Capacitors, both small- and large-volume, are found in nearly all forms of electronic equipment. These components perform two vital actions in any electronic circuit: they store electricity, and they filter out direct current (DC) while passing through only alternating current (AC). Electrolytic capacitors are designed to store a larger amount of electricity, and they have polarity--meaning they have a positive side and a negative side. Electronics engineering standards stipulate that such capacitors must be manufactured with polarity markings on them so as to assist in proper placement of the capacitors into a circuit.
Place your capacitor on a shop workbench or a table in a well-lit area. Hold your magnifying glass over the capacitor to magnify the markings on the external body of the component.
Rotate the capacitor slowly while watching for the markings and graphical images on the capacitor. Some electrolytic capacitors will have only the negative side marked; the symbols will appear as ovals that look like minus signs (-), indicating the negative side of the capacitor. Some capacitors will have a white or a black stripe on the negative side. In such a case, the opposite side of the electrolytic capacitor is the positive side. If you cannot find the minus sign but you are able to see a plus sign (+), then the plus sign indicates the positive side and the other is the negative side.
Look at the two wire leads that come from the bottom of your capacitor, and make special note of whether one lead is shorter than the other. Some manufacturers will deliberately make the negative-polarity lead shorter than the positive lead, which may also assist in easily spotting the polarity. Double-check the markings on the capacitor body to verify that shorter lead is indeed the negative side of the capacitor.
Once you have located the polarity markings on your capacitor, be certain that you install the negative wire lead onto the side of your circuit from which the negative current is coming and the positive wire lead on the side toward which the positive current flows. The capacitor will be damaged and may even explode if inserted backward into the circuit. Unless the capacitor is brand new, never touch both of the wire leads with your fingers, as some capacitors store a high voltage, sometimes for months, after a power source has been removed from them.