Unbalance voltage from a potentiometer is amplified by a simple circuit employing a 60‐cycle polarized interrupter, and used to control the extent of the on‐off cycle of a furnace. Automatic cold‐junction compensation and a sensitive, temperature‐indicating circuit are incorporated. The circuit responds to a change in input voltage of less than 4 microvolts. The long‐term stability is limited by changes in 60‐cycle pick‐up and aging of components to about 40 microvolts.