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Lewis Latimer once invented a switch that was meant to control an electric light bulb. The switch mechanism was a bit unusual: When the knob was turned, a metal pin came between two metal strips to complete the circuit (see the sketch). You can build a switch that works in much the same way . . . and use it in the electrical experiments that we’ll talk about later. Since you will need three switches for the parallel circuit experiment, gather together enough material to build three switches before you begin. Our switch does not use a metal pin. Instead, a metal foil surface on its “knob” comes between two metal contact strips to complete the circuit. Here’s how to build it: Use an empty thread spool as the knob. As shown in the drawing, glue a small piece of aluminum foil between the two end flanges. The piece of foil should run a little over halfway around the spool. This is important: Apply cement all over the back of the piece of foil so that all of the foil is held tightly against the wooden surface. The foil must fit smoothly in place with no bubbles or tears. Make the two contact strips out of metal cut from a tin can. Clean the can thoroughly, then remove both ends. Next, cut the can open lengthwise with tin snips and press it flat. The cut edges will be sharp, so work carefully. Each contact strip ‘should measure about 1/2 inch by 1 inch. Bend each strip into an “L” — each leg of the “L” should be about 1/2 inch long. The switch’s base is a piece of wood about 2 inches square. Mount the knob on the center of the base with a long wood screw. Place a washer at the top and bottom of the spool. Tighten the screw just enough so that the knob can’t jiggle around but will still turn freely.