A pipeline pig is provided with a wheel mounted thereon to roll along the inner wall of the pipeline. A magnet is carried by the wheel for movement past a magnetic flux responsive switch carried by the wheel mounting mechanism so that as the wheel revolves, the magnet moves past the switch to actuate it and thereby provide a signal proportioned to the distance moved by the pig without any appreciable drag or torsional load on the wheel so as to cause it to tend to skid without rotation along the pipeline.