A simple temperature-controlled laboratory furnace
Robert S. Barnes · Journal of Scientific Instruments · 1951
A simple and robust laboratory furnace using an on/off method of control is described. The temperature-sensitive element (or "thermometer") is a platinum resistor which forms one part of the heater winding of the furnace, the other part, which is in series, being a nickel-chromium resistor. The simplicity of the furnace control derives from this combination of heater winding and thermometer, since such an arrangement calls for little amplification in the control circuit. The resistors of the heater circuit act as two arms of a Wheatstone bridge network which is balanced at a temperature determined only by the setting of the inductive ratio arms. The control circuit, which maintains the bridge in balance, consists of a thyratron operated switch, the only amplification of the out-of-balance signal from the bridge being provided by an audio-frequency transformer with a tuned secondary. For more than six months the apparatus has given satisfactory service at temperatures up to 800° C. For example, in a run at 750° C the temperature was maintained constant to ± 0.1° C for a day.