LED's are polarised. They need to be connected the right way round to work. Plugging one into GND & pin 13 is not a good idea on an arduino made in the last 5 years (including the Uno). There is no current limiting resister in circuit. Its completely unnecessary any way, any arduino made in the last 5 years (The Uno) will have one on board, already wired and working. Its usually labelled 'L'. The tutorial you've found is out of date and doing it today can damage the Arduino. You may have already damaged the output on Pin 13.
The tutorial on this site has you connecting the longer leg of the LED to a resistor, but as you say, not really necessary anyway.
Quote from: iOSyum on Oct 28, 2012, 05:33 pmThe tutorial on this site has you connecting the longer leg of the LED to a resistor, but as you say, not really necessary anyway. No. A resistor is ABSOLUTELY necessary, unless you want to buy new ATmega328 chips every so often.
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