Topological groups and Dugundji compact spaces

A pipeline pig for locating constrictions or irregularities on the inner surface of a pipeline consists of a fluid-filled envelope, such as a rubber or rubberlike spherical or toroidal ball, arranged to be a close fit within the pipeline, and means for sensing pressure variations within the envelope. The distance travelled along the pipeline by the pig may be measured by means of a contact wheel running on the pipeline surface and provided with a revolution counter.