Thermostats control the temperature of a system—such as an oven, car engine, or room—so that it remains close to a preset value. The control is achieved by means of a temperature-sensitive switch that operates heating or cooling devices. Many of these switches are activated by monitoring the expansion of metals, waxes, or gases. More recently, thermostatic devices have relied on thermistors in electrical circuits. The thermistor is usually a ceramic or polymer electrical resistance that changes its value significantly as a function of temperature. Thermistors were patented by Samuel Ruben in 1930.
Dr. Andrew Ure (1778-1857) was a Scottish medic and chemist who was also greatly interested in the factory system, free trade, and steam-driven machines. Realizing that textile mills needed constant temperatures to ensure uniformity in product manufacture, Ure patented a thermostat for this purpose. This was not an entirely new device. The Dutchman Cornelis Drebbel (1572-1633) had used the expansion and contraction of a vessel of mercury to control the airflow, and thus the temperature, inside a chicken incubator.
Animals have very efficient biological thermostats. The human body uses the hypothalamus to both regulate temperature and detect temperature. The typical human temperature is about 99.32°F (37.4°C) when we are awake and working, and 97.34°F (36.3°C) when we are asleep. When the skin temperature increases above 98.6°F (37°C), sweating begins, and body heat loss is enhanced by the evaporation of perspiration. At temperatures below 98.6°F (37°C), heat flow to the skin is reduced by vasoconstriction. Shivering starts, hairs become erect to increase insulation, and heat production is increased.