Thermostat
For many years, heat was an insoluble mystery. The first step toward understanding the mystery came in the mid-1600s, when scientists began constructing the first thermometers. Once they began to realize that temperature was measureable, they determined that anything that could be measured could be controlled. So began the effort to design a device to govern the temperature of a system.
That effort culminated around 1660 with the invention of the thermostat. Named from the Greek words for "constant temperature," the thermostat was invented by Dutch scientist Cornelius Drebbel. His device, used to regulate the temperature within a duck-and chicken-egg incubator, was actually an elegant combination of several different technologies. Placed above a furnace, the incubator was surrounded by a water-filled jacket. Inside the incubator was a container of alcohol that would expand as it was heated--this expansion pushed down on a U-shaped tube filled with mercury, and as one end of the U was pushed down, the other end would rise up, raising with it a rod. The rising rod would raise a lever connected to the furnace's flue, so that the airway would be partially closed and the temperature would begin to fall. When the temperature fell beyond a certain point the alcohol once again contracted, thus releasing the mercury, lowering the lever, and re-opening the furnaces airway. This type of thermostat underwent a series of modifications over the next two centuries before it found its way into industry.
Thermostats were an important part of the Industrial Revolution, and today are essential to the heating and air conditioning systems of homes, automobiles, and heavy machinery. Modern thermostats fall into two general categories: metallic and electronic.
The metallic thermostat has at its heart a bimetallic strip--a thin strip of two metals fused together. The two metals have different coefficients of thermal expansion; that is, they expand or contract by different amounts as the temperature rises or falls. When a bimetallic strip is heated, it will bend to one side. In a thermostat, that bending can be used to push a lever or a valve, or to complete an electrical circuit. Many household thermostats use a bimetallic strip. The thermostat that is becoming more common is the electrical thermostat. In electronics, the resistance of most metals will increase as the temperature does. By running an electrical current through a circuit, temperature can be measured by observing the change in the circuit's resistance. This type of thermostat is particularly useful when very precise temperatures must be maintained, such as in scientific laboratories and hospitals. Modern electronics are further combined with thermostats to produce programmable thermostats, essentially small microcomputers that control the temperature. Such units keep full track of the time and date, have a digital display readout, and can be preset to change to certain temperatures, such as an automatic lowering of a house temperature at night. It can then be programmed to raise the temperature of the house a hour or so before waking, saving money overnight that would have otherwise been spent heating an essentially empty house. Such units can save between 5% and 15% of a household's energy usage.
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