Voltage is measured across a resistance. An open circuit will show a full potential voltage, and zero amperage flow. A short circuit(like a piece of wire) will show no voltage drop and maximum amperage flow.
To measure voltage, you put one lead on a ground and the other lead on a test point which has a resistive element in between, like a light bulb filament. Given enough amperage flow from the supply, you will measure the supply voltage across the gauge terminals. In a car, that would be ~12VDC. To measure the current, you must put the meter leads in series with the resistance so all the flow passes through the meter:
When the connection is made, the bulb will light up, and the current passing through the ammeter will deflect the gauge, indicating all the current being delivered to the bulb filament(there is a tiny amount of current used by the meter).
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