10. SI Units of Quantities Measured by These Instruments
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Exam papers frequently pair an instrument with the SI unit of the quantity it measures, so it is worth memorising both together rather than separately. The table below lists the standard SI unit for each quantity discussed in this chapter, alongside the instrument used to measure it.
Quantity | SI Unit (Symbol) | Instrument Used |
Length/Distance | Metre (m) | Vernier callipers, screw gauge, metre scale |
Mass | Kilogram (kg) | Common/beam balance |
Time | Second (s) | Chronometer, clock, stopwatch |
Electric Current | Ampere (A) | Ammeter |
Temperature | Kelvin (K); Celsius (°C) commonly used | Thermometer, pyrometer |
Pressure | Pascal (Pa); mm of Hg / bar also used | Barometer, manometer |
Potential Difference | Volt (V) | Voltmeter |
Resistance | Ohm (Ω) | Ohmmeter, multimeter |
Power | Watt (W) | Wattmeter, dynamometer |
Frequency | Hertz (Hz) | Sonometer (of a vibrating string), audiometer |
Speed/Velocity | Metre per second (m/s); km/h commonly used | Speedometer, anemometer (wind speed) |
Force/Weight | Newton (N) | Spring balance |
Density (Relative) | No unit (ratio); kg/m³ for absolute density | Hydrometer |
Heat Energy | Joule (J); calorie also used | Calorimeter |
Sound Intensity/Loudness | Decibel (dB) | Audiometer, sound level meter |
Note that many instruments in this chapter, such as the sextant, compass, seismograph, and endoscope, do not report their result in a single fixed SI unit but instead as an angle (degrees), a direction, a magnitude on a specific scale (like the Richter scale), or simply a visual image — students should be careful not to force-fit an SI unit onto every instrument in the chapter.
10.1 Historical Timeline — Key Instrument Inventions
A short historical timeline helps in remembering inventor-related questions, since many of these instruments were developed within a relatively well-defined period of the history of science, often building on each other's underlying discoveries.
Approximate Period | Instrument | Associated Scientist |
1608–1609 | Telescope (refracting) | Invented in the Netherlands; developed astronomically by Galileo Galilei |
1631 | Vernier Callipers (vernier scale) | Pierre Vernier |
1643 | Barometer | Evangelista Torricelli |
18th century | Marine Chronometer | John Harrison |
1668 | Reflecting Telescope | Isaac Newton |
Early 18th century | Mercury Thermometer / Fahrenheit Scale | Daniel Gabriel Fahrenheit |
1816 | Stethoscope | René Laennec |
1846 | Cup Anemometer | John Thomas Romney Robinson |
Late 19th century | Bolometer | Samuel Pierpont Langley |
Late 19th – early 20th century | Electrocardiograph (ECG) | Willem Einthoven |
1930s | Modern Richter Scale for Earthquakes | Charles Richter |