Basic Circuitry
Necessary elements of a lamp's circuit are the electrical source, the lighting element and the switch. When you plug a lamp into an electrical outlet, metal prongs in the plug make contact with the conductor in the outlet, allowing household current to flow through the bulb. The switch, when in "closed" position, permits the current to flow through the bulb, where electrical energy is transformed into electromagnetic energy, which in this case is light.
Turning the Lamp On
When the lamp is plugged into the wall outlet, but the switch is "off," no electricity can pass through the bulb because the circuit is broken or incomplete, just as if the lamp cord was cut in two. Turning the lamp "on" by closing the switch is essentially the same as reconnecting the cut lamp cord or placing a conductor between the separated pieces, allowing electrical current to pass through the bulb.
Dimmer Switches
Dimmer switches for incandescent lamps have been around since the 1960s. Older models used a rheostat or autotransformer to manually control electrical current through the bulb. More recent models provide pulsed light and are more energy-efficient than early light dimmers.