which size is the one on Arduino Uno ? ... did not find it on www.arduino.cc
Thanks for not answering too quickly. ( better than a "search and you'll find " type reply )
Selecting "Hardware" on the ain page, then the board type, leads me to http://arduino.cc/en/Main/ArduinoBoardUno
which says rather clearly:
External (non-USB) power can come either from an AC-to-DC adapter (wall-wart) or battery. The adapter can be connected by plugging a 2.1mm center-positive
plug into the board's power jack.
Leads from a battery can be inserted in the Gnd and Vin pin headers of the POWER connector.