Editing Resistor (section)

==Variable resistors==
{{Main article|List of resistors}}

===Adjustable resistors===
A resistor may have one or more fixed tapping points so that the resistance can be changed by moving the connecting wires to different terminals.  Some wirewound power resistors have a tapping point that can slide along the resistance element, allowing a larger or smaller part of the resistance to be used.

Where continuous adjustment of the resistance value during operation of equipment is required, the sliding resistance tap can be connected to a knob accessible to an operator. Such a device is called a [[rheostat]] and has two terminals.

===Potentiometers===

[[File:Potentiometer cutaway drawing.png|thumb|Potentiometer with case cut away, showing parts: (''A'') shaft, (''B'') stationary carbon composition resistance element, (''C'') phosphor bronze wiper, (''D'') shaft attached to wiper, (''E, G'') terminals connected to ends of resistance element, (''F'') terminal connected to wiper.]]
A [[potentiometer]] (colloquially, ''pot'') is a three-terminal resistor with a continuously adjustable tapping point controlled by rotation of a shaft or knob or by a linear slider.<ref name="Mazda">{{cite book
 | last1  = Mazda
 | first1 = F. F.
 | title  = Discrete Electronic Components
 | publisher = CUP Archive
 | date   = 1981
 | pages  = 57–61
 | url    = https://books.google.com/books?id=3qk8AAAAIAAJ&pg=PA57
 | isbn   = 0521234700
 }}</ref>  The name ''potentiometer'' comes from its function as an adjustable [[voltage divider]] to provide a variable [[potential]] at the terminal connected to the tapping point.  Volume control in an audio device is a common application of a potentiometer.  A typical low power potentiometer ''(see drawing)'' is constructed of a flat resistance element ''<span style="color:red;">(B)</span>'' of carbon composition, metal film, or conductive plastic, with a springy [[phosphor bronze]] wiper contact ''<span style="color:red;">(C)</span>'' which moves along the surface.  An alternate construction is resistance wire wound on a form, with the wiper sliding axially along the coil.<ref name="Mazda" />  These have lower resolution, since as the wiper moves the resistance changes in steps equal to the resistance of a single turn.<ref name="Mazda" />

High-resolution multiturn potentiometers are used in precision applications.  These have wire-wound resistance elements typically wound on a helical mandrel, with the wiper moving on a helical track as the control is turned, making continuous contact with the wire.   Some include a conductive-plastic resistance coating over the wire to improve resolution.  These typically offer ten turns of their shafts to cover their full range. They are usually set with dials that include a simple turns counter and a graduated dial, and can typically achieve three-digit resolution. Electronic analog computers used them in quantity for setting coefficients and delayed-sweep oscilloscopes of recent decades included one on their panels.

<gallery mode=packed heights=160>
File:Potentiometer.jpg|Typical panel mount potentiometer
File:12 board mounted potentiometers.jpg|An assortment of small through-hole potentiometers designed for mounting on [[printed circuit board]]s.
</gallery>

===Resistance decade boxes===
{{Main article|Decade box}}
[[File:Odporová dekáda KURBELWIDERSTAND (A).jpg|thumb|Resistance decade box]]
A resistance decade box or resistor substitution box is a unit containing resistors of many values, with one or more mechanical switches which allow any one of various discrete resistances offered by the box to be dialed in. Usually the resistance is accurate to high precision, ranging from laboratory/calibration grade accuracy of 20 parts per million, to field grade at 1%. Inexpensive boxes with lesser accuracy are also available. All types offer a convenient way of selecting and quickly changing a resistance in laboratory, experimental and development work without needing to attach resistors one by one, or even stock each value. The range of resistance provided, the maximum resolution, and the accuracy characterize the box. For example, one box offers resistances from 0 to 100 megohms, maximum resolution 0.1 ohm, accuracy 0.1%.<ref>{{cite web|url=http://www.ietlabs.com/decaderes.html |title=Decade Box – Resistance Decade Boxes |publisher=Ietlabs.com |access-date=2008-09-22}}</ref>

===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.