Many electrical devices use direct current

Direct current

Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

 (DC) power, often provided by a AC (alternating current

Alternating current

In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

) to DC adapter

Adapter

An adapter or adaptor is a person that adapts or a device that converts attributes of one device or system to those of an otherwise incompatible device or system.Some adapters may only affect physical attributes:...

, also known as a wall wart

Wall wart

The AC adapter, AC/DC adapter or AC/DC converter is a type of external power supply, often enclosed in what looks like an over-sized AC plug. Other names include plug pack, plug-in adapter, adapter block, domestic mains adapter, line power adapter, or power adapter...

or power brick. The polarity of the adapter cord 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.

Centre Positive.

Indicates that the centre (tip) of the output plug is Positive (+)
and the barrel of the output plug is Negative (-).
Centre Negative.

Indicates that the centre (tip) of the output plug is Negative (-)
and the barrel of the output plug is Positive (+).

External links

Examples of this symbol may be found at the following sites. The first two show the symbol and more or less state the obvious about its relation to the sockets and plugs of devices and adapters. The third shows the symbol and specifies which arrangement of the plus and minus signs constitutes which polarity.

• http://www.accesscomms.com.au/reference/polarity.htm
• http://www.stagebeat.co.uk/index.php?page_id=30006