Potentiometer. Basically an adjustable resistor.
12 volts at the ammeter. The gauges are actually 5 volts. The voltage is stepped down with a voltage regulator so if you hook 12 volts to the ammeter connection with the gauges still in the cluster you will be ok. If the gauges are out and you are checking them individually, then you want 5 volts.
I have all the gauges individually! I will check them with 5 volts then! Thanks.