Sistema
A thermostat is a system ensuring a constant temperature. That can be a passive device (standard Thermos bottle) or one apparatus which is useful has to control a Température.
Insulating wall
Certain materials are heat insulators. A container whose wall is made in such a material makes it possible to preserve a constant temperature inside whatever the temperature variations external.
In fact, the insulation is never perfect, the wall transmits heat all the same. Such a device only allows in fact to slow down the thermal balance which will end in any case up being established between the container of the container and outside.
The best insulating with respect to the thermal Conductivité is the vacuum: the absence of Atome does not allow the kinetic transmission of energy gradually, the only possible transmission is the Radiation. An ideal insulator is thus a double-wall with the vacuum inside, and which reflects the radiations Infrarouge S (for example the external wall is metallized on its inner face); there will exist always matter bridges between the two walls to maintain them remote.
It is very difficult to maintain a vacuum, that requires an active pumping in general. One is often satisfied with a layer of Air between the walls: the air is indeed an excellent heat insulator as long as it is motionless (principle of the Fourrure, of the Laine, the double glazing), on the other hand, it can transmit heat by Convection. It is the solution in general adopted for the Thermos bottles: a bulb of glass metallized in a plastic bottle of , plastic playing the part of second wall and shock protection.
The best example of passive thermostat is the calorimetric Bombe.
Apparatus of active regulation
Historically a thermostat consisted a long time of a system with Bilame which opened an electrical circuit when heat reached a certain stage and completed this as soon as one went down in lower part from this same Température. In practice, all occurred as if there had existed in fact two temperatures close, the thermal system having itself a certain inertia (comparable with the Hystérésis in the field of magnetism).
This system still is very much used, but electronics took the step on mechanics. Most of the time, electronic measurement consists in measuring an electrical resistance which varies according to the temperature.
The old cars of the SNCF used for the heating another system of regulation not having discontinuity nor the problems of reliability of the thermostat, and which was based on a system of the kind Wheatstone bridge.
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