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Measurement

2007 Schools Wikipedia Selection. Related subjects: Mathematics

   Various meters
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   Various meters

   Measurement is the estimation or determination of extent, dimension or
   capacity, usually in relation to some standard or unit of measurement.
   The measurement is expressed as a number of units of the standard (a
   real number times a unit), such as distance being indicated by a number
   of miles or kilometers.

   The process of measuring involves estimating the ratio of the magnitude
   of a quantity to the magnitude of a unit of the same type (e.g. length,
   time, mass, etc.). A measurement is the result of such a process,
   expressed as the product of a real number and a unit, where the real
   number is the estimated ratio. An example is 9 metres, which is an
   estimate of an object's length relative to a unit of length, the metre.
   Unlike a count, or integer quantity of items that is known exactly,
   every measurement is an estimate that has some uncertainty.

Overview

   In the natural sciences, the act of measuring an object normally
   involves comparing the magnitude of a quantity possessed by an object
   with a standard unit by using an instrument under controlled
   conditions. Examples of measuring instruments include the thermometer,
   speedometer, weighing scale and voltmeter. In order to measure
   accurately, measuring instruments must be carefully constructed and
   calibrated. However, all measurements have some degree of uncertainty
   associated with them, which is usually expressed as a standard error of
   measurement. This means that while a measurement is usually given as a
   number followed by a unit, every measurement has three components; the
   estimate, an error bound, and a probability that the actual magnitude
   lies within the error bound of the estimate. For example, a measurement
   of a plank might result in a measurement of 9 meters plus or minus 0.01
   meters, with a probability of 0.95.

   A measurement is usually distinguished from a count. A measurement is a
   real number, and is never exact. A count is a natural number and may be
   exact. For example, we can determine that there are exactly 12 eggs in
   a carton by counting them. However some groups are not so easily
   counted, and estimating their numbers can involve similar issues to
   physical measurement. For example, figures for the number of people
   with HIV or the number of stars in the Milky Way will have associated
   standard errors, and can be viewed as estimates rather than exact
   counts.

   Measurement is fundamental to most fields of science, including
   physics, chemistry and biology. Measurement is also essential to a
   diverse range of industries and commercial applications such as in
   engineering, construction, manufacturing, pharmaceutical production and
   electronics.

Other uses of the term

   In addition to the definition of measurement given above, the term is
   also often used in a looser fashion to refer to any process which
   numbers are assigned to entities to represent increasing amount or
   degree in some sense. For example, counts of raw scores on tests are
   sometimes referred to as measurements. Other examples include consumer
   confidence and the rate of increase in the price of a good or service.

History

   Laws to regulate measurement were originally developed to prevent
   fraud. However, units of measurement are now generally defined on a
   scientific basis, and are established by international treaties. In the
   United States, commercial measurements are regulated by the National
   Institute of Standards and Technology NIST, a division of the United
   States Department of Commerce.

   The history of measurements is a topic within the history of science
   and technology. The metre (us: meter) was standardized as the unit for
   length after the French revolution, and has since been adopted
   throughout most of the world. The United States and the UK are in the
   process of converting to the SI system. This process is known as
   metrication.

Units and systems of measurement

   Because measurement involves the estimation of magnitudes of quantities
   relative to particular quantities, called units, the specification of
   units is of fundamental importance to measurement. The definition or
   specification of precise standards of measurement involves two key
   features, which are evident in the International System of Units (SI).
   Specifically, in this system the definition of each of the base units
   makes reference to specific empirical conditions and, with the
   exception of the kilogram, also to other quantitative attributes. Each
   derived SI unit is defined purely in terms of a relationship involving
   itself and other units; for example, the unit of velocity is 1 m/s. Due
   to the fact that derived units make reference to base units, the
   specification of empirical conditions is an implied component of the
   definition of all units.

   The measurement of a specific entity or relation results in at least
   two numbers for the relationship between the entity or relation under
   study and the referenced unit of measurement, where at least one number
   estimates the statistical uncertainty in the measurement, also referred
   to as measurement error. Measuring instruments are used to estimate
   ratios of magnitudes to units. Prior comparisons underlie the
   calibration, in terms of standard units, of commonly used instruments
   constructed to measure physical quantities.

Imperial system

   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 distance, weight and time. It is interesting to note that many
   Imperial units remain in use in Britain despite the fact that it has
   mostly switched to the SI system. Road signs are still in miles, yards,
   miles per hour, etc, people tend to measure their own height in feet
   and inches and beer is sold in pints, to give just a few examples.
   Imperial units are used in many other places, for example, in many
   Commonwealth countries which are considered metricated, land area is
   measured in acres and floor space in square feet, particularly for
   commercial transactions (rather than government statistics). Similarly,
   the imperial gallon is used in many countries that are considered
   metricated at gas/petrol stations, an example being the United Arab
   Emirates.

Metric system

   The metric system is a decimalised system of measurement based on the
   meter and the gram. 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 (SI), explained
   further below, is the internationally recognised 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 is has a single base unit for
   each physical quantity. All other units are powers of ten 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 meters, or
   thousandths of a metre (millimeters), or thousands of meters
   (kilometres), and so on. There is no profusion of different units with
   different conversion factors as in the Imperial system (e.g. inches,
   feet, yards, fathoms, rods). 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, such as 2/5
   of a meter, is not prohibited, but uncommon.

SI

   The International System of Units (abbreviated SI from the French
   language name Système International d'Unités) is the modern, revised
   form 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 (MKS) system, rather than the
   centimetre-gram-second (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.

   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/m^3.

Converting Prefixes

   The SI allows easy multiplication when switching among units having the
   same base but different prefixes. If you are working with meters and
   want to convert to centimeters, you only need to multiply the number of
   meters by 100 because there are 100 centimeters in a meter. Inversely,
   to switch from centimeters to meters you multiply the number of
   centimeters by .01.

Length

   A 2 metre carpenter's rule
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   A 2 metre carpenter's rule

   A ruler or rule is a tool used in geometry, technical drawing and
   engineering/building to measure distances and/or to rule straight
   lines. Strictly speaking, the ruler is the instrument used to rule
   lines and the calibrated instrument used for determining measurement is
   called a measure. However, common usage is that a ruler is calibrated
   so that it can measure.

   Several different designs of flexible instruments are used to determine
   length, such as the carpenter's rule, the ribbon-like tape measure used
   by tailors, and the retractable rule used especially in the
   construction trades and by home handyman, also known as a tape measure.
   As can be seen by the photos on this page, a 2 metre carpenter's rule
   can be folded down to a length of only 20 centimetres to easily fit in
   a pocket, and the 5 metre long tape easily retracts to fit within a
   small-sized housing.

Time

   The most common devices for measuring time are the clock, for periods
   less than a day, and the calendar, for periods longer than a day.
   Clocks can range from watches, to more exotic varieties such as the
   Clock of the Long Now. They can be driven by a variety of means,
   including a pendulum. There are also a variety of different calendars,
   for example the Lunar calendar and the Solar calendar, although the
   Gregorian calendar is the most commonly used.

   A chronometer is a timekeeper precise enough to be used as a portable
   time standard, usually in order to determine longitude by means of
   celestial navigation.

   The most accurate type of measuring devices for time is the atomic
   clock. More archaic devices include the hourglass, the sundial, the
   tempometer and the water clock.

Mass

   A weighing scale is a device for measuring the weight of an object.
   Until digital scales, the most accurate means of measuring the weight
   or mass of an object was using a balance. In its conventional form,
   this class of measuring instrument compares the sample, placed in a
   weighing pan (weighing basin) and suspended from one end of a beam with
   a standard mass or combination of standard masses in a scale pan (scale
   basin) suspended from the other end. To weigh an object in the
   measuring pan, standard weights are added to the scale pan until the
   beam is in equilibrium as closely as possible. Less accurate, but very
   versitile is the spring-based scale which has a calibrated spring that
   deforms linearly as more weight is put on it. Mass can also refer to
   how much inertia an object has.

Metrology

   Metrology is the study of measurement. In general, a metric is a scale
   of measurement defined in terms of a standard: i.e. in terms of
   well-defined unit. The quantification of phenomena through the process
   of measurement relies on the existence of an explicit or implicit
   metric, which is the standard to which measurements are referenced. If
   one says I am 5, that person is indicating a measurement without
   supplying an applicable standard. He could mean I am 5 years old or I
   am 5 feet high, however the implicit metric is that he is 5 years old.

Probabilistic measurement

   Measurement is not limited to physical quantities and relations but can
   extend to the quantification of a magnitude of any kind. In the social
   sciences and other fields such as health, biology and market research,
   probabilistic models such as the Rasch model for measurement are
   applied in order to measure using instruments such as questionnaires
   and assessments which enable comparisons between persons. The field of
   psychometrics is concerned with the theory and technique of measurement
   of psychological and mental phenomena.

Difficulties in measurement

   For physical quantities gaining accurate measurement can be difficult.
   It is not possible to be exact; instead, repeated measurements will
   vary due to various factors affecting the quantity such as temperature,
   time, electromagnetic fields, and especially measurement method. As an
   example in the measurement of the speed of light, the quantity is now
   known to a high degree of precision due to modern methods, but even
   with those methods there is some variability in the measurement.
   Statistical techniques are applied to the measurement samples to
   estimate the speed. In earlier sets of measurements, the variability
   was greater, and comparing the results shows that the variability and
   bias in the measurement methods was not properly taken into account.
   Proof of this is that when various group's measurements are plotted
   with the estimated speed and error bars showing the expected
   variability of the estimated speed from the actual number, the error
   bars from each of the experiments did not all overlap. This means a
   number of groups incorrectly accounted for the true sources of error
   and overestimated the accuracy of their methods.
   Retrieved from " http://en.wikipedia.org/wiki/Measurement"
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