The law of a variable resistor or potentiometer must be specified in addition to the quantities that are specified for any fixed resistor.
The potentiometer law (called taper in the USA) describes the way in which resistance between the slider and one contact varies as the slider is rotated; the law is illustrated by plotting a graph of resistance against shaft rotation angle (Figure).
A linear law potentiometer (Figure) produces a straight-line graph, hence the name linear. Logarithmic (log) law potentiometers are extensively used as volume controls and have the graph shape shown in Figure.
Potentiometer laws: (a) linear, (b) logarithmic. In the USA the word ‘taper’ is used in place of ‘law’, and ‘audio’ in place of ‘log’. Broken lines show tolerance limits.
Less common laws are anti-log and B-law, and specialized potentiometers with sine or cosine laws are also available.