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# Definition of the ampere

## Fundamental properties and fundamental units

Charge is a fundamental property of matter, like mass.  You would have thought that the ampere would have been defined in terms of the coulomb but in fact it's the other way round.

In the SI system the ampere is a base unit and the coulomb is just the charge that passes a point when 1 ampere flows for 1 second.

## How to define a unit

How do you define a metre or a kilogram or a second?  One way of doing it is just to have a block of metal kept in a safe somewhere and say 'This is a kilogram.'  And then everyone else in the world builds their own kilogram reference masses to match it.

With the kilogram this is exactly what happens.  The reference kilogram is a platinum-iridium alloy block manufactured in the 1880's and kept with six official copies in a vault at the Bureau International des Poids et Mesures (BIPM) near Paris.

It's much more convenient to have a definition that involves an experiment that's fairly easy to do.  Then all you need to do is repeat the experiment and the result shouldn't matter who did the experiment or when.

For example the metre used to be defined in terms of the distance between two marks on another platinum-iridium block at the BIPM but is now defined as the distance travelled by light in an absolute vacuum in 1/299,792,458 of a second.

The second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.

It's harder to find a good experiment to define the coulomb than it is to define the ampere so the ampere becomes the more fundamental unit.

## The definition of the ampere

All electric currents produce a magnetic field.  So two wires carrying currents in opposite directions will tend to attract each other as their two magnetic fields interact.  It's this effect that gives us the definition of the ampere:

That constant current, which when maintained in two infinitely long, straight, parallel wires of negligible circular cross-section held one metre apart in a vacuum, produces between them a force of 2 x 10-7 newtons per metre of length.

back to Lesson 5: Voltage and Current