Transformers change the voltage of your system to suit your electronics. If you travel to a country that uses a different voltage than the U.S., you will need to bring your transformer with you. To use your hair dryer, electric shaver, charger for cellphones and other electronic equipment, simply plug the transformer into the electrical socket and then plug your electronic item into the transformer. It's a good idea to test that your transformer is working before heading out on a long trip. A multimeter is used to test your transformer while you are still at home.
Plug the transformer into the electric socket. Unravel the cord and inspect the jack at the end of the cord.
Check the transformer's label to find which part of the jack is the positive and which is the negative. You will usually find that the thicker part of the jack closest to the cord is positive, while the slimmer part of the jack is negative.
Place the positive (red) probe from the multimeter on the tip of the jack and the negative (black) probe on the larger inside area.
Turn the dial on the multimeter to read DC current and set the range to 20V.
Working with electricity is extremely dangerous; do so with due caution. Do not plug in transformers that have frayed cords or are visibly damaged as this may result in shock or injury.