The following figure shows a temperature automatic control circuit composed of an electric contact mercury thermometer. In the figure, A and B are the two contacts of the electric contact thermometer, and the KM is the AC contactor whose coil rated voltage is 36V, and RL is an electric heater. When the power switch S is closed, the electric heater starts to heat up. When the temperature rises to the preset temperature, the two contacts A and B of the electric contact mercury thermometer are turned on, so that the contactor KM is electrically connected, and the normally closed contact cuts off the electric heater power supply and stops heating.

When the temperature is lower than the preset temperature, the two contacts A and B of the electric contact mercury thermometer are disconnected, the KM is de-energized, the contact is reset, and the electric heater starts heating again. In this way, the heating is repeated and the temperature is automatically controlled.