Measurement is a comparison of something to be measured with some agreed stand. For example, let us suppose that we want to take the pen as a unit of length. Then the lengths of all the objects could be related to the length of the pen. One object may be twice as long as the length of the pen. A second may be two and half times as long and so on. Once the unit is established, measurement becomes simply a matter of counting. So, if we say that a tree is 1000 "pen lengths" high, you will have some idea of how tall it is even though you do not see it. But you must know what the unit of measurement is. Thus, we need standards, and these standards must be know by everyone with whom we wish to communicate. Now, let us see the standards we choose.
Fundamental Units:
We have already stated that in making a measurement, we have to express the quantities in terms of known standard quantities of length, mass and time. The basic idea of selecting standard unit is that they should be constants independent of time and location, easily obtainable and reproducible so that all countries in the world can duplicate them and keep them as references.
Standard of length:
The standard unit of length is the meter. This is defined as the distance between two points made on a bar of platinum-Iridium alloy kept at 0C at the International Bureau of weights and Measures in Paris and France.
Today, all distances are measured in terms of the metre.
However, it was difficult ta make replicas and maintain them in the laboratory. So, in 1960, a new standard was adopted which was easier to reproduce and measure experimentally. When an electron jumps between the energy levels 2p and 5d of an isotope of Krypton-86, it emits a characteristic radiation. (Orange-red light). In a comparison of the wave length of this radiation with the standard metre it was seen that the standard meter is equal to 1650763.73 wave lengths of the krypton line.
Therefore, the metre is redefined as "the length equal to 1650763.73 wave lengths of a orange -red light emitted by the Krypton-86 atom"
Standard mass:
The standard unit of mass is the kilogram. This is also kept at the international Bureau of weights and measures in paris and denotes the mass of a standard cylinder made of platinum-iridium ally. All masses are compared with this mass which is called the standard kilogram.
Standard time:
Until recent times, the standard unit of the solar second. This was 1/86400 part of the mean solar day.
A better standard was then introduced and a second was redefined as 1/31556925.9 of the tropical year 1900. As the length of the year is not constant, this definition was discarded.
In 1967, the standard was equated again to agree with more precise and easily available data.
Thus, the standard of time, "the second" was defined as the time taken for a Cesium atom of atomic mass 133 to vibrate exactly 9 192637770 times.
As stated above the Fundamental Units of length mass and time are metre, kilogram and second. the system using these units, for measurements is called the M. K. S. (meter, kilogram, Second) . But in our study in physics we require to define several more units for basic quantities. This is called the SI system of units.
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