How can amperage in a low voltage circuit be measured?

The question should be, "How should electrical current measured?"

Current is measured in Amperes using an Ammeter placed in series with the conductor of the current. This is impractical, so most ammeters have windings of wire that pick up the field generated around the conductor in question. This winding, or coil, is very carefully sized and the meter very carefully calibrated to yield correct readings of current without a direct connection.