What is SI Unit

Definition of SI Units

Système International d'unités, more commonly known as “The International System of Units” is a basic form of measurement for different natural phenomena. It is abbreviated from the French term as “SI”.

• SI unit stands for  International System of Units

The SI units contain 7 basic forms of measuring the natural phenomenon:

1. Distance is measured through “Meters”.
2. Mass is measured through “Kilograms”
3. Current is measured through “Ampere”.
4. Temperature is measured through “Kelvin”.
5. Amount of substance is measured through “Moles”
6. Intensity is measured through “Candela”
7. Time is measured through “Seconds”.

All countries in the world follow the SI unit’s system; however, some countries do deviate a bit for certain measurements. For example, The United States measures distance in miles rather than meters.

Where did SI units come from?

In the past, many countries measured things differently. For example, some countries measured distances in feet while other countries measured distance in miles. This often caused many problems in the scientific community as scientists had to regularly convert every value that they found in books to their own value.

Distance Measurement: Around the mid-18th century, scientists in France decided to introduce the “Meter Convention” in which all scientists should stick to only certain types of measuring units. 

Weight Measurement: The Meter Convention asked scientists to measure their weight and distance in kilograms and meters. The agreement was signed by 17 countries – which of whom were mostly Europeans based?

Around 1860, a famous scientist by the name of James Clerk Maxwell decided to try and grow the Meter Convention by adding other forms of measurements as well. Mr. Maxwell wanted to add measurements such as Pascal (measures pressure) and Erg (measures energy, later changed to Jules). Unfortunately, Mr. Maxwell’s request was denied. 

Electricity Measurement: For the next 100 years, a lot of attempts were done to improve the Meter Convention. Many more requests to increase the convention were added such as the request to add Ohms (a unit for resistance) and volts (a unit for energy in batteries).

The Second World War disrupted scientific progress. Each scientist started using their own form of measurements and some said that the Metric Convention had come to an end.

In 1948, an organization called “General Conference on Weights and Measures” (GCWM) decided to conduct a 12-year study on these measuring units. The result of this study was published in 1960.

According to the GCWM, there are only 7 measuring units. All other values in the world are composed of these 7 measuring units. The GCWM decided to call these measuring units, the SI units.

1. The first SI Unit - Seconds: In the list of SI Units, time is perhaps the most difficult to measure. This is due to the fact that it is not something physical that one can hold with their own hands. The earliest measurements of time were quite simple:

1/86400 of a 24 hours day.

The above definition was quite simple and worked for almost 300 years. It was only in 1967 that scientists decided to make the definition of “one second” very complex.

In order to understand this definition, you first have to understand the term “hyperfine”. According to the Merriam Webster, hyperfine is something that happens when an atom split in two very temporarily. 

Therefore, "one second is defined as the time taken for a cesium atom to do two hyperfine".

2. The second SI Unit - Meters: Shortly after establishing the original definition of 1 second, people started figuring out the definition of 1 meter.

One of the oldest forms of measuring distance was meters. The word “Meters” – often written in the commonwealth as “Metres” – originally derives from the Greek word “Metro”, meaning “To measure”.

The system of measuring distance originally came from the renowned French scientist, Jean Picard. The Frenchmen was a renowned scientist who defined 1 meter as twice the length of a second pendulum (a pendulum that takes two seconds to tick). 

Although Mr. Picard’s system was accurate in the location of his measurements, Paris, it varied slightly across the world. This is due to the fact that the gravity of the Earth is not precise everywhere.

The French revolution that came a few years later made everybody forget about Mr. Picard’s system. It was only a few decades later that scientists decided to re-define 1 meter as a certain wavelength emitted by an atom of Krypton.

Today, a wavelength is officially described as 1/299792458 of a second.

3. The third SI Unit - Kilograms: To say, Kilograms was the third SI unit established will be a false statement, as 1 kilogram was defined in the same year as the measurement of 1 meter.

The kilogram was originally defined in 1773 as the mass of one liter of water at the freezing point. The measurement of 1kg changed over time and has changed again very recently.

In 2019, 1 kg got to be defined as  Where his the plank constant set at a specific value – Why could they not keep it simple ;)

4. The fourth SI Unit - Ampere: Ampere is the unit of charge that comes from the namesake of its discoverer, Andre-Marie Ampere. The Frenchman defined 1 ampere through finding the electromagnetism of current that flowed through a circle with a radius of 1 cm. 

This complicated definition got even more difficult in 2019 when it was changed to the flow of one atom of current in seconds.

5. The fifth SI unit - Kelvin: Many people are often surprised to find that the SI unit of temperature in Celsius or Fahrenheit and not Kelvin. The reason for this is that Kelvin is called the “differential” temperature.

    For example, if water boiled from 0 degrees Celsius to 100 degrees Celsius, we would say it is a change in 100 degrees Celsius or 100 kelvins.
The recent value of 1 Kelvin is by setting the Boltzmann constant (k) to 1.380649×10−23.

6. The sixth SI unit - Mole: Famous English scientist John Dalton was the first scientist to coin the term “mole”. 

Moles is the amount of substance in an atom. Mole is a very important metrics in chemistry and is the fundamentals of all chemical calculations.

The value of 1 mole (recently changed in 2018) was the substance in 6.02214076×1023 elementary entities. Elementary entities are electrons, protons, and neutrons.

7. The final SI unit - Candela: Candela is a relatively new SI unit. It is seen as the brightness of the radiation of an object with 5.4 x 1014 hertz.

The future of SI units
The pursuit of precision in science was the main reason for the creation of SI units. It was in order to have a universal language of scientific communication.
Yet, what should be remembered is that languages change – most of the time for the better. Similarly, the language of SI units should also evolve and the measurement of one specific value of the SI units should become more specific.
In this way, the scientific community can progress in a unified and collaborative way.

People also search for OK, SOS, PH, GMT, TBD, CID, MSME, LMAO, etc.