# Why is the Ampere a base unit and not the Coulomb?

I always thought of current as the time derivative of charge, $\frac{dq}{dt}$. However, I found out recently that it is the Ampere that is the base unit and not the Coulomb. Why is this? It seems to me that charge can exist without current, but current cannot exist without charge. So the logical choice for a base unit would be the Coulomb. Right?

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Fyi, this question has been previous asked on electronics stackexchange: 1. electronics.stackexchange.com/q/23449 2. electronics.stackexchange.com/q/62483 –  The Photon Jul 11 '13 at 4:59

Because it was defined by measurements (the force between to wire segments) that could be easily made in the laboratory at the time. The phrase is "operational definition", and it is the cause of many (most? all?) of the seemingly weird decision about fundamental units.

It is why we define the second and the speed of light but derive the meter these days.

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To amplify further on this answer, we have instruments (ammeters) that can measure current very accurately. But it's extremely difficult to do high-precision experiments with static electricity, i.e., it's relatively hard to measure charge. –  Ben Crowell Jul 11 '13 at 3:10
Well, you can always measure charge by measuring current and measuring the time for which the current was flowing. So I don't buy the measurement argument. It is true that any set of 4 quantities can be made as 'fundamental' and others would be 'derived'. But we should ideally choose quantities which appeal to our notion of fundamental - something which is a basic property or a primitive notion. Charge is a basic property of all matter, unlike current which is defined only with respect to a surface hence I feel that charge needs to be chosen as a fundamental quantity rather than current. –  guru Jul 11 '13 at 9:57
The reason why current is chosen may be just due to historical reasons. –  guru Jul 11 '13 at 9:58
Of course it is historical. That's what I mean about it depending on what was easy at the time. The time the decision was taken. And when you say*"you can always measure charge by measuring current and measuring the time"* you have explained why the decision was made to have charge a derived unit. –  dmckee Jul 11 '13 at 11:44

## protected by Qmechanic♦Nov 1 '13 at 21:17

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