Conduction, convection and radiation are all involved in temperature sensing, for temperature to be measured or sensed, Temperature sensing however differs among different types of devices, some require close proximity to the measurement or detection point while some necessarily doesn't require proximity. Below is an illustration of how individual sensing device works to detect temperature change:
The Thermistor
This works basically by exploiting the a variation in resistance to detect temperature. The circuit below illustrates how a thermistor circuitry works
Figure 1
From the above diagram it can be noted that when the thermistor warms up as a result of heat from the hair dryer, resistance of the thermistor drops which result in current flowing from the 9v potential to the 0v, this current makes way to the base of transistor which lights up the led connected to it.
The Thermocouple:
The thermocouple senses temperature as a result of the combination of two dissimilar metals or alloy connected via a joint. Temperature sensing occurs at the point called the joint, the ends of both metals or alloys are connected to a voltmeter.
The two junctions exhibits a difference in potential, which is indicated by the voltmeter, which is used to find the temperature of the heated junction, usually from a table in a handbook.
to be continued.....
http://www.technologystudent.com/elec1/therm1.htm
http://www.kitronik.co.uk/blog/how-a-thermistor-works