In an electric furnace control system, a thermocouple is used to measure the internal temperature of the furnace and transmit real-time temperature signals to the controller. The controller adjusts the heating element's power output by controlling the triggering angle of a thyristor based on feedback from the thermocouple and the set temperature. Together, the thermocouple and thyristor ensure precise temperature control in the electric furnace, maintaining stability and accuracy during industrial processes.