A switch is a way to turn the electrical current off and on. The diagram for a switch is which is a picture of how it works: as drawn, the two wires are not electrically connected and no current can go through the device; but if you bring the two ends together, the wires are connected electrically. Some switches actually look exactly like this inside.

When the switch closes, it connects the yellow wire on the right to the green wire on the left

With these hints, design and construct a switch for your circuits. Of course, you could have a pair of wires with bare ends and a little card that says, "Twist these wires together to turn on the light," but that isn't very elegant. See what you can make using alligator clips and household or office materials (paper clips, tape, index cards or a piece of cardboard, foil, staples, straws,...). Test your switch by using it to turn on a light bulb or a motor.

Most of the electrical components that we have studied serve to turn electrical energy into other forms: motion, light, or sound. The current through the device provides the energy, and determines what will happen. A switch is different: it lets us control the circuit. It lets us put information into the system: stop, go, yes, no! When it is too cold, the thermostat turns on the switch that makes the furnace go. A century ago, the telegraph was a revolutionary new means of communication which consisted of just a switch and a circuit. Closing a switch in New York would cause a light to blink in San Francisco; the pattern of blinks would encode a message.

Still more interesting is to have a switch that can be turned on and off electrically. We could make one, with a motor and some wires, but transistors accomplish this with no moving parts. This allows one current to control another. Connect together a few hundred million of these, and you can make your own computer.

Check the box when you are done:  
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