How do thermostats work?
Thermostat
A thermostat exerts control by switching heating or cooling devices on or off, or by regulating the flow of a heat transfer fluid as needed, to maintain the correct temperature. A thermostat can often be the main control unit for a heating or cooling system, in applications ranging from ambient air control, to such as automotive coolant control. Thermostats are used in any device or system that heats or cools to a setpoint temperature, examples include building heating, central heating, air conditioners, as well as kitchen equipment including ovens and refrigerators and medical and scientific incubators.
Warren S. Johnson
Johnson had an inquisitive mind and was particularly interested in electricity. In 1883, he developed a thermostat, which he deployed at the State Normal School. He called the instrument an "electric tele-thermoscope" in the patent application. It was a bi-metal coiled thermostat with a mercury switch, which could be used to ring a bell to alert the fireman to open or close the heating damper. While not the first bi-metal thermostat, Johnson received a patent for the device and interested William Plankinton, heir to the Plankinton Packing Company, to provide financial backing to manufacture the device. 
The Johnson Electric Service Company was established in 1885 in Milwaukee, Wisconsin. Johnson’s most notable contribution to temperature control was the automatic multi-zone temperature control system – a pneumatic system that used a bi-metal thermostat to control air flow through a nozzle and thereby operate a pilot regulator. The amplified air signal from the regulator was then used to control a steam or hot water valve on a heat exchanger, or to control a damper of a forced air system. He received a patent for the system in 1895. 