Essentials of the SI: Base & Derived Units

Previously, scientists from various countries used different measurement methods. Until recently, three such systems, the CGS, the FPS (or British) system, and the MKS system, were widely used.

The following were the basic units for length, mass, and time in these systems:

• In the CGS system, they were centimetre, gram, and second; 
• In the FPS system, they were foot, pound, and second; and 
• In the MKS system, they were a metre, kilogram, and second.

The Système Internationale d’ Unites (French for International System of Units), abbreviated as SI, is the system of units that is now universally recognised for measurement.

SI base quantities and units

The SI was designed by the Bureau International des Poids et Measures (BIPM) in 1971 and included a standard set of symbols, units, and abbreviations. It was recently amended by the General Conference on Weights and Measures in November 2018.

• Length measurement

Some straightforward ways for measuring length are already familiar to you. A metre scale is employed for lengths ranging from 10-3m to 102 m. For lengths with an accuracy of 10-4 m, vernier callipers are utilised. Using a screw gauge and a spherometer, you may measure lengths as short as 10-5m. We also employ certain specific indirect methods to measure lengths outside of these ranges.

• Large distance measurement

Large distances, such as the distance between a planet and the earth, are impossible to measure directly with a metre scale. A parallax approach is a helpful tool in such situations. When you hold a pencil against a certain point on the backdrop (a wall) and look at it through your left eye (closing the right eye) and then through your right eye (closing the left eye), you’ll notice that the location of the pencil changes about the point on the wall. This is known as parallax.

• Estimation of extremely short distances: molecule size

We need to use specific technologies to measure a very small size, such as a molecule (10-8 m to 10-10 m). We won’t be able to use a screw gauge or similar tools. A microscope, too, has its limitations. An optical microscope ‘looks’ at the object under investigation with visible light. Because light has wave-like properties, the wavelength of light determines the resolution of an optical microscope. The wavelength range for visible light is around 4000 to 7000 nm.

• Measurement of mass

Mass is a basic property of matter. It does not depend on the object’s temperature, pressure, or location in space. The SI unit of mass is the kilogram (kg). 

• Time measurement

Second (s) The basic unit of time is the second. The second is defined as the duration of 9,192,631,770 oscillations of radiation corresponding to the transition between the two hyperfine levels of cesium-133.

There are seven basic units of measurement used by scientists worldwide. These include – 

• Length – Metre (m)
• Time – Second (s)
• Mass – Kilogram (kg)
• Electric current – Ampere (A)
• Temperature – Kelvin (K)
• Amount of a substance – Mole (mol)
• Luminous Intensity – Candela (cd)

Derived Units of Measurement 

Evolution of measurements

One of the essential principles in scientific theory is that of measurement. Without the capacity to quantify their outcomes, scientists would have difficulty conducting experiments and formulating theories. The term “measurement” comes from the Greek word “metron,” which means “restricted ratio” in English.

Measurement needs the application of instruments to give scientists a quantity. A quantity is a technique to express “how much and how many”. Researchers utilise a measurement technique known as the “metric system,” which is still widespread today. It was the world’s first uniform measurement system, founded in France in the 1790s. Except for the United States, this is the standard unit of measurement today. The qualities of an object may be determined by comparing it to a standard, which is accomplished using measuring procedures.

Conclusion 

SI derived units are measurement units derived from the International System of Units’ seven basic units. They are used in scientific and technical applications. Depending on the context, they can be stated as a product of one or more base units, which an appropriate power of exponentiation can then scale. The names of SI derived units are usually written in lowercase letters when written in full. Units are named after persons; on the other hand, units named after persons have their logo written with an uppercase initial letter.