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|As an example, let’s say that for a particular card you
decide that “B” shall be the correct answer. After writing
the questions and answers on the card (the correct answer “B” and two false answers “A” and “C”), you
glue a piece of foil to the card at location “B” (see the second diagram).
When you insert the card into the switch contacts, the foil completes the series circuit that controls answer light “B.” Thus, when your friend touches answer bar “B,” the “B” bulb lights up. But, if he touches bar “A” or “C,” nothing happens — their circuits are not completed by a piece of foil.
The same sort of thing happens if you make “A” or “C” the correct answer; the foil completes the appropriate circuit so that only answer light “A” or “C” will light. Here’s how to build the machine:
You’ll need three pairs of switch contacts. Make each contact from a strip of tin-can metal about 1 inch by 3 inch. Follow the instructions for cutting open an old tin can given in the first experiment; be careful of sharp edges. Bend each strip into an “L” — the long leg of each “L” should be about 2 inches long.
Make the machine’s base from a piece of wood about 6 inches by 8 inches. Mount the pairs of switch contacts at the locations shown in the drawings. Use short wood screws to fasten the short legs of the “Ls” to the base.
This is important: Each pair of contacts must be mounted so that the long legs are back-to-back and very close to each other. But, the long legs must not touch each other. Use a thin piece of cardboard as a spacing guide.
Next, fasten the three flashlight bulb sockets to the base with small screws. Then add the interconnecting wires: To make connections to the switch contacts, loop the ends of the connecting wires underneath the mounting screws and then tighten the screws.