Chapter 2: Systems of Units

2.1 Why Do We Need a System of Units?

Measurement is meaningless unless everyone agrees on a common standard against which comparisons are made. If one scientist measured length in hand-spans and another in foot-lengths, their results could never be compared or reproduced. To avoid this chaos, nations and scientific bodies historically evolved fixed, reproducible standards for basic quantities such as length, mass, and time, and then built a consistent set of units — a system of units — around those standards. Over the eighteenth, nineteenth, and twentieth centuries, several such systems came into use, and it is useful for exam purposes to know the older systems even though the SI has now replaced them almost everywhere.

2.2 The Four Classical Systems of Units

Before the adoption of SI, four systems of units were commonly referenced in physics: CGS, FPS, MKS, and the British Engineering (or Imperial) system. Each system chose its own base units for length, mass, and time, and derived units followed from those choices.

(a) CGS System (Centimetre-Gram-Second)

The CGS system uses the centimetre as the unit of length, the gram as the unit of mass, and the second as the unit of time. It was widely used in scientific work, particularly in electromagnetism and physical chemistry, throughout the nineteenth and much of the twentieth century, and several named CGS units are still occasionally referenced today — for example, the erg (unit of energy), the dyne (unit of force), the gauss (unit of magnetic flux density), and the poise (unit of viscosity).

(b) FPS System (Foot-Pound-Second)

The FPS system, also called the British or Imperial system, uses the foot as the unit of length, the pound as the unit of mass, and the second as the unit of time. It developed in Britain and was historically used throughout the British Empire, including in India before metrication. It survives today mainly in the United States for everyday and some engineering purposes (miles, pounds, gallons), though scientific work everywhere now uses SI.

(c) MKS System (Metre-Kilogram-Second)

The MKS system uses the metre as the unit of length, the kilogram as the unit of mass, and the second as the unit of time. Proposed by the Italian scientist Giovanni Giorgi in 1901, it was designed to be more convenient for everyday and engineering use than CGS (whose units, like the dyne and erg, are inconveniently small for practical purposes) while still being decimal (metric) in nature, unlike FPS. The MKS system is the direct ancestor of the modern SI system — SI essentially extends MKS by adding four more base units (for current, temperature, amount of substance, and luminous intensity) to cover electricity, heat, chemistry, and photometry.

(d) SI System (International System of Units)

The SI system is the modern, internationally agreed, coherent system of units adopted by the General Conference on Weights and Measures (Conférence Générale des Poids et Mesures, CGPM) in 1960 and refined multiple times since, most recently and comprehensively in the major redefinition of 2019. SI is built on seven base units and is now used almost universally in science, industry, medicine, and international trade. It replaced the older CGS, FPS, and MKS systems for scientific and most practical purposes because it offers a single coherent, decimal, and universally reproducible framework for every branch of measurement.

2.3 Comparison of the Four Systems

2.3.1 India and the Metric System

India officially adopted the metric system of weights and measures through the Standards of Weights and Measures Act, 1956, replacing the diverse and inconsistent traditional units (such as seer, maund, and tola) that were in local use across different regions and trades. The metric system was made compulsory for all trade, commerce, and official purposes in India in stages during the 1960s, and today the SI system is the legally recognised system of measurement in the country. The National Physical Laboratory (NPL) of India, located in New Delhi and functioning under the Council of Scientific and Industrial Research (CSIR), is the country's National Metrology Institute — it maintains and realises the national standards of measurement and ensures they remain traceable to the international SI standards maintained by the BIPM in France.

2.4 History and Rationale for SI

The metric idea itself dates back to the French Revolution, when in the 1790s French scientists proposed the metre (defined originally as one ten-millionth of the distance from the North Pole to the Equator along a meridian passing through Paris) and the kilogram as scientifically reasoned, decimal standards to replace the confusing patchwork of local units then in use across Europe. This metric system spread gradually across the world through the nineteenth and twentieth centuries. In 1875, seventeen nations (including major scientific powers) signed the Metre Convention (Convention du Mètre) in Paris, establishing the International Bureau of Weights and Measures (BIPM), headquartered at Sèvres near Paris, to maintain international standards. India became a signatory to the Metre Convention and adopted the metric system officially, with the SI system now used throughout Indian scientific, industrial, and educational institutions.

The SI system as we know it today was formally established by the 11th General Conference on Weights and Measures (CGPM) in 1960, extending the earlier MKS system by adding base units for electric current (ampere), thermodynamic temperature (kelvin), amount of substance (mole), and luminous intensity (candela), thereby providing a single coherent framework covering the whole of physical science: mechanics, heat, electricity and magnetism, chemistry, and photometry. Since 1960, the CGPM (which meets periodically in Paris) has periodically refined and modernised the definitions of the base units. The most significant recent change was the 2019 redefinition, which took effect on 20 May 2019 (World Metrology Day), in which every SI base unit was redefined in terms of fixed values of fundamental physical constants of nature (such as the speed of light, the Planck constant, and the elementary charge) rather than in terms of physical artefacts or specific experiments. This was a landmark change because it removed the last physical object-based standard (the International Prototype of the Kilogram, a platinum-iridium cylinder kept at BIPM since 1889) and made every SI unit reproducible anywhere in the world, at any time, using nothing but the laws of nature — a fact frequently tested in SSC/RRB general science sections.

Why SI is preferred over the older systems

• It is coherent: derived units are formed simply by multiplying or dividing base units without extra numerical conversion factors (unlike, for example, converting horsepower to watts).
• It is a single unified system covering mechanics, heat, electricity, chemistry, and light, replacing the need for separate CGS, FPS, and other specialised systems.
• It is decimal, so unit conversions within SI only involve powers of ten (using standard prefixes such as kilo, milli, micro), unlike the FPS system's irregular conversion factors (12 inches to a foot, 3 feet to a yard, and so on).
• Since 2019, every base unit is defined using an invariant constant of nature, making the standards permanently stable, universally reproducible, and not dependent on any physical artefact that could be damaged, lost, or drift in mass over time.
• It is accepted and used internationally, which is essential for global scientific communication, trade, and engineering collaboration.