Digital thermometers work on the principle that changes in temperature cause the resistance of a sensor to change allowing a computer to convert the difference to a digital read-out. The sensor is a thermoresistor, commonly called a thermistor, according to HowStuffWorks.com.
About.com describes a thermistor as a rugged and robust low-cost sensor. It provides good accuracy and high sensitivity. However, the thermistor is non-linear in its response to temperature, limiting its accuracy for use over a wide temperature range unless the computer has the ability to make the necessary corrections. The most common way these devices fail is damage to one of the two lead wires. Selecting a metal probe offers greater protection.
Thermistors are available to support a large variety of applications. The most common type is a small glass bead from 0.5 to 5 millimeters in diameter, fitted with two wires. However, a particular application sometimes calls for a surface mountable disc or a tubular metal probe.
Thermistors find many uses in the electronics field. They replace fuses or circuit breakers and shut down sensitive electronics to prevent damage from overheating. In thermostats, they replace mercury switches and bimetallic strips to provide an electronic control rather than a mechanical one.