Thermistors are temperature sensors which have a sensing element usually made of polymers or ceramics.  Thermistors function by changing their resistance when the temperature increases. 

Thermistors find wide application in the industry, in automobiles and in electric appliances.  Their small size makes them ideal for use in electronic circuit boards and digital thermostats.

The principle of the Thermistor was first discovered by Michael Faraday in 1833.  However, the first practical thermistor was constructed by Samuel Ruben in the year 1930.

When the temperature of the Thermistor changes, the resistance of the Thermistor also changes.  The change can be either positive or negative.  Thus, we have PTC Thermistors (Positive Temperature Coefficient) and NTC Thermistors (Negative Temperature Coefficient).

NTC thermistors are used in temperature measurement while PTC thermistors are used in Electric current control. 

Thermistors are generally formed into a disc or bead and sealed in an enclosure made of plastic or gas.

Thermistors are highly accurate and have a quick response.  However, they have a limited range of measurement.  Another downside is that they do not have a linear response. 

Thermistors have high stability and are not affected by ageing.  This means that they need not be calibrated for long periods of time.    They are cheaper, rugged and are easy to produce.