Electric Switch, a device for completing and breaking an electric current, or for changing the path of a current. Electric switches are among the most common types of control devices and are in wide use wherever electricity is available. There are two basic types of switches, electromechanical and electronic. Only electromechanical switches are discussed here; electronic switches are described in Electronics, section “Some Basic Electronic Devices and Circuits,” subtitle Logic Circuits.
In its simplest form, a switch consists of two contacts, one fixed and one movable. When the contacts are brought together, the switch and the circuit are closed and current flows through the circuit. Operating the switch to disconnect the contacts opens the circuit and stops the flow of current. This type of switch, known as a single-pole switch, is commonly used in the home for turning lights on and off.
Switches are available in many types and sizes for a vast number of uses. Household wall switches include the familiar snap-action type, which contains a spring to give positive opening and closing action, and the mercury switch, in which a drop of mercury in a sealed glass tube carries current from one contact to the other when the switch is closed. The mercury switch is also used in thermostats.
Three-way switches are of two different types. In one, current is directed to one or both filaments of a double-filament (threeway) light bulb. The other type is used in pairs to control a single light from two locations, such as the top and bottom of a staircase.
In one type of photoelectric switch, light striking a photoelectric cell generates a current that causes an electromagnet to hold the switch open. When the light fails or is interrupted, the switch closes. Such switches are used in some types of automatic door openers and for automatic control of outdoor lighting.
Membrane, or touch, switches are commonly used in electronic calculators and other low-voltage devices containing microelectronic chips. The switching circuit is printed in two parts; each part is on a plastic film and the two films are separated from each other by a sheet of insulating material. The circuit is closed by lightly pressing one of the plastic films (or a key located directly above the film) so that the two parts make contact through an opening in the insulating sheet.