The unit of electric current is the Ampere.
1 Ampere of current means that if you set up your chair and
stare at the electrons flowing past one point in the circuit, you'll
see 1 coulomb of charge passing that point every second.
How will you recognize 1 coulomb of charge ?
Well, every electron carries the same amount of charge, and
we know how much that is. (Read about the Millikan oil-drop
experiment in 1909.) So we know how many electrons it takes
to carry 1 coulomb of charge past the point you're watching.
All you have to do is count the electrons as they zip past.
Every time you count 6,241,509,343,000,000,000 electrons,
you can tick off 1 coulomb of total charge that they're carrying.
If you reach that count every second, you know the current
passing that point is 1 Ampere.
