External power supply polarity?

Hi Simple question (and hopefully not too stupid) but i cant find the answer anywhere. When Hooking up an external battery (in this case a 9v battery) do I use a positive or negative center? Could take a chance but would prefer to know first so I dont pop something!



the centre pin is positive... and there is also a blocking diode in the Arduino USB to prevent reverse polarity connections.


Thanks Daniel. Thought that there might be a diode but didn't want to risk it as it takes awhile to get the boards send to NZ.