You found an AC/DC power adaptor at a thrift shop, but you are not sure about the voltage polarities at the device's tip. Most manufacturers use the centre pin for positive voltage and the outer shield for negative voltage. Some manufacturers use the opposite convention. This manufacturing standard uses the centre pin for the negative voltage. Connecting an adaptor with the wrong polarity could cause damage to your electronic device. A simple test with a voltmeter will reveal the adaptor's polarity.
Insert the voltmeter's red test lead into the red jack marked "V." Insert the black test lead into the blackjack marked "COM."
Turn the voltmeter on. Set the range selector knob to the highest range marked "VDC."
Plug the AC/DC adaptor into a wall outlet.
Place the red test lead into the centre pin on the AC/DC's output connector.
Place the black test lead onto the outer shield of the AC/DC's output connector.
Read the voltage on the voltmeter's display. Do not let the two test leads touch each other and cause a short circuit. If the voltage read is positive, the centre conductor is the positive voltage. If the voltage read is negative, the centre conductor is the negative voltage.
Use a small piece of wire to measure the centre conductor's voltage, if the test lead is too large to insert.