Overview

Scientific instruments, units of measurement, and landmark inventions form a consistently tested area in UPSC Prelims. Questions range from matching instruments with their functions to identifying inventors and understanding the SI system. This chapter provides a structured, table-heavy reference covering the International System of Units (SI), commonly tested scientific instruments, major inventions and their inventors, and the contributions of Indian scientists to global science.

Exam Strategy: Prelims questions on this topic are factual and direct — instrument-function matching, inventor-invention pairing, and SI unit definitions are common. Prepare using the tables below as flash-card material. Mains GS3 (Science & Technology) may ask about India's contributions to science or the significance of specific discoveries.

The International System of Units (SI)

The International System of Units (SI) — from the French Systeme International d'Unites — is the modern form of the metric system and the world's most widely used system of measurement.

The 7 SI Base Units

Quantity SI Unit Symbol Defined By (Post-2019)
Length metre m Speed of light in vacuum (c = 299,792,458 m/s)
Mass kilogram kg Planck constant (h = 6.62607015 x 10⁻³⁴ J·s)
Time second s Caesium-133 atom hyperfine transition (9,192,631,770 periods)
Electric Current ampere A Elementary charge (e = 1.602176634 x 10⁻¹⁹ C)
Temperature kelvin K Boltzmann constant (k_B = 1.380649 x 10⁻²³ J/K)
Amount of Substance mole mol Avogadro constant (N_A = 6.02214076 x 10²³ mol⁻¹)
Luminous Intensity candela cd Luminous efficacy of 540 THz radiation (683 lm/W)

Prelims Tip: The 2019 SI Redefinition (effective 20 May 2019) redefined four units — kilogram, ampere, kelvin, and mole — in terms of fundamental physical constants, replacing the old definitions based on physical artefacts (like the International Prototype Kilogram stored in Paris). The second, metre, and candela had been previously redefined using physical constants.

SI vs CGS System

Feature SI System CGS System
Full Name International System of Units Centimetre-Gram-Second system
Base Units for Length, Mass, Time metre, kilogram, second centimetre, gram, second
Usage Standard worldwide for science and commerce Older system; still used in some physics sub-fields
Force Unit Newton (N) = kg·m/s² Dyne = g·cm/s²
Energy Unit Joule (J) = kg·m²/s² Erg = g·cm²/s²

Important Derived SI Units

Quantity Unit Symbol Expressed As
Force Newton N kg·m/s²
Pressure Pascal Pa N/m²
Energy / Work Joule J N·m
Power Watt W J/s
Electric Charge Coulomb C A·s
Voltage Volt V W/A
Resistance Ohm Ohm V/A
Frequency Hertz Hz s⁻¹
Magnetic Flux Weber Wb V·s
Radioactivity Becquerel Bq s⁻¹ (disintegrations per second)

Scientific Instruments and Their Uses

This is one of the most frequently tested areas in competitive exams. The following table lists instruments commonly asked in UPSC and state-level exams.

Instruments for Measuring Physical Quantities

Instrument Measures / Function
Altimeter Altitude (height above sea level)
Ammeter Electric current
Anemometer Wind speed
Barometer Atmospheric pressure
Calorimeter Quantity of heat
Chronometer Time with extreme precision (used in navigation)
Dynamometer Force, torque, or power
Galvanometer Small electric currents and their direction
Hydrometer Relative density (specific gravity) of liquids
Hygrometer / Psychrometer Humidity in the atmosphere
Lactometer Purity (specific gravity) of milk
Manometer Pressure of gases
Odometer Distance travelled by a vehicle
Pyrometer Very high temperatures (e.g., in furnaces, molten metals)
Rain Gauge (Pluviometer) Rainfall
Seismograph Intensity and origin of earthquake waves
Sphygmomanometer Blood pressure
Tachometer Speed of rotation (RPM)
Thermometer Temperature
Voltmeter Electrical potential difference (voltage)

Instruments for Observation and Detection

Instrument Function
Binoculars Viewing distant objects with both eyes
Endoscope Viewing internal organs of the body
Geiger-Muller Counter Detecting and measuring ionising radiation
Microscope Magnifying very small objects
Periscope Viewing objects above the line of sight (used in submarines)
Spectroscope Analysing the spectrum of light emitted by a source
Stethoscope Listening to internal sounds of the body (heart, lungs)
Telescope Viewing distant celestial objects

Instruments for Specific Applications

Instrument Function
Audiometer Testing hearing ability
Cardiograph (ECG) Recording electrical activity of the heart
Electroencephalograph (EEG) Recording electrical activity of the brain
Fathometer Measuring depth of the ocean
Gyroscope Maintaining orientation; used in navigation and stabilisation
Radar Detecting objects using radio waves (Radio Detection and Ranging)
Sonar Detecting underwater objects using sound waves (Sound Navigation and Ranging)
Spectrometer Measuring wavelength and intensity of light
Transformer Changing voltage of alternating current

Prelims Tip: Common traps in MCQs: Hydrometer measures density of liquids (not humidity — that is hygrometer). Pyrometer measures very high temperatures (not pressure). Sphygmomanometer measures blood pressure (not heart rate).

Important Inventions and Inventors

Invention Inventor Year Country
Printing Press (movable type) Johannes Gutenberg c. 1440 Germany
Telescope (refracting) Hans Lippershey; improved by Galileo 1608 Netherlands
Steam Engine (practical) James Watt 1769 Scotland
Vaccination (smallpox) Edward Jenner 1796 England
Electric Battery (voltaic pile) Alessandro Volta 1800 Italy
Electromagnetic Induction Michael Faraday 1831 England
Telegraph Samuel Morse 1837 USA
Telephone Alexander Graham Bell 1876 USA/Scotland
Phonograph Thomas Edison 1877 USA
Electric Bulb (practical) Thomas Edison 1879 USA
Radio (wireless telegraphy) Guglielmo Marconi 1895 Italy
X-Rays Wilhelm Rontgen 1895 Germany
Radioactivity Henri Becquerel 1896 France
Radium Marie & Pierre Curie 1898 France/Poland
Quantum Theory Max Planck 1900 Germany
Theory of Relativity Albert Einstein 1905 (Special), 1915 (General) Germany
Penicillin Alexander Fleming 1928 Scotland
Nuclear Fission Otto Hahn & Fritz Strassmann 1938 Germany
Transistor John Bardeen, Walter Brattain, William Shockley 1947 USA
Structure of DNA James Watson & Francis Crick 1953 UK/USA
Polio Vaccine (injectable) Jonas Salk 1955 USA
Laser Theodore Maiman 1960 USA
World Wide Web Tim Berners-Lee 1989 UK

Indian Scientists and Their Contributions

Major Indian Scientists

Scientist Field Key Contribution Recognition
C.V. Raman (1888-1970) Physics Discovered the Raman Effect (1928) — when light passes through a transparent medium, a fraction of scattered light changes wavelength due to interaction with molecules Nobel Prize in Physics (1930) — first Asian and first non-White person to win a Nobel in Physics; 28 February celebrated as National Science Day
Satyendra Nath Bose (1894-1974) Physics Developed Bose-Einstein Statistics (1924) — a new way of counting quantum states for identical particles; led to the prediction of Bose-Einstein Condensate (experimentally confirmed 1995); the class of particles called bosons is named after him Padma Vibhushan (1954); seven Nobel Prizes were awarded for research related to his concepts, though Bose himself never received the Nobel
Meghnad Saha (1893-1956) Astrophysics Formulated the Saha Ionization Equation (1920) — relates the ionisation state of a gas to temperature and pressure; fundamental to interpreting stellar spectra Called one of the "top ten achievements of 20th century Indian science"; nominated for the Nobel Prize but never awarded
Har Gobind Khorana (1922-2011) Biochemistry Decoded the genetic code — showed how nucleotide sequences in nucleic acids control protein synthesis; first scientist to chemically synthesise oligonucleotides Nobel Prize in Physiology or Medicine (1968) (shared with Nirenberg and Holley); Padma Vibhushan (1969)
Srinivasa Ramanujan (1887-1920) Mathematics Made extraordinary contributions to mathematical analysis, number theory, infinite series, and continued fractions; his notebooks contain thousands of results, many still being studied Fellow of the Royal Society (FRS) at age 31 — among the youngest in history
Jagadish Chandra Bose (1858-1937) Physics / Biology Pioneer of radio and microwave optics; demonstrated wireless communication before Marconi (1895); proved that plants respond to stimuli using the crescograph Knighted 1917; regarded as father of Bengali science fiction
Homi Bhabha (1909-1966) Nuclear Physics Founded India's nuclear programme; established the Tata Institute of Fundamental Research (TIFR, 1945) and the Atomic Energy Establishment, Trombay (now BARC) Known as the "father of Indian nuclear programme"
Vikram Sarabhai (1919-1971) Space Science Founded the Indian National Committee for Space Research (INCOSPAR, 1962), precursor to ISRO; established the Physical Research Laboratory (PRL), Ahmedabad Known as the "father of Indian space programme"
A.P.J. Abdul Kalam (1931-2015) Aerospace Led India's SLV-III (first indigenous satellite launch vehicle); key role in the Pokhran-II nuclear tests (1998); developed the Agni and Prithvi missile programmes Known as the "Missile Man of India"; 11th President of India (2002-2007); Bharat Ratna (1997)

Prelims Tip: C.V. Raman is the only Indian citizen to win a Nobel Prize in Physics (Subrahmanyan Chandrasekhar won in 1983 but was a US citizen). Har Gobind Khorana won in Medicine (1968) but had become a US citizen in 1966.

Indian-Origin Nobel Laureates in Science

Scientist Prize Year Field Key Work
C.V. Raman Physics 1930 Light scattering Raman Effect
Har Gobind Khorana Physiology/Medicine 1968 Genetics Interpretation of genetic code
Subrahmanyan Chandrasekhar Physics 1983 Astrophysics Chandrasekhar Limit — mass limit for white dwarf stars
Venkatraman Ramakrishnan Chemistry 2009 Structural biology Structure of the ribosome
Abhijit Banerjee Economics 2019 Development economics Experimental approach to alleviating poverty

Select Nobel Prize Highlights in Science

Year Prize Laureate(s) Discovery
1901 Physics Wilhelm Rontgen X-Rays
1903 Physics Henri Becquerel, Marie & Pierre Curie Radioactivity
1905 Physics Philipp Lenard Cathode rays
1918 Physics Max Planck Energy quanta
1921 Physics Albert Einstein Photoelectric effect
1922 Physics Niels Bohr Atomic structure
1928 Medicine Charles Nicolle Typhus transmission research
1929 Medicine Christiaan Eijkman, Frederick Hopkins Vitamins
1930 Physics C.V. Raman Raman Effect
1932 Physics Werner Heisenberg Quantum mechanics
1945 Medicine Alexander Fleming, Howard Florey, Ernst Chain Penicillin
1953 Medicine Hans Krebs Citric acid cycle
1962 Medicine Watson, Crick, Wilkins DNA structure
1965 Physics Richard Feynman, Julian Schwinger, Sin-Itiro Tomonaga Quantum electrodynamics
2020 Chemistry Emmanuelle Charpentier, Jennifer Doudna CRISPR-Cas9 gene editing

Important Physical Constants

Constant Symbol Value Significance
Speed of light in vacuum c 299,792,458 m/s (exact) Defines the metre; nothing can travel faster
Planck constant h 6.626 x 10⁻³⁴ J·s Defines the kilogram (since 2019); fundamental to quantum mechanics
Gravitational constant G 6.674 x 10⁻¹¹ N·m²/kg² Used in Newton's law of gravitation
Boltzmann constant k_B 1.381 x 10⁻²³ J/K Defines the kelvin (since 2019); relates temperature to energy
Avogadro constant N_A 6.022 x 10²³ mol⁻¹ Defines the mole (since 2019); number of particles in a mole
Elementary charge e 1.602 x 10⁻¹⁹ C Defines the ampere (since 2019); charge of a proton
Electron mass m_e 9.109 x 10⁻³¹ kg Mass of an electron; used in atomic calculations

Commonly Confused Instruments — Exam Traps

Often Confused Instrument A Instrument B
Hydrometer vs Hygrometer Hydrometer: measures density of liquids Hygrometer: measures atmospheric humidity
Barometer vs Manometer Barometer: measures atmospheric pressure Manometer: measures gas pressure in closed systems
Pyrometer vs Thermometer Pyrometer: very high temperatures (furnaces, molten metals) — non-contact Thermometer: moderate temperatures — contact-based
Ammeter vs Voltmeter Ammeter: measures current (connected in series) Voltmeter: measures voltage (connected in parallel)
Seismograph vs Richter Scale Seismograph: instrument that records earthquake waves Richter Scale: logarithmic scale measuring earthquake magnitude
Radar vs Sonar Radar: uses radio waves (air/space detection) Sonar: uses sound waves (underwater detection)
Galvanometer vs Ammeter Galvanometer: detects small currents and their direction Ammeter: measures magnitude of larger currents

Frequently Asked Prelims Questions — Pattern

Question Pattern Answer
How many SI base units? 7
What is the SI unit of luminous intensity? Candela (cd)
Which instrument measures blood pressure? Sphygmomanometer
Which instrument measures earthquake intensity? Seismograph
Who discovered the Raman Effect? C.V. Raman (1928); Nobel Prize 1930
National Science Day is celebrated on? 28 February (discovery of Raman Effect)
First Indian Nobel laureate in Physics? C.V. Raman (1930)
Bosons are named after? Satyendra Nath Bose
Who invented the telephone? Alexander Graham Bell (1876)
Who discovered penicillin? Alexander Fleming (1928)
What did the 2019 SI redefinition change? Kilogram, ampere, kelvin, and mole — now defined by fundamental constants

Key Terms for Quick Revision

Term Meaning
SI Unit International System of Units — 7 base units from which all other units are derived
Raman Effect Change in wavelength of light scattered through a medium due to molecular interaction
Bose-Einstein Condensate State of matter formed when bosons are cooled to near absolute zero, causing them to occupy the same quantum state
Saha Equation Relates ionisation state of a gas to temperature and pressure; fundamental in stellar astrophysics
Chandrasekhar Limit Maximum mass of a stable white dwarf star (~1.4 solar masses)
Crescograph Instrument invented by J.C. Bose to measure plant growth and response to stimuli
Boson Class of particles obeying Bose-Einstein statistics (e.g., photon, Higgs boson) — named after S.N. Bose
Planck Constant Fundamental constant relating photon energy to frequency; used to define the kilogram since 2019
Avogadro Constant Number of particles in one mole (6.022 x 10²³); used to define the mole since 2019
Derived Unit SI unit formed by combining base units (e.g., Newton = kg·m/s²)
CGS System Centimetre-Gram-Second system — older measurement system, predecessor to SI

Sources: NIST (nist.gov); NobelPrize.org; Britannica; Wikipedia; India Science, Technology & Innovation Portal (indiascienceandtechnology.gov.in).