Test your LED to determine color, brightness and functionality. Pinch the LED legs, or leads, to the battery terminals. The longer LED lead, called the anode, should be touching the positive terminal (+) of the battery and the shorter LED lead, called the cathode, should be touching the negative terminal (-) of the battery.
Note that the positive terminal on the battery has a larger contact surface than the negative terminal. The positive terminal extends around the sides of the battery. Don't let the cathode lead of the LED accidentally touch the positive terminal of the battery. This will create a short and cause the LED to function improperly.
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