Ruth Jovanovich, age 13, of Fort Wayne, Ind., for her question:

HOW DOES A THERMOSTAT WORK?

A thermostat is a device that helps control the temperature of an indoor area or of an appliance. Most thermostats used in home heaters have a bimetallic strip that actually senses changes in temperature.

A bimetallic strip is made up of two metals fastened together. When the temperature rises, each metal expands at a different rate, causing the strip to bend. The metals contract unequally when the temperature drops. These actions cause the strip to bend in the opposite direction.

The bending action of the bimetallic strip opens or closes the electric circuit that controls heating or cooling equipment.

In some thermostats, the bending of the strip tilts a bulb filled with mercury. When the mercury bulb tilts, it completes or breaks the circuit.

Other thermostats use the expension or contraction of a gas or a liquid to control heating or cooling equipment. Still other types use electric devices or infrared detectors that sense temperature changes.

Most thermostats turn heating or cooling equipment completely on or completely off. But some use a method called proportional control. These thermostats measure the difference between the actual temperature and the desired temperatures. They change the amount of heating or cooling in proportion to this temperature difference.

Proportional control thermostats can provide an extremely even temperature. They are used in industry and scientific research to control the temperature so that certain chemical processes can take place.

A thermostat measures temperature changes and automatically controls the heating or cooling unit of the equipment being used.

In a home heater, the thermostat turns on the furnace if the temperature drops below the desired level and it shuts off the furnace when the temperature reaches that level.

Thermostats are also used to keep refrigerators and freezers at the necessary temperature to keep food from spoiling. And they are used in ovens to keep the temperatures at exactly the right point.

In homes, thermostats also operate temperature control over irons, hot water heaters and heaters for fish tanks.

Thermostats are also used to control the flow of water in automobile cooling systems. The thermostat operates a valve that opens when the water reaches a certain temperature. The open valve allows water to circulate through the radiator and through the water jacket that surrounds the engine.

Thermostats are also used to keep an even heat in your electric blanket.