V variable resistors.
Presets and potentiometers are commonly used types of variable resistors. These are mostly used for voltage division and setting the sensitivity of sensors. These have a sliding contact or wiper which can be rotated with the help of a screw driver to change the resistance value. In the linear type, the change in resistance is linear as the wiper rotates. In the logarithmic type, the resistance changes exponentially as the wiper slides. The value is meant to be set correctly when installed in some device, and is not adjusted by the device's user.
The variable may have three tabs where the middle tab is the wiper. If all the three tabs are used, it behaves as a voltage divider. If only wiper tab is used along with another tab, it becomes a variable resistor or rheostat. If only the side tabs are used, then it behaves as a fixed resistor. These are mostly used for tuning, voltage division and adjusting sensitivity of sensors.
The variable can have one or two switches in-built where the resistor operates for the ON state of the switch(s). Such resistors were mostly used for volume control in older TV and radio circuits. There may also be four-tab variables where the fourth lead is for feedback signal and placed near the first tab. Wire wound variable resistors are used for very precise control of resistance.
The wiper may also be rotary (as in most presets), sliding or disc shaped (as used in pocket radios for volume control).
source:engineersgarage,com
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