Explaining Variable Resistors
The above explanation discussed resistor in their fixed forms, which meant that the parts could be introduced with a certain fixed values and the corresponding fixed current and voltage levels could be passed.
However in many applications the voltage and current levels in a circuit are required to be set in response to some external parameter. During such occasions variable resistors become very handy and many are made specifically for such applications.
These are also called potentiometer and presets. Potentiometers are the ones which are bigger in size, incorporate knobs, and are normally kept protruding externally over a circuit enclosure so that the necessary adjustments can be done whenever required by the user (e.g. a dimmer switch or volume control).
Presets are rather smaller in appearance, are required to be soldered over the PCB, and are normally used for prior fixed settings through a screwdriver.
Other forms include variable resistors which change their physical characteristic with the change in the ambient levels of heat or light.
All the above forms of variable resistors are rigorously explained below through examples:
How do Potentiometers Work?
What are Potentiometer Transducers?
How to Identify a Potentiometer
What are the Types of Potentiometers?
How do Thermistors Work?
What is a Thermocouple?