CoachAn electric circuit is a complete loop for electricity: out of one terminal of the battery, through the wires and the bulb, and back into the other terminal. When the loop is complete we say the circuit is closed; if it has a gap anywhere, it is open.
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Example 16.7 (The switch)
A switch is a door in the circuit: a little metal bridge that the click lays down (closed — the loop is complete, light!) or lifts open (a gap — dark). The flashlight’s button, the light switch on the wall, the button of a doorbell: all of them are doors in a loop. You can build one from a clothes-peg and a strip of kitchen foil folded over its jaws.