LED electroscope

In preparation for teaching electrostatics next week, I was searching the web for some demonstrations that were a little more exciting than combs attracting bits of paper. The LED electroscope is one of the ones I found, and I think it's really cool.

Its beauty comes from its sheer simplicity of construction. First, you take a normal 9V PP7 battery connector, slice off the top moulding and remove the wires (if you're careful, you can keep the plastic that you slice off and re-insert it later for a neater finish). You then solder an LED and a a FET (field effect transisitor) in series across the two terminals (making sure you have the correct polarity). The gate pin of the transisitor remains unconnected and acts as the antenna.

When you connect the battery, the LED glows with around 80% brightness (there's plenty of resistance across the transistor, that's also why you don't need a protective resistor for the LED). If you charge a plastic ruler and bring it within around 15-20 cm of the transistor, the LED goes out.

I built one with a highly directional green LED, but I'm going to try a super-bright, wide-angle blue LED to see if I can get a more striking demonstration, possibly pannel mounted, if I get time.