Many electrical devices use direct current (DC) power, often provided by an AC (alternating current) to DC adapter, also known as a "wall wart" or "power brick". The adapter typically supplies power to the device through a thin electrical cord which terminates in a coaxial power connector often referred to as a "barrel plug" (so-named because of its cylindrical shape). The polarity of the adapter cord and plug must match the polarity of the device, meaning that the positive contact of the plug must mate with the positive contact in the receptacle, and the negative plug contact must mate with the negative receptacle contact. Since there is no standardization of these plugs, a polarity symbol is printed on the case indicating which type of plug is needed.
The commonly used symbol denoting the polarity of a device or adapter consists of a black dot with a line leading to the right and a broken circle (like the letter "C") surrounding the dot and with a line leading to the left. At the ends of the lines leading right and left are found a plus sign (+), meaning positive, and a minus sign (-), meaning negative.
The symbol connected to the dot (usually the symbol found to the right) denotes the polarity of the device or adapter. Thus a device with a plus sign at the end of the line leading from the center dot is said to have "positive polarity" and requires an adapter that has positive polarity. A device with a minus sign at the end of the line leading from the center dot has "negative polarity" and requires an adapter of negative polarity.
Indicates that the center (tip) of the output plug is Positive (+) and the barrel (ring) of the output plug is Negative (-).
Indicates that the center (tip) of the output plug is Negative (-) and the barrel (ring) of the output plug is Positive (+).
Examples of these symbols may be found at the following site.