Overview
A thermistor is a thermal resistor - a resistor that changes its resistance with temperature.
Technically, all resistors are thermistors - their resistance changes slightly with temperature -
but the change is usually very very small and difficult to measure. Thermistors are made so that
the resistance changes drastically with temperature so that it can be 100 ohms or more of
change per degree!
There are two kinds of of thermistors, NTC (negative temperature coefficient) and PTC
(positive temperature coefficient). In general, you will see NTC sensors used for temperature
measurement. PTC's are often used as resettable fuses - an increase in temperature increases
the resistance which means that as more current passes thru them, they heat up and 'choke
back' the current, quite handy for protecting circuits!
Thermistors have some benefits over other kinds of temperature sensors such as analog
output chips (LM35/TMP36 (http://adafru.it/aK2) ) or digital temperature sensor chips (DS18B20)
or thermocouples (http://adafru.it/cl0).
First off, they are much much cheaper than all the above! A bare 5% thermistor is only 10
cents in bulk.
They are also much easier to waterproof since its just a resistor.
They work at any voltage (digital sensors require 3 or 5V logic).
Compared to a thermocouple, they don't require an amplifier to read the minute voltages -
you can use any microcontroller to read a thermistor.
They can also be incredibly accurate for the price. For example, the 10K 1% thermistor in
the shop is good for measuring with ±0.25°C accuracy! (Assuming you have an accurate
enough analog converter)
They are difficult to break or damage - they are much simpler and more reliable
