is designed to follow a logarithmic taper, aka a mathematical exponent or "squared" profile. A logarithmic taper potentiometer is constructed with a resistive element that either "tapers" in from one end to the other, or is made from a material whose resistivity varies from one end to the other. This results in a device where output voltage is a logarithmic function of the slider position. Most (cheaper) "log" potentiometers are not accurately logarithmic, but use two regions of different resistance (but constant resistivity) to approximate a logarithmic law. The two resistive tracks overlap at approximately 50% of the potentiometer rotation; this gives a stepwise logarithmic taper. A logarithmic potentiometer can also be simulated (not very accurately) with a linear one and an external resistor. True logarithmic potentiometers are significantly more expensive. Logarithmic taper potentiometers are often used for volume or signal level in audio systems, as human perception of audio volume is logarithmic, according to the Weber–Fechner law. Contactless potentiometer Unlike mechanical potentiometers, non-contact potentiometers use an optical disk to trigger an infrared sensor, or a magnet to trigger a magnetic sensor (as long as there are other types of sensors, such as capacitive, other types of non-contact potentiometers can probably be built), and then an electronic circuit does the signal processing to provide an output signal that can be analogue or digital. An example of a non-contact potentiometer can be found with the AS5600 integrated circuit. However, absolute encoders must also use similar principles, although being for industrial use, certainly the cost must be unfeasible for use in domestic appliances. Rheostat The most common way to vary the resistance in a circuit continuously is to use a rheostat. It is basically used to adjust magnitude of current in a circuit by changing length. The word rheostat was coined about 1845 by Sir Charles Wheatstone, from the Greek rheos meaning "stream", and - -states (from histanai, " to set, to cause to stand") meaning "setter, regulating device", which is a two-terminal variable resistor. For low-power applications (less than about 1 watt) a three-terminal potentiometer is often used, with one terminal unconnected or connected to the wiper. Where the rheostat must be rated for higher power (more than about 1 watt), it may be built with a resistance wire wound around a semi-circular insulator, with the wiper sliding from one turn of the wire to the next. Sometimes a rheostat is