1.1. THE INTERNATIONAL SYSTEM OF UNITS

In earlier time scientists of different countries were using different systems of units for measurement. Three such systems, the CGS, the FPS (or British) system and the MKS system were in use extensively till recently.

Before the adoption of a standard system, scientists used various measurement systems. The CGS (centimetre-gram-second) system was based on centimetres for length, grams for mass, and seconds for time. The FPS (foot-pound-second) system used feet, pounds, and seconds, while the MKS (metre-kilogram-second) system employed metres, kilograms, and seconds. These varying systems often complicated communication and calculations in scientific work.

Imagine trying to buy fabric: if one store measures in yards and another in meters, it could lead to confusion and mistakes. This is similar to how scientists faced challenges when using different systems of measurement.

The system of units which is at present internationally accepted for measurement is the Système Internationale d’ Unites (French for International System of Units), abbreviated as SI. The SI, with a standard scheme of symbols, units and abbreviations, developed by the Bureau International des Poids et measures (The International Bureau of Weights and Measures, BIPM) in 1971 were recently revised by the General Conference on Weights and Measures in November 2018.

The SI is the globally accepted system for measurement that provides consistency and clarity. It simplifies scientific communication by standardizing units of measurement, which were previously inconsistent. Established in 1971, it was revised to ensure that definitions reflect the latest scientific advancements, making it relevant and accurate.

Using SI units is like having a universal language; just as language facilitates easier communication between people, SI units enable scientists worldwide to share and understand results without confusion.

In SI, there are seven base units as given in Table 1.1. Besides the seven base units, there are two more units that are defined for (a) plane angle d θ as the ratio of length of arc d s to the radius r and (b) solid angle dΩ as the ratio of the intercepted area d A of the spherical surface, described about the apex O as the centre, to the square of its radius r. The unit for plane angle is radian with the symbol rad and the unit for the solid angle is steradian with the symbol sr. Both these are dimensionless quantities.

The SI system comprises seven fundamental units: metre (length), kilogram (mass), second (time), ampere (electric current), kelvin (thermodynamic temperature), mole (amount of substance), and candela (luminous intensity). Additionally, radians and steradians measure angles, although they do not have dimensions. The definitions of these units are precise, which allows for standard measurements across various fields of science and technology.

Think of the SI base units as the building blocks of measurement. Just as you need bricks, cement, and wood to build a house, these base units serve as the foundation for all scientific measurements, creating a strong and shared framework for discovery.

Base quantity Name Symbol Definition Length metre m The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299792458 when expressed in the unit m s–1, where the second is defined in terms of the caesium frequency ∆νcs. Mass kilogram kg The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10–34 when expressed in the unit J s, which is equal to kg m2 s–1, where the metre and the second are defined in terms of c and ∆νcs. Time second s The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency ∆νcs, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s–1.

The exact definitions of the base quantities provide a scientific standard. For instance, the metre is defined by the speed of light, which is a universal constant, assuring that the unit remains consistent. Similarly, the kilogram is tied to the Planck constant, and the second is based on a specific property of the caesium atom. This reliance on fundamental constants ensures that the measures are stable over time.

Using these definitions is like using a specific recipe to make a dish. Just as each ingredient must be measured precisely to achieve the desired flavor, using defined constants guarantees that measurements are accurate and universally understood across the scientific community.

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