Aluminum wiring is electrical wiring made of aluminum, instead of the usual copper. It is widely and successfully used for electricity transmission and distribution. It is lighter and stronger than copper wire, which helps prevent the cables from sagging. It is also less expensive than copper wire. It has also been successfully used in aircraft wiring. In the 1960s, with the rising price of copper, electricians started to use it to wire homes and other buildings. Aluminum wiring was not successful in this application and it is a major safety problem. Because of the safety problems with aluminum wiring, it was discontinued for building wiring in the mid-1970s. Over time, aluminum wiring can separate from the screws on electrical outlets, switches, or lights, creating a poor connection, which causes the wire to heat up. The heat can cause the aluminum to oxidize, creating an even worse connection and more heat, which can eventually start a fire. Incorrect repairs to aluminum wiring, such as connecting aluminum wire to copper wire with twist-on wire connectors, can make the problem even worse. Even twist-on connectors designed to connect aluminum to copper can be dangerous. The only proper way to repair aluminum wiring is to connect it to copper "tails" (short pieces of copper wire) with crimped or screw-on connectors specifically designed for this purpose.