An electronic temperature controller

An instrument is described in which a thermistor, immersed in the fluid system to be controlled, is connected in an a.c. Wheatstone bridge operated at 50 c/s. An error signal results when the thermistor is not at the required temperature, and is passed, after amplification, to a phasesensitive detector which operates a relay controlling the heating and/or cooling of the fluid. Special attention is paid to the mounting of the thermistor. Tests have shown that the unit can control a water bath of some 8 gallons capacity within ±0.02° C over a period of weeks.