI unit of temperature

There are two types of SI units, base and derived units. Base units are the simple measurements for time, length, mass, temperature, amount of substance, electric current and light intensity. Derived units are made up of base units, for example, density is kg/m³.

Converting prefixes

The SI allows easy multiplication when switching among units having the same base but different prefixes. To convert from metres to centimetres it is only necessary to multiply the number of metres by 100, since there are 100 centimetres in a metre. Inversely, to switch from centimetres to metres one multiplies the number of centimetres by 0.01 or divide centimetres by 100.

Distance

A ruler A ruler, sometimes called a rule or line gauge, is an instrument used in geometry, technical drawing, printing and engineering/building to measure distances and/or to rule straight lines. Strictly speaking, the ruler is essentially a straightedge used to rule lines[citation needed], but typically the ruler also contains calibrated lines to measure or rule is a tool used in, for example, geometry Geometry "Earth-measuring" is a part of mathematics concerned with questions of size, shape, relative position of figures, and the properties of space. Geometry is one of the oldest sciences. Initially a body of practical knowledge concerning lengths, areas, and volumes, in the 3rd century BC geometry was put into an axiomatic form by, technical drawing Technical drawing, also known as drafting, is the academic discipline of creating standardized technical drawings by architects, interior designers, drafters, design engineers, and related professionals. Standards and conventions for layout, line thickness, text size, symbols, view projections, descriptive geometry, dimensioning, and notation are, engineering, and carpentry, to measure distances or to draw straight lines. Strictly speaking, the ruler is the instrument used to rule straight lines and the calibrated instrument used for determining length is called a measure, however common usage calls both instruments rulers and the special name straightedge is used for an unmarked rule. The use of the word measure, in the sense of a measuring instrument, only survives in the phrase tape measure, an instrument that can be used to measure but cannot be used to draw straight lines. As can be seen in the photographs on this page, a two-metre carpenter's rule can be folded down to a length of only 20 centimetres, to easily fit in a pocket, and a five-metre long tape measure easily retracts to fit within a small housing.

Some special names

We also use some special names for some multiples of some units.

• 100 kilograms = 1 quintal;1000 kilogram = 1 metric tonne;
• 10 years = 1 decade; 100 years = 1 century; 1000 years = 1 millennium

Building trades

The Australian building trades adopted the metric system The metric system is an international decimalised system of measurement, first adopted by France in 1791, that is the common system of measuring units used by most of the world. It exists in several variations, with different choices of fundamental units, though the choice of base units does not affect its day-to-day use. Over the last two in 1966 and the units used for measurement of length are metres (m) and millimetres (mm). Centimetres (cm) are avoided as they cause confusion when reading plans. For example, the length two and a half metres is usually recorded as 2500 mm or 2.5 m; it would be considered non-standard to record this length as 250 cm.

Time

Mass

Mass refers to the intrinsic property of all material objects to resist changes in their momentum. Weight, on the other hand, refers to the downward force produced when a mass is in a gravitational field. In free fall, (no net gravitational forces) objects lack weight but retain their mass. The Imperial units of mass include the ounce, pound, and ton. The metric units gram and kilogram are units of mass.

One device for measuring weight or mass is called a weighing scale or, often, simply a scale. A spring scale measures force but not mass, a balance compares masses, but requires a gravitational field to operate. Some of the most accurate instruments for measuring weight or mass are based on load cells with a digital read-out, but require a gravitational field to function and would not work in free fall.

Economics

The measures used in economics are physical measures, nominal price value measures and fixed price value measures. These measures differ from one another by the variables they measure and by the variables excluded from measurements. The measurable variables in economics are quantity, quality and distribution. By excluding variables from measurement makes it possible to better focus the measurement on a given variable, yet, this means a narrower approach.

Difficulties

Since accurate measurement is essential in many fields, and since all measurements are necessarily approximations, a great deal of effort must be taken to make measurements as accurate as possible. For example, consider the problem of measuring the time it takes an object to fall a distance of one metre (39 in). Using physics, it can be shown that, in the gravitational field of the Earth, it should take any object about 0.45 second to fall one metre. However, the following are just some of the sources of error that arise. First, this computation used for the acceleration of gravity 9.8 metres per second per second (32.2 ft/s²). But this measurement is not exact, but only precise to two significant digits. Also, the Earth's gravitational field varies slightly depending on height above sea level and other factors. Next, the computation of .45 seconds involved extracting a square root, a mathematical operation that required rounding off to some number of significant digits, in this case two significant digits.

So far, we have only considered scientific sources of error. In actual practice, dropping an object from a height of a metre stick and using a stopwatch to time its fall, we have other sources of error. First, and most common, is simple carelessness. Then there is the problem of determining the exact time at which the object is released and the exact time it hits the ground. There is also the problem that the measurement of the height and the measurement of the time both involve some error. Finally, there is the problem of air resistance.

Scientific experiments must be carried out with great care to eliminate as much error as possible, and to keep error estimates realistic.

Definitions and theories

Classical definition

In the classical definition, which is standard throughout the physical sciences, measurement is the determination or estimation of ratios of quantities. Quantity and measurement are mutually defined: quantitative attributes are those possible to measure, at least in principle. The classical concept of quantity can be traced back to John Wallis and Isaac Newton, and was foreshadowed in Euclid's Elements.^[1]

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