What do the 3 pins on a 2.1mm DC power jack do?

What do the 3 pins on a 2.1mm DC power jack do? I am trying to hook one up to my breadboard but have had no success. I've soldered a wire to each port and tried hooking them into the power rails but no combination seems to work. I started with the two I was told were positive and negative.

Which prongs do I use for vmot and gnd? I've linked to the specific port I am using bellow.

link: http://www.allelectronics.com/make-a-store/item/dcj-1/2.1mm-dc-power-jack/1.html?extra=a%3A2%3A{i%3A0%3Bs%3A40%3A%2203823345592a403b2f4a37a59384e7ab28f02be1%22%3Bi%3A1%3BN%3B}

I got it working! I had the positive and negative backwards!

What do the 3 pins on a 2.1mm DC power jack do?

Ok, then answer your own question.

Hint: sometimes you want to run a device from batteries when the mains supply is not connected.