Why electric current is a SI base unit?

1. Dec 5, 2012

wasi-uz-zaman

hi, why electric current is a SI base unit - why not couloumb?

2. Dec 5, 2012

phinds

Wikipedia explains it quite well.

3. Dec 5, 2012

D H

Staff Emeritus
In the past, realizability. A system of units needs to have base units that are realizable and portable. How do you measure charge? A flowing current induces a force on the wires through which the current is flowing. That force is measurable (but there are better realizations).

In the future, it's a bit fuzzier. The proposed change to the International System makes the elementary charge (the charge of one electron) a defined constant, exactly 1/(6.2415093×1018) coulombs (or something to that effect). The ampere will be defined in terms of a current that has specific number of elementary charges flowing past a point per second. So which is the base unit now, the ampere or the coulomb? It's still the ampere. The reason: Because they said so. Bureaucratic inertia is an incredibly powerful force, even though it cannot be expressed in Newtons.