...

Basically any material above absolute zero temperature radiates in the infrared even ice is known to emit infrared radiation.Sensing is carried out by collecting a small amount of energy (usually 0.0001 watt) which is a radiation(black body) from the target, and generates electrical signal that is amplified  transformed into voltage output.[5]

...

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.[6]

                                                                    Figure 5: A simple bimetallic strip when heated and when cooled[7]

It should be noted that the metal with high thermal expansion coefficient is usually placed on the outer part, when heated it expands hence causing the inner metal to contract, this in effect would cause the shape of the strip to bend in a "C" shape.

...

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.[8]

REFERENCES

1. V. Ryan. THE THERMISTOR : http://www.technologystudent.com/elec1/therm1.htm  Last accessed 15-4-2014
   
   
2. Kitronik Ltd . HOW A THERMISTOR WORKS : http://www.kitronik.co.uk/blog/how-a-thermistor-works  Last accessed 15-4-2014
   
   
3. HowStuffWorks Inc. THERMOCOUPLE : http://science.howstuffworks.com/thermocouple-info.htm  Last accessed 15-4-2014
   
   
4. Analogue Dialogue. Matthew Duff and Joseph Towey 
   http://www.analog.com/library/analogdialogue/archives/44-10/thermocouple.pdf  Last accessed 15-4-2014
   
   
5. Metris Instruments. INFRARED TUTORIAL : http://www.metrisinst.com/faqs-and-tutorials   Last accessed 15-4-2014
   
   
6. Wikipedia : http://en.wikipedia.org/wiki/Bimetallic_strip . Last accessed 15-4-2014
   
   
7. Wikimedia common : http://commons.wikimedia.org/wiki/File:Bimetal.gif .Last accessed 15-4-2014
   
   
8. Answers  : http://wiki.answers.com/Q/Why_is_mercury_used_as_a_thermometric_fluid?#slide=1 Last accessed 15-4-2014

...