This could also be a specialised bendable pipe, known as a conduit, or one in all a number of types of steel (rigid steel or aluminium) or non-metallic (PVC or HDPE) tubing. A considerably comparable system known as "concentric wiring" was introduced within the United States around 1905. On this system, an insulated electrical wire was wrapped with copper tape which was then soldered, forming the grounded (return) conductor of the wiring system.