Standards of Measurement
There are various methods for measurement of different physical quantities like length, mass, temperature, time etc. There are also wide ranges of instruments available for measuring these quantities. However, the unit of measurement of these quantities remains same and there is certain basis for the comparison of the standard unit.
Say for instance, we have measured the length of the wall as one meter by the measuring tape. But what and who decides that the measurement I have taken is indeed one meter. For people in US it may be one meter long, but the people from Europe may consider the same wall to be one feet long and similarly for Asian people it may be something else.
The standards of measurement specify the standard units of the measurement of various physical quantities and the exact value of each standard. Again say for instance, the standard unit for the measurement of length is considered to be meter. As per the standards adopted by the Eleventh General Conference on Weights and Measure, one meter of length is considered to be equivalent to 1,65,763.73 wavelengths, in vacuum, of the radiation corresponding to the transition between the levels 2p10, and 5d5 of the krypton atom (wavelength of radiations emitted by atoms between these two levels). This standard has also been adopted by National Bureau of Standards of United States and accordingly one inch is equivalent to 41,929.399 wavelengths of above krypton light.
Requirements of Standard Units of Measurement
1) For the standard to be successful there must be general agreement among all the countries about the exact valve of the standard. While devising the standards it is crucial to consider the system of measurement in all the concerned countries and also the standard values adopted by them.
2) The value of the standard should remain constant and it should not be affected by any parameter. It should be not be affected by the climatic and other changes on the surface of the earth. For instance, earlier, one meter length was considered to be equivalent to distance between two finely scribed lines on a platinum-iridium bar under specified conditions, but later it was found the wavelengths of the krypton radiations remains constant so they were chosen as the standard for the measurement of meter.
3) Another important point to note is that the standard units are converted into the other units, like meter is converted into centimeter, millimeter, and inches in different countries. For the international trade to function properly it is important that standards for the various countries must be convertible on the same basis mutually agreed by all. Thus a definite relation must be established between meter and centimeter, millimeter, and inches. The same applies to kilogram and grams and other units of measurement.
4) Defining the zero for any standard is vital. For most of the physical quantities like length, mass, temperature etc. there is certain starting point, which should be defined else the whole process of measurement will go wrong. For instance freezing temperature of water is defined at zero degrees Celsius.
The Birth of National Bureau of Standards (NBS)
In United States the congress has the power to fix the standards of weights and measures and they are the authority to control the matters pertinent to the standards of measurement. In year 1866 it was declared that, “It shall be lawful to throughout the United States of America to employ the weights and measures of metric system," which showed that metric system could be used for measurement.
In the year 1875, agreement was signed in Paris, France to establish and maintain scientific and permanent international bureau of weights and measures at Paris. Though the bureau was established, it did not bind the US to use or adopt the standards developed by the bureau.
Since the congress did not take any further action, in the year 1893, Superintendent Mendenhall of the Coast and Geodetic Survey issued the order as follows:
“The office of Weights and Measures with the approval of the Secretary of the Treasury, will in the future regard the international prototype meter and the kilogram as fundamental standards, and the customary units, the yard and the pound, will be derived therefrom in accordance with the Act of July 1866."
The order from Mendenhall was a historic one and it turned out to be crucial in the further developments of the units of measurement. Two important decisions were taken by this order. First, the order made it clear that the meter and kilogram would be the fundamental units for the measurement of length and mass respectively. Secondly, it tied together the metric and English system of measurement of length and mass and helped establishing the definite relationship between the two systems thus making the possible the international exchange on a more exact basis.
Meanwhile, in a number of countries including Great Britain and Germany, special bureaus were developed for the standards of measurement. Further, there were lots of requests from the scientific and industrial community. Finally, on March 3, 1901, the US Congress passed the act providing that, “The office of Standard Weights and Measures shall hereafter be known as ‘The National Bureau of Standards’". The Congress expanded the functions of the new bureau.
The functions of National Standards of Bureau (NBS) were to develop standards, do the research of the basic to standards and the calibration of the standards and devices. The NBS was formally established in the year 1910 and was further expanded by the amendment in the year 1950.
1) Book: Mechanical Measurements by Thomas G. Beckwith and N. Lewis Buck, Oxford and IBH Publishing Co. Pvt. Ltd.