WHEN CHOOSING POTENTIOMETERS

When choosing potentiometers and their values, another consideration is either a linear type or a logarithmic type, if using for example; to vary a motors speed or fan on say a unidirectional (one way), generally a linear type is usually best suited in a very basic circuit, as turning the potentiometer from one end increases/decreases the resistance evenly and is proportional to the amount turned, so at midway it is half of the total value of the pot. This could be a 50% speed increase or decrease, of a motor, which could be 0 RPM through to max RPM, The potentiometer would generally drive the base of a  darlington pair transistor or gate of a  mosfet for large current amplification, and could be wired in common collector mode so as the transistor follows the voltage at its base (less the 1.4 volt drop across base - emitter junction for a darlington pair) as there are two transistors in one package so .7v and another .7v are lost across each transistor in the package and the products of their gains are multiplied together, for large current amplification.

Logarithmic potentiometers are suited for audio and volume controls as they are for the human ear which hears logarithmically and not in a linear fashion. The log pots at midway are of moderate power then increase sharply as it keeps moving up, to match human hearing. They can still be used for general use but don't exhibit the proportional increase to movement, for example increasing a light from low to max brightness  would be a bit jittery. And similarly a linear pot used for audio would seem to have the most increase in sound at the lower volume settings, and not much apparent change at the higher end.  Some potentiometers have lin, or log written on them to identify which type it is.