Its called an anticipator. The anticipator is a ring of resistive wire on the dial.
This thermostat has a neat device called a heat anticipator. The heat anticipator shuts off the heater before the air inside the thermostat actually reaches the set temperature. Often, some parts of the house will reach the set temperature before the part of the house containing the thermostat does. The anticipator shuts the heater off a little early to give the heat time to reach the thermostat
Now the freaks are on television, the freaks are in the movies. And its no longer the sideshow, its the whole show. The colorful circus and the clowns and the elephants, for all intents and purposes, are gone, and were dealing only with the freaks. - Jonathan Winters