This may be a specialised bendable pipe, referred to as a conduit, or one among several forms of metal (inflexible steel or aluminium) or non-metallic (PVC or HDPE) tubing. A considerably related system known as "concentric wiring" was introduced in the United States around 1905. In this system, an insulated electrical wire was wrapped with copper tape which was then soldered, forming the grounded (return) conductor of the wiring system.