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A potentiometer (pot) is a variable resistor with an adjustable slider. You can think of it as two resistors (one on each side of the slider). The diagram above shows the commonest arrangement. There is another straight version often called a fader. Both types are used as volume and tone controls. They are notorious for being unreliable. Dirt gets into the moving parts and corrosion causes problems. Digital pots are available. These don't suffer from these mechanical problems.
When used for position sensing, potentiometers are classed as input transducers.
Voltage Divider Lab Exercise
Build this voltage divider circuit and measure the voltages at A, B, C, D and E without and with the LED. Note what happens when the potentiometer is adjusted.
CTRL+Click for the Falstad Circuit Simulaton: LED Circuit In options, check European Resistors and uncheck Conventional Current. Adjust the slider to alter the Potentiometer setting.