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An ammeter is a completely different type of meter than the various pressure, temperature, and quantity units made by King Seeley. Ammeters have a d'arsonval type meter movement that measures current difference between two sides of a circuit through either a shunt or a shunt wire arrangement. Ford's use of ammeters was spotty since they found that an induction type meter worked more reliably for the purpose of a charge or discharge meter since that is basically all they are. This way they didn't have to worry about a shunt and it was more simple plus less expensive to produce.
King Seeley units were more of a variable amperage device with the device on the other end mirroring it's position. Since it uses temperature sensitive bimetallic switches, it could be easily adapted for both temperature and pressure sensitive reading devices. On the fuel quantity it was a pressure sensitive device with the float arm providing different pressures depending on the level in the tank. A very simple but ingenious device.
Ohmic type rheostats start wearing out the second you start to rub the brush on the wire wound resistor. It's just a matter of time before the brush wears down or the wire wears through making an open circuit. The more wear they have the less reliable they become due to wear and crud build up.