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This type of temperature sensor works by converting a temperature change into mechanical displacement.It contains two jointed strip which differ in expansivity.It works on two basic principle, one which is that a metal would expand when heated and contract when cooled, another is that the two different metals would expand and contract at different rates.
Liquid in Glass Thermometer
The sensing element in most thermometers is the liquid placed in between a scaled capillary tube which exploits the expansivity of such elements, for example mercury or alcohol, Mercury is usually preferred because of its versatility, it is a metal in liquid form, it requires only one calibration temperature and has a very stable coefficient of expansivity.
REFERENCESto be continued.....
http://www.technologystudent.com/elec1/therm1.htm
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http://science.howstuffworks.com/thermocouple-info.htm
http://wiki.answers.com/Q/Why_is_mercury_used_as_a_thermometric_fluid?#slide=1
http://www.slideshare.net/harshadd/temperature-sensors-828567
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