Polarity Detector

You have a battery (you peeled the labels off, for reasons only you know). You are about to put it in your camera, but suddenly doubt falls over you. Which side is positive, and which is negative? With no label, you will never know.

Or will you?

Introducing the  
Polarity Detector! 

Just attach the electrodes to your battery and a light will light up on the positive side! great!

This is how it works: