A: When it's defined and measured differently, by global consensus.
The scientific community has redefined several System Internationale units of measure, including the ampere, in an historic agreement signed at Versailles in Paris this week.
Previously defined in theoretical terms involving measuring the forces of attraction or repulsion between two infinitely long conductors in a vacuum, the amp is now defined more pragmatically in terms of the passage of a coulomb per second, in other words a specific number of elementary charges (electrons or protons) passing through a conductor in a precisely specified period.
The charges and the time are both defined as SI units and are measurable, hence the amp is also measurable ... although it's not exactly easy. Whereas the measurement of time is pretty well sewn up by measuring the frequency of energy changes in caesium, scientists are currently developing solid state devices to control and count the passage of individual electrons.
At least, that's my understanding as a radio amateur, a former scientist with an interest in measurement. I'm not a theoretical physicist. I don't count electrons, I put them to use.
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