Basic Switch Mechanism

• The light switch consists of a small metal box containing the switch mechanism and wiring. Most electricians use Romex, a thin plastic sheath containing two insulated wires and a bare grounding wire, to run 120-volt AC (alternating) current from the electrical service panel (formerly the fuse box) to the switch's top or first terminal. A ground wire also runs directly between the service panel and the light outlet. In the "on" or "closed" position, electricity flows between the terminals via a pair of metal contacts. A single wire from the bottom or second terminal then carries the current to the light. The circuit is completed as the current returns to the service panel via the neutral grounding wire. Moving the switch to the "off" position breaks the circuit and interrupts the flow of electricity.
  
  

On and Off Around the World

• A decorative wall plate, made of plastic or metal, covers the switch itself to protect it from damage or to protect anyone from having contact with the switch's live wires and terminals. One of the most common wall switches is operated by a small toggle lever which moves between up and down positions. In the United States, the up represents the "on" position. In other nations, including the United Kingdom, Ireland and Australia, the down position creates the closed circuit or "on."
  
  

Toggles and Quiet Switches

• The exterior toggle operates a mechanism of springs and levers within the switch. When functioning properly, the toggle rapidly snaps the contacts to open or closed position just after it is moved halfway to the new position. "Quiet switches," used in the United States since the 1990s, silence the mechanism so there is no sudden click or snap as the toggle is manipulated.
  
  

Contact Surfaces

• The contact area itself is a small brass, silver or aluminum surface. The switches are designed to wipe clean the contact area each time they are used, which prevents a buildup of dust and corrosion on the contact surface. This design allows wall switches to remain in use for many years without any regular maintenance. Functioning wall switches can last decades without a breakdown, and some of the earlier designs are still in use.
  
  

New Switch Designs

• Since the invention of the light switch, there have been a great variety of switch and wall-plate designs. Push-button switches were common in the mid-twentieth century. Mercury switches in the 1960s employed a glass vial in which the movement of mercury opened and closed the circuit. These have gone out of fashion with concerns over the release of toxic mercury. Modern rocker switches use thin, rectangular "paddles" that lie flush to the switch plate. A dimmer switch controls a continuous flow of electricity, allowing the light to gradually dim from full brightness. More expensive devices use touch plates or motion sensors to open and close the circuit.