## Metric offers units from the very large to the very small

...from the tiniest particles such as the electron with a classical radius of approximately 2.8 fm (femtometre) to the theoretical size of the known universe which is of the order of 140 Ym (Yottametre).

Even in more familiar realms we encounter objects from cars and lorries that are weighed in tonnes (Megagram) to the minutest quantities in pharmacy where doses may be measured in micrograms.

By comparison, in imperial the largest unit of distance is the mile and the smallest the inch.

Smaller measures are normally achieved by fractions but they become unwieldy below 1/64. In general fractions are harder to use than decimal for the purposes of measurement.

Precision engineers who work - or used to - in imperial decimalise the inch and borrow from the metric system, hence the mil (one-thousandth of an inch).

## Imperial has no units for electricity and magnetism

The volt and the ampère are metric units with no equivalents in imperial.

The volt is equal to 1 joule per coulomb. The joule is the metric unit of energy and the coulomb a fixed amount of electric charge.

When an electric current of 1 amp flows from a power source of 1 volt, energy is dissipated in the circuit at the rate of 1 watt. The watt (1 joule per second) is the metric unit of power.

We all encounter these units when we buy batteries, fuses or lightbulbs. Thus virtually no one alive in today's Britain can remember a time completely before metric units.