BS 546 outlets are often put on lighting circuits to allow control of free-standing lamps from the room's light switches. In North American site-built and mobile homes, often living rooms and bedrooms have a switched receptacle for a floor or table lamp. Such receptacles are referred to as or . Internal operation The contacts of a switch are under their greatest stress while opening or closing. As the switch is closed, the resistance between the contacts changes from almost infinite to almost zero. At infinite resistance, no current flows and no power is dissipated. At zero resistance, there is no voltage drop and no power is dissipated. However, while the contacts change state, there is a brief instant of partial contact when resistance is neither zero nor infinite, and electrical power is converted into heat. If the heating is excessive, the contacts may be damaged, or may even weld themselves closed. A switch should be designed to make its transition as swiftly as possible. This is achieved by the initial operation of the switch lever mechanism storing potential energy, usually as mechanical stress in a spring. When sufficient mechanical energy is stored, the mechanism in the switch "breaks over", and quickly drives the contacts through the transition from open to closed, or closed to open, without further action by the switch operator. This quick-break action of the switch is essential to a long life for the switch contacts, as described in Holmes' 1884 patent. While the contacts are separating, any electrical energy stored in the inductance of the circuit being disconnected is dissipated as an arc within the switch, prolonging the transition and worsening the heating effect on the contacts. Switches are commonly rated by the current they are designed to break, under specified voltage and power factor conditions, as this is the most stringent limitation. The arc that results when the switch opens erodes the switch contacts. Therefore, any switch has a finite life, often rated at a given number of cycles of disconnection at a specified current. Operation outside of its specified operating capacity will drastically shorten the life of the switch. To combat contact corrosion, a switch is usually designed to have a "wipe" action so that the contacts are cleaned. Large switches may be designed with a supplemental replaceable contact that closes and opens before the main contact, protecting the main current-carrying contacts from wear due