<P> Standardized 12 volt DC automobile plugs are defined in the United States by UL Standard 2089 regarding vehicle battery adapters . This standard covers plugs and cord sets that insert into cigarette lighter receptacles . In Europe, 12 volt plugs and sockets are not specially regulated, and do not require approvals for the CE mark . </P> <P> The male plug is sometimes used to feed power into a vehicle to recharge its battery . For instance, portable solar battery maintainers generally connect to a vehicle's battery in this manner . Trickle chargers also sometimes connect in this way, eliminating the need to leave a vehicle's hood open, as well as eliminating the possibility of reversed polarity . In some models, the cigarette lighter outlet is not powered when the ignition key is removed and charging is not possible . </P> <P> The sockets and mating plugs are defined in the ANSI / SAE J563 specification . For the 12 volt systems, the "contact point", which is the center part of the plug when viewed end - on, carries the positive voltage, whereas the "can" part, which is the outer part of the connector, carries the negative voltage (which is the "ground" connection for most automobiles, which have a negative ground electrical system). </P> <P> 12 volt auto connectors are made to comply with a standard by Underwriters Laboratories for safety . UL2089 was developed to cover the requirements for portable adapters rated 24 V DC or less that are intended to be supplied from the battery powered electrical system of a vehicle . Products covered by the standard include cord assemblies of a plug that mates with the standard cigarette receptacle found in automobiles . </P>

Where is the positive on a cigarette lighter
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