Create a variable capacitor on your own to save the expense of purchasing one. Capacitors store energy for use by devices in electrical circuits. For example, many motors incorporate a capacitor to help them start. The higher the capacitance, measured in farads, the more energy the device can store. In its simplest form, a capacitor consists of two parallel metal plates separated by a distance less than a millimetre. A variable capacitor has moving parts to change its capacitance by changing the area of the metal plates facing each other.
Mark the centre of both pieces of cardboard with an "X." Set the compass legs 3 inches apart, then draw a 6-inch diameter circle on each piece of cardboard.
Use a razor knife to cut out each cardboard circle. Apply contact cement to 1/2 of each circle-shaped piece of cardboard. Adhere aluminium foil to the contact-cement-covered regions. Be sure to lay the foil as flat as possible. Remove excess foil from the edges of the cardboard. Set the compass legs 1/2 inch apart and draw a circle centred on each circle 1 inch in diameter. Remove the aluminium foil from these circles.
Cut one of the circular pieces of cardboard in half such that the portion without foil is removed. Discard the chunk with no foil. The full circle is the bottom of the variable capacitor while the semicircle is the top.
Strip one end of each length of wire. Using the sharp end of the compass punch a hole 1 inch left of the centre of the bottom capacitor portion and 1/4 inch down from the foil. Tape one striped wire end to the foil above the hole and feed the rest of the wire through the hole.
Poke a hole into the top portion of the capacitor 1/2 inch from its curved, left edge and 1/2 inch above its straight edge. Adhere the stripped end of a wire just above the hole and push the rest of the wire through the hole.
Apply contact cement to the foil side of the top of the capacitor. Glue the plastic protective sheet to the semicircle and remove excess plastic from its edges. This plastic creates a gap between the capacitor plates, so they may store energy.
Place the top of the capacitor on its bottom so that the foil sides face each other. Make sure the edges are flush. Insert the screw into the centre of the non-foil covered side of the capacitor's bottom. Make sure the screw comes up through the top part of the capacitor and screw on the nut. The variable capacitor is complete. The other ends of the wires are used to wire to a device or battery. Vary the capacitance of the device by rotating the top part of the capacitor. When the foil of both portions completely overlap the capacitance is greatest and should be near 300 picofarads. Pico stands for 10 to the power minus 12.