Joey Green's Mad Scientist

Switching Switch

What You Need

Light bulb base
Plastic project enclosure box (6 by 3 by 2 inches)
Electric drill with 3/8-inch bit and 1/16-inch bit
Precision screwdrivers
Needle-nose pliers
Three push-on/push-off switches
Sharpie marker
Krazy Glue
AA battery holder
Wire cutters
Wire
Soldering iron
Solder
AA battery
2-volt light bulb

What to Do
Mount the light bulb base on the cover panel of the project enclosure box. With adult supervision, use the drill with the 3/8-inch bit to mount the three push-on/push-off switches under the light bulb base. Use the Sharpie marker to label them A, B, and C. Using Krazy Glue, adhere the battery holder to the underside of the cover panel. Wire the switches in series (as illustrated in the schematic diagram) to the battery holder and light bulb base. With an adult watching, solder the connections. Insert the battery and light bulb. Screw the cover panel into place on the project enclosure box.

Press all the buttons so the bulb lights. Push button A several times to show that it controls the bulb. Leave the switch off. Push button B an odd number of times to show that it does not affect the bulb. Then push button C an odd number of times to show that it does not affect the bulb. This leaves all the buttons turned off.

Ask a volunteer to push whatever button he thinks will turn on the bulb. He naturally pushes button A, but the bulb does not light. Give him a second chance. He pushes button B, but again the bulb does not light. You push button C, which miraculously lights the bulb.

Repeat the trick by pushing button C several times to show it does indeed light the bulb, then leave it off. Push the other two buttons an odd number of times to show that they do not light the bulb. Once again, ask the volunteer to push whatever button he thinks will turn on the bulb.

What Happens
No matter how many times you repeat the trick, the volunteer continues failing on his first two tries. The third button selected always lights the bulb. Most people assume there is a perplexing circuit pattern under the board.

Why It Works
The switches and the bulb are joined in series to the battery.

All the switches are turned off when the volunteer is asked to push a button. His first choice turns on the first button, his second choice turns on the second button, and the third button (which you push) completes the circuit and lights the bulb.

Bizarre Facts

The trick of making a pea vanish under one of three thimbles originated in Egypt some five thousand years ago using cups and balls. The trick is done with sleight of hand. The conjurer lets the pea roll out from under the cup, quickly catches it in the hand holding the cup, and then drops it into another thimble.
Three Card Monte—a card trick in which a gullible spectator is asked to wager money on which one of the three cards lying face down on a table is the queen—is rigged by a fake throw. The operator, previously throwing the bottom card from his right hand before the top card, unexpectedly throws the top card from his right hand before the bottom one.

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