Thermistors are inexpensive, easily-obtainable temperature sensors.  They are easy to use and adaptable.  Circuits with thermistors can have reasonable outout voltages – not the millivolt outputs thermocouples have.  Because of these qualities, thermistors are widely used for simple temperature measurements.  They’re not used for high temperatures, but in the temperature ranges where they work they are widely used.

A Thermistor is a temperature dependent resistor.  When temperature changes, the resistance of the thermistor changes in a predictable way.

Source:http://www.facstaff.bucknell.edu/mastascu/elessonsHTML/Sensors/TempR.html

Link to report of this instrument: TP