Show 0 an exchange says an ordinary potato may be used to tell which is the positive and which is the negative terminal of a circuit insert the two current carrying wires into the freshly cut surface A green stain due to dissolved copper indicates the positive wire if both wires are surrounded by dark colored stains the current is alternating if you a potato handy place both terminals in water bubbles will collect at ahe the end of the negative wire if the water is in a metal vessel be very careful not to let the wire we touch the metal or a short circuit willae will be formed |