Your home electricity supply is alternating current (AC), while electricity from generators or transformers is direct current (DC). AC electricity from your home outlet automatically reverses about 60 times per second. Given the high voltage supplied to your home, AC is safer than DC; if you were to accidentally touch a hot wire, the alternating effect of the current means it repels you so you don’t stick to the wire. However, DC electricity flows in a single direction, so if you were to touch a high-voltage hot wire, you would stick to the wire and continue to receive an electric shock. You can reverse DC electricity by changing the polarity.
Turn off the device that is producing the electricity you want to reverse. If it’s a transformer, remove the plug from the wall socket; if it’s a generator, press the stop button, or turn the switch to the “Off” position.
Locate the two terminals on the transformer or generator. Wires extend from the terminals to the device it powers, such as a motor. One of the terminals is labeled "+", meaning positive and the other "-", meaning negative. You’re likely to find that the wires are colored red and black for positive and negative respectively.
Make a note of the colors of the wires that connect to the positive and negative terminals. Remove the wires from the terminals. Use a screwdriver to loosen the terminal screws holding the wires in place, then slide the wires out from under the screws using your fingers. If the wires have metal rings on the end, you need to completely remove the screws in order to disconnect the wires.
Attach the wire that was connected to the positive terminal to the negative terminal. Either slide the wire under the terminal screw using your fingers or place the ring on the terminal then replace the screw. Tighten the screw using a screwdriver.
Attach the wire that was connected to the negative terminal to the positive terminal. Slide the wire under the screw and tighten using a screwdriver, or place the metal ring on the terminal and replace the screw then tighten.
Turn on the transformer or generator. Either insert the plug into the wall socket, or if it’s a generator, press the start button or turn the switch to the “On” position. The electrical supply is reversed and you notice that if the device you’re powering is motorized, it’s rotating the opposite way than it had before.
If your electrical device is battery powered, simply disconnect the wire from the positive terminal and attach it to the negative terminal and disconnect the wire from the negative terminal and attach it to the positive terminal. All batteries produce DC current so you will reverse the flow of electricity.
This article is intended for use with low-voltage power supplies; up to about 12 volts. Never reverse the polarity if the voltage is high; it can be dangerous.