The heat anticipator is an electrical resistor device mounted in the center of many mechanical thermostats that fine tunes the point at which the thermostat turns off the furnace burners. It anticipates the flywheel effect of a space heating up and turns off the burners a short period of time before the space reaches the desired temperature.
The heat anticipator essentially consists of a simple thin wire mounted to a disc which is attached to the bimetallic coil. There is an adjustment arm which touches the wire and functions to vary the electrical resistance of the wire which in turn varies how hot the wire gets. This in turn warms the bimetallic coil causing it to shut down the gas burners early as determined by the heat anticipator.