Water Temperature Indicator Sender Unit
WARNING: This page is about a different car, the 2002 Ford Ranger. However, it is still accessible from the selected car via links, so may be relevant.
When the engine temperature is low, the resistance of the water temperature indicator sender unit is high, thus restricting the flow of current through the gauge and moving the pointer only a short distance. As the temperature of the coolant increases, the resistance decreases, allowing more current to flow through the gauge and resulting in a corresponding movement of the pointer.