[fixit-p]

It's an external shunt type ammeter it doesn't really measure amps it is more of a charge indicator. The charge wire is the shunt in the system and the gauge is connected at either end of the wire and is calibrated to the wires resistance make sure you fuse both wires (3-4 amp). When the ammeter sees x voltage on the battery end of the shunt (wire) it expects x voltage on the alt. end because of the shunts fixed resistance and depending on the voltage differential the ammeter moves accordinly. On smaller GM cars there was an actual shunt installed the pic below is a shunt from a chevyII.