Editing Resistor (section)

===Special devices===
There are various devices whose resistance changes with various quantities. The resistance of NTC [[thermistor]]s exhibit a strong negative temperature coefficient, making them useful for measuring temperatures. Since their resistance can be large until they are allowed to heat up due to the passage of current, they are also commonly used to prevent excessive [[Inrush current|current surges]] when equipment is powered on. Similarly, the resistance of a [[humistor]] varies with humidity. One sort of photodetector, the [[photoresistor]], has a resistance which varies with illumination.

The [[strain gauge]], invented by [[Edward E. Simmons]] and [[Arthur C. Ruge]] in 1938, is a type of resistor that changes value with applied strain. A single resistor may be used, or a pair (half bridge), or four resistors connected in a [[Wheatstone bridge]] configuration. The strain resistor is bonded with adhesive to an object that is subjected to [[Infinitesimal strain theory|mechanical strain]]. With the strain gauge and a filter, amplifier, and analog/digital converter, the strain on an object can be measured.

A related but more recent invention uses a [[Quantum Tunnelling Composite]] to sense mechanical stress. It passes a current whose magnitude can vary by a factor of 10<sup>12</sup> in response to changes in applied pressure.