A precision thermostat has been developed, based upon that of Roberts, for the control of high temperatures (1000°—1550°C). With the instrument described, temperatures are readily maintained constant within ±0.1°C for several hours, and within ±1.0°C for several days.In this method of control a thyratron is used in conjunction with an a.c. bridge and high gain amplifier. The phase angle of the unbalanced bridge regulates the firing of the thyratron. The control is highly sensitive and independent of normal fluctuations in line voltage. The thermostat is convenient to operate, easy to adjust for any temperature, and reliable for periods of continuous operation from a few minutes to several days.