Logging linear movement can be a complicated and expensive business. A variety of measuring systems can be used, depending on the accuracy required and distance to be measured. The simplest of these is the sliding potentiometer, which supplies a variable resistance proportional to the distance of travel. Incremental or absolute linear encoders can have accuracies of <1 ?m at lengths of several meters. Simple applications, such as level controllers, lift magnets, seat adjusters, motion sensors, or noncontact sliding switches, however, require the measurement of distances from a few millimeters to several hundred centimeters-as inexpensively as possible and without contact between the components. Magnetic encoders based on Hall sensors have long been successfully deployed for the detection of rotational motion .