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Copy of Standards of Measurements
Transcript of Copy of Standards of Measurements
**The National Institute of Standards and Technology (NIST)
***The National Physical Laboratory (NPL) -- Great Britain Secondary Standards : Basic reference standards used in industrial measurement laboratories . Working Standards : They are the principle tools of a measurement laboratory . They are used to check and calibrate general laboratory instruments for accuracy and performance . ex : quality control departments . Absolute Ampere ampere is that it is the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one meter apart in vacuum, would produce between those conductors a force equal to (2 x 10^-7) newtons per meter of length. However the ampere is difficult to realize in practice with sufficient accuracy, so it is realized via the watt (the SI unit for power). The electrical power generated in a controlled experiment is compared to mechanical power, and using an accurate measurement of resistance the ampere can be calculated via:
Power = (Current)2 x Resistance. Ohm's law is important . V=IR Resistance : Mass : The metric unit of mass was originally defined as the mass of a cubic decimeter at its temperature of maximum density.
- International standards :- international prototype kilogram, preserved at the international Bureau of weights and measures near Paris.
- Primary standards :- united states prototype kilogram, preserved by the NBS to an accuracy of 1 part in (10^8).
- Secondary standards :- kept by industrial laboratories and have accuracy 1 ppm.
- working standards :- are available in a wide range of values to suit almost any application. Their accuracy is in the order of 5 ppm. LENGTH : Definitions of meter :-
- 1/(10 million) of distance from Equator to north pole.
The international standards for it represented by the distance between two lines engraved on a platinum-iridium bar preserved at the international Bureau of weights and measures near pairs.
- The meter is the length of the path traveled by light in vacuum during a time interval of 1299,792,458 of a second
**The working standards :- are precision gage blocks , made of steel with accuracy tolerances in the 0.5-0.25 micron range ( 1 micron = one millionth of 1 m ). TIME AND FREQUENCY STANDARDS Time : 1- Earth's rotation on its axis for the sun .
2- mean solar day (A rate of days in a year).
**The mean solar second equal to 1/86400 of mean solar day .
3- Atomic Clock . FREQUENCY Depending on the time was defined frequency.
F=1/T The standard resistor is a coil of wire of some alloy like manganin ( high resistance + low temp coefficient of resistance Voltage Standards : At the National Physical Laboratory the volt is realized from the AC Josephson effect. Due to this effect the potential difference between two superconductors separated by a narrow gap and exposed to electromagnetic radiation, takes discrete values dependent on the Josephson constant (483597.9 gigahertz per volt) and the frequency of radiation. This gives the volt to an accuracy of 1 hundred millionth of a volt (0.000 000 01 volts). V=h f/2e Standards of Temperature and Luminous The kelvin (K) : The 'standard temperature' we use is the temperature of the triple point of water, which is the unique temperature at which the three phases of water (solid, liquid and vapor) co-exist in equilibrium. We define this temperature to be 273.16 K exactly and hence determine the size of the unit of temperature to be:
The fraction 1/273.16 of the thermodynamic temperature of the triple point of water. The international temperature community is working towards a redefinition of the kelvin in terms of this microscopic motion, and a new definition will be based on a fundamental constant known as the Boltzmann constant that measures how much energy of motion corresponds to one kelvin. The candela (cd) The realization of the candela at the National Physical Laboratory is based on the use of a cryogenic radiometer which, by equating the heating effect of optical radiation with that of electric power, can provide measurements of optical radiant power at specific wavelengths with an uncertainty of better than 0.01%. A solid-state photometer has been developed to evaluate light of other wavelengths according to the V() function, enabling the candela to be realised with an uncertainty of 0.2%.
The current definition of the candela was made in 1979, in terms of the watt at only one wavelength of light. It is defined as:The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watts per steradian (a unit of solid angle). 1- Ancient Egyptian science, a Source Book. Volume Three: Ancient Egyptian Mathematics..
2- 17th General Conference on Weights and Measures. (1983). International Bureau of Weights and Measures.
4- "Brief history of the SI". BIPM. Retrieved 2009-04-21.
5- http://www.bipm.org/en/scientific/ Thank You Done by :
Ayman Abdullah v=hf/2e