**SI** stands for ** ‘Système Internationale d’Unités’** which translates into

*‘International System of Units’*.

SI is the latest of several historical variations of the metric system. Previous versions include the ‘**cgsA**‘ (centimetre-gram-second-ampere) **system** and the ‘**mksA**‘ (metre-kilogram-second-ampere) **system**. SI is based on the mksA system.

**Base Units**

SI defines *seven* fundamental or ‘**base units**‘, from which *all* other units are derived. Not surprisingly, these other units are termed, ‘**derived units**‘.

The **base units** for SI are: **metre** (length), **kilogram** (mass), **second** (time), **ampere** (electric current), **kelvin** (thermodynamic temperature), **candela** (luminous intensity), and **mole** (amount of substance).

Note, in particular, that the base unit for mass is the **kilogram** (**kg**), and *not* the **gram** (**g**). The gram is considered to be a sub-multiple of the kilogram base unit. Note, too, that the kilogram is a measure of **mass**, and *not* weight. Mass and weight are related, but *different*, quantities; with weight being a **force** due to the effect of gravity upon a mass. In SI, force and, therefore, weight, are measured in **newtons** (symbol: **N**), a derived unit.

Since these base units were established in the late 1940s, they have been defined in terms of both **physical constants** and **artifacts**. For example, the second has been defined in terms of the speed of light, *c, *while the kilogram has been defined in terms of the mass of a metal cylinder kept in a laboratory in the suberbs of Paris.

However, since May of 2019, all this has changed and, now, *all* SI base units are defined in terms of **physical constants**. These constants include the speed of light, the charge on an elementary particle, Planck’s Constant, and others. This fully-explained in my Page on ‘Understanding the New SI‘, elsewhere in this blog.

### Derived Units

SI derived units are each defined in terms of base units. For example, the SI unit for electric potential difference, the **volt** (symbol: **V**) is defined as *‘the potential-difference between two points such that the energy used in conveying a charge of one coulomb from one point to the other is one joule’*.

So the **volt** is defined in terms of the **coulomb** and the **joule**. The **coulomb**, in turn, is defined in term of the ampere and the second (both base units). The **joule** is defined in terms of the newton and the metre (a base unit). Finally, the **newton** is defined in terms of the kilogram, the metre, and the second (all base units).

So, by ‘deconstructing’ the **volt**, we find that it is ultimately derived from *a combination of the base units:* **ampere**, **second**, **kilogram**, and **metre**.

Most derived units have been given **special names** in honour of famous physicists whose research has contributed to our knowledge of the quantity concerned —for example, as we have learnt, the derived unit for potential difference is the ‘**volt**’, which is simply *a special name* given to a ‘**joule per coulomb**’, and is named after the Italian nobleman and professor of physics, Count Alessandro Volta (1745 – 1827).

### Non-SI Metric Units

Not all metric units are SI units, although many may be ‘used alongside’ SI units.

Commonly-used, but non-SI metric, units include:

**watt second**(symbol:**W·s**), used to measure energy.**tonne**(symbol:**t**), spoken as*‘metric ton’*, used to measure mass.**litre**(symbol:**L**or**l**), used to measure volume.**Celsius**(symbol:**ºC),**used to measure temperature.

### Multiples and Sub-Multiples

Frequently, we have to deal with *very large*, or *very small*, quantities. For example, the resistance of insulation is measured in **millions of ohms**, while the resistance of a conductor is measured in **thousandths of an ohm**.

To avoid having to express very large or very small values in this way, we use, instead, **multiples** and **submultiples**. These are indicated by assigning a prefix to the SI unit. The more common are listed below:

**terra**(symbol:**T**), meaning

${10}^{12}$, e.g. terrawatt (TW).

**giga**(symbol:**G**), meaning

${10}^{9}$. e.g. gigawatt (GW).

**mega**(symbol:**M**) meaning

${10}^{6}$, e.g. megavolt (MV).

**kilo**(symbol:**k**), meaning

${10}^{3}$, e.g. kilovolt (kV).

**milli**(symbol:**m**), meaning

${10}^{\u20133}$, e.g. milliampere (mA).

**micro**(symbol:**μ**), meaning

${10}^{\u20136}$, e.g. microvolt (μV).

**pico**(symbol;**p**), meaning

${10}^{\u201312}$, e.g. picofarad (pF).

Note that SI recommends using the ‘engineering system’ of multiples —i.e. ten, raised to the power of multiples of 3. This means that multiples, such as **centi**, **deca**, etc., should not normally be used with SI units. For example, we should *avoid using* centimetres, and use millimetres instead.