Treebeard's Homepage : Stumpers

Treebeard's Stumper Answer
6 Mar 98

An Electric Stumper

We've been building electric circuits in science class at school, so here's an electric stumper. Below is a picture of a simple circuit with an old-fashioned knife switch, a light bulb, and a couple of posts to connect to a battery. But in this circuit, the light goes ON when the switch is open, and it goes OFF when the switch is closed! Of course the circuit is a trick. There is some extra wiring hidden in the box. But there are only hidden wires, no relays or secret switches. How does this stumper circuit work?


In the stumper circuit, the light goes ON when the switch is open, and OFF when the switch is closed, just the opposite of what we expect. Justin was the first student to figure out how this works. The wire between the switch and the bulb is a fake that's broken inside the insulation. When the switch is open, a hidden wire completes the circuit to the bulb. But when the switch is closed, another hidden wire makes a direct short circuit that bypasses the bulb. Just don't leave it "off" for long!

This is a fun project to build! The knife switch, 6 volt bulb, and bulb holder are all available at Radio Shack. I built mine on a scrap of 2x2 pine wood. Use two bolts for the binding posts. Drill holes through the block to pass the bolts and hidden wires, and carve a groove along the bottom to hold them. Then glue a piece of thin wood over the bottom to cover the evidence. I used 1/8" basswood from the hobby shop. Strip an inch of insulation from a piece of wire, and reinsert bits of bare wire on each end to make the fake wire between the switch and the bulb. This "fake" wire is the real stumper, so make it good. Everyone expects the hidden wires, but this trick is in plain view. This is an example of a magician's misdirection.

Use temporary jumpers to connect the bolts to the battery so you don't forget that "off" is really ON. It's probably best to use an old battery!

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Copyright © 1998 by Marc Kummel / mkummel@rain.org