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The Central Importance of Measurement in Science
The mass of an object is a measure of the quantity of matter present in it and remains constant. The S. I. Unit of mass is the kilogram (Kg). The weight of an object varies with the gravitational field influencing it. While the S.I. Unit of length is the meter (m), length units on the atomic scale are nanometer (nm) and picometre (pm). Volume units are derived from length units, the most important volume units in Chemistry are the cubic meter (m3) and the liter (L). Density (d) is the ratio of mass to volume of a substance and is one of its characteristics physical properties. Temperature is a measure of relative hotness of an object. Heat is energy that flows from an object at higher temperature to one at lower temperature. Temperature scales differ in the size of the degree unit and/or zero point. In chemistry temperature is measured in Kelvin’s (k) or degrees Celsius (oC). Extensive properties, such as mass, volume and energy, depend on an objects size (extent). Intensive properties, such as density and temperature are independent of size. Nowadays measurement is so common that it is taken as for granted, but it has a long and fascinating history characterized by the search for exact, invariable standards.
System of measurement began thousands of years ago as trade, building and land surveying spread through the civilized world. For most of that time, however, measurements were based on inexact physical standards. For example: an inch was the length of three barley corns (seeds) placed end-to-end a yard was the distance from the tip of king Edgar’s nose to the tip of his thumb with his arm outstretched, and an acre was the area tilled by an man working with a pair of oxen in a day.
The standard meter is now based on two quantities, the speed of light in vacuum and the second. The history of the meter points up the ongoing drive to define units based unchanging standards. The French scientists who set up the metric system defined the meter as 1/10,000,000, the distance from the equator (through Paris) to the North Pole. The meter was later defined as the distance between the two five lines engraved on a corrosion resistant metal bar kept at the international Bureau of weights and Measures in France. Fear that the bar would be destroyed / damaged by war led to an exact, unchanging, universally available atomic standard, 1,650,763.73 wavelength of orange red light from electrically excited Krypton atoms. The current standard is even more reliable; 1 meter is the distance light travels in a vacuum in 1/299,792,458 second.
The U.S. copy of the master kilogram is kept in a vault at the National Institute of Standards and Technology near Washington, D.C. since 1889, it has been taken to France only twice for comparison with the master kilograms which is removed from its vault only once a year for such purposes. Two people are always present when it is moved, one to carry with forceps, the other to catch it, if the first person stumbles. The kilogram is unusual in another way; it is only base unit whose name has a prefix. Unlike the practice with other base units, however, we attach prefixes to the word “Gram” as in ‘microgram’ rather than to the word “kilogram”. Thus we never say “micro-kilogram”.
The S.I. Unit of time is the second(s). Although time was once measured by the day and year, it is now based in an atomic standard, microwave, and radiation from excited cesium atoms. In the laboratory, we study the speed of a reaction by measuring the time it takes a fixed amount of substance to undergo a chemical change. The range of reaction speeds is enormous a fast chemical change in over in a few nanoseconds (10-9s), whereas slow ones, such as rusting or biological aging, take years. Chemists use lasers to study changes that occur in a few picoseconds (10-12s) or even femto-seconds (10-15s). The accuracy of the best pendulum clock is to within 3 seconds per year and that of the best quartz clock is 1000 times greater. The latest version of the atomic clock, NIST-7, is over 2000 times more accurate still, to within 1 second in 6 million year. Rather than using the oscillations of a pendulum, the atomic clock measures the natural oscillation frequency of cesium atoms absorbing microwave radiations. 1 second is defined as 9,192,631,770 of these oscillations. Current plausto place an advanced laser-cooled cesium atomic clock on the international space station will allow longer observation times of the atoms, for atleast a 10 fold increase in accuracy.
About the Author
Dr.Badruddin Khan teaches Chemistry in the University of kashmir, Srinagar, india.
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